Reionization with galaxies and active galactic nuclei

Pratika Dayal; Marta Volonteri; Tirthankar Roy Choudhury; Raffaella Schneider; Maxime Trebitsch; Nickolay Y Gnedin; Hakim Atek; Michaela Hirschmann; Amy Reines

doi:10.1093/mnras/staa1138

Reionization with galaxies and active galactic nuclei

Dayal, Pratika; Volonteri, Marta; Choudhury, Tirthankar Roy; Schneider, Raffaella; Trebitsch, Maxime; Gnedin, Nickolay Y; Atek, Hakim; Hirschmann, Michaela; Reines, Amy 2020-07-01 00:00:00 Downloaded from https://academic.oup.com/mnras/article/495/3/3065/5843276 by DeepDyve user on 13 July 2022 MNRAS 495, 3065–3078 (2020) doi:10.1093/mnras/staa1138 Advance Access publication 2020 May 23 1‹ 2 3 Pratika Dayal , Marta Volonteri, Tirthankar Roy Choudhury , 4,5,6 2,7,8 9,10,11 Raffaella Schneider, Maxime Trebitsch , Nickolay Y. Gnedin, 2 12 13 Hakim Atek, Michaela Hirschmann and Amy Reines Kapteyn Astronomical Institute, University of Groningen, PO Box 800, NL-9700 AV Groningen, the Netherlands Institut d’Astrophysique de Paris, Sorbonne Universite, CNRS, UMR 7095, 98 bis bd Arago, F-75014 Paris, France National Centre for Radio Astrophysics, Tata Institute of Fundamental Research, Pune 411007, India Dipartimento di Fisica, ‘Sapienza’ Universita ` di Roma, Piazzale Aldo Moro 5, I-00185 Roma, Italy INAF/Osservatorio Astronomico di Roma, Via di Frascati 33, I-00040 Monte Porzio Catone, Italy INFN, Sezione Roma 1, Dipartimento di Fisica, ‘Sapienza’ Universita ` di Roma, Piazzale Aldo Moro 5, I-00185 Roma, Italy Max-Planck-Institut fur ¨ Astronomie, Konigstuhl ¨ 17, D-69117 Heidelberg, Germany Zentrum fur ¨ Astronomie der Universitat ¨ Heidelberg, Institut fur ¨ Theoretische Astrophysik, Albert-Ueberle-Str. 2, D-69120 Heidelberg, Germany Particle Astrophysics Center, Fermi National Accelerator Laboratory, Batavia, IL 60510, USA Kavli Institute for Cosmological Physics, The University of Chicago, Chicago, IL 60637, USA Department of Astronomy and Astrophysics, The University of Chicago, Chicago, IL 60637, USA DARK, Niels Bohr Institute, University of Copenhagen, Lyngbyvej 2, DK-2100 Copenhagen, Denmark eXtreme Gravity Institute, Department of Physics, Montana State University, Bozeman, MT 59717, USA Accepted 2020 April 21. Received 2020 April 16; in original form 2020 January 16 ABSTRACT In this work we investigate the properties of the sources that reionized the intergalactic medium (IGM) in the high-redshift Universe. Using a semi-analytical model aimed at reproducing galaxies and black holes in the ﬁrst ∼1.5 Gyr of the Universe, we revisit the relative role of star formation and black hole accretion in producing ionizing photons that can escape into the IGM. Both star formation and black hole accretion are regulated by supernova feedback, resulting in black hole accretion being stunted in low-mass haloes. We explore a wide range of combinations for the escape fraction of ionizing photons (redshift-dependent, constant, sf bh and scaling with stellar mass) from both star formation (f ) and AGN (f ) to ﬁnd: (i) the esc esc ionizing budget is dominated by stellar radiation from low stellar mass (M < 10 M ) galaxies 6 9 at z> 6 with the AGN contribution (driven by M > 10 M black holes in M 10 M bh ∗ galaxies) dominating at lower redshifts; (ii) AGN only contribute 10 − 25 per cent to the cumulative ionizing emissivity by z = 4 for the models that match the observed reionization sf bh constraints; (iii) if the stellar mass dependence of f is shallower than f ,at z< 7 esc esc a transition stellar mass exists above which AGN dominate the escaping ionizing photon production rate; (iv) the transition stellar mass decreases with decreasing redshift. While AGN dominate the escaping emissivity above the knee of the stellar mass function at z ∼ 6.8, they take-over at stellar masses that are a tenth of the knee mass by z = 4. Key words: galaxies: evolution – galaxies: high-redshift – intergalactic medium – quasars: general – reionization. ﬁrst metal-free (population III; PopIII) stars at z 30, with the 1 INTRODUCTION key sources gradually shifting to larger metal-enriched haloes, The epoch of (hydrogen) reionization (EoR) begins when the ﬁrst powered by population II (PopII) stars and accreting black holes. stars start producing neutral hydrogen (H I) ionizing photons and However, this picture is complicated by the fact that the progress and carving out ionized regions in the intergalactic medium (IGM). sources of reionization depend on a number of (poorly constrained) In the simplest picture, the EoR starts with the formation of the parameters including the minimum halo mass of star-forming galaxies, the star formation/black hole accretion rates, the escape fraction (f )ofH I ionizing photons from the galactic environment, esc E-mail: [email protected] the impact of the reionization ultraviolet background (UVB) on the The Author(s) 2020. Published by Oxford University Press on behalf of The Royal Astronomical Society. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited. Downloaded from https://academic.oup.com/mnras/article/495/3/3065/5843276 by DeepDyve user on 13 July 2022 3066 P. Dayal et al. gas content of low-mass haloes and the clumping factor of the IGM this model lie in that: (i) it is seeded with two types of black hole (see e.g. Dayal & Ferrara 2018). seeds (stellar and direct collapse); (ii) the black hole accretion rate Observationally, a number of works have used a variety of data is primarily regulated by the host halo mass; (iii) it uses a minimal sets and trends – e.g. the UV luminosity density, the faint-end set of free parameters for star formation and black holes and their slope of the Lyman Break Galaxy (LBG) luminosity function, f associated feedback. esc increasing with bluer UV slopes, and the abundance and luminosity The cosmological parameters used in this work correspond to , distribution of galaxies – to conclude that star formation in low- , , h, n , σ = 0.3089, 0.6911, 0.049, 0.67, 0.96, 0.81 (Planck b s 8 mass galaxies with an absolute magnitude M −10 to −15 Collaboration XIII 2016). We quote all quantities in comoving units UV alone can reionize the IGM (Bouwens et al. 2012; Finkelstein et al. unless stated otherwise and express all magnitudes in the standard 2012; Duncan & Conselice 2015; Robertson et al. 2015), although AB system (Oke & Gunn 1983). Naidu et al. (2019) assume f ∝ the star formation rate surface The paper is organized as follows. In Section 2, we detail our esc density and infer that high stellar mass (M 10 M ) galaxies code for the galaxy-BH (co)-evolution, our calculation of f and the ∗ esc dominate the reionization budget (see also Sharma et al. 2016). The progress of reionization. The results of the ﬁducial and of alternative bulk of the observational results are in agreement with theoretical models are presented in Sections 3 and 4. Finally, we discuss our 9.5 results that converge on stars in low-mass haloes (M 10 M results and present our main conclusions in Section 6. and M −17) providing the bulk of H I ionizing photons at z 7 UV (e.g. Choudhury & Ferrara 2007; Salvaterra, Ferrara & Dayal 2011; 2 THEORETICAL MODEL Yajima, Choi & Nagamine 2011;Wiseetal. 2014; Paardekooper, We start by introducing the galaxy formation model in Section 2.1 Khochfar & Dalla Vecchia 2015; Liu et al. 2016; Dayal et al. 2017a). before discussing the escape fraction of ionizing radiation from A key caveat in the results, however, is that the redshift-dependent galaxies and AGN in the ﬁducial model in Section 2.2. These are reionization contribution from star formation in galaxies of different used to calculate the reionization history and electron scattering masses crucially depends on the strength of UVB feedback, the trend optical depth in Section 2.3. Our ﬁducial model parameters are of f with mass and redshift and the evolution of the clumping esc described in Table 1. factor (for details see Section 7, Dayal & Ferrara 2018). In addition, the contribution of Active Galactic Nuclei (AGNs) to reionization and its dependence on redshift and on the host 2.1 Galaxy formation at high-z galaxy stellar mass still remain key open questions. A number of works show AGN can only have a minor reionization contribution In this work, we use the semi-analytic code Delphi (Dark matter and (Onoue et al. 2017; Yoshiura et al. 2017;Hassanetal. 2018). the emergence of galaxies in the epoch of reionization) that aims at Contrary to these studies, a number of results show that radiation simulating the assembly of the dark matter, baryonic and black hole from AGN/quasars might contribute signiﬁcantly to reionization components of high-redshift (z 5) galaxies (Dayal et al. 2014, (Volonteri & Gnedin 2009; Madau & Haardt 2015; Mitra, Choud- 2019). In brief, starting at z = 4 we build analytic merger trees up hury & Ferrara 2015, 2018; Grazian et al. 2018; Finkelstein et al. to z = 20, in time-steps of 20 Myr, for 550 haloes equally separated 8 13.5 in log space between 10 and 10 M . Each halo is assigned a 2019), especially at z 8 if ionizations by secondary electrons are number density according to the Sheth–Tormen halo mass function accounted for, with stars taking over as the dominant reionization (HMF) which is propagated throughout its merger tree; the resulting sources at z 6 (Volonteri & Gnedin 2009). The question of the HMFs have been conﬁrmed to be in accord with the Sheth–Tormen contribution of AGN to reionization has witnessed a resurgence HMF at all z ∼ 5–20. after recent claims of extremely high number densities of faint The very ﬁrst progenitors of any galaxy are assigned an initial AGN measured by Giallongo et al. (2015, 2019)at z 4. While gas mass as per the cosmological baryon-to-dark matter ratio such other direct searches for high-redshift AGN have found lower that M = ( / )M ,where M is the halo mass. The effective number densities (Weigel et al. 2015; McGreer et al. 2018), the gi b m h h eff star formation efﬁciency, f ,for anyhaloiscalculatedasthe integrated H I ionizing emissivities can be signiﬁcantly affected by minimum between the efﬁciency that produces enough type II the inhomogeneous selection and analysis of the data and by the ej supernova (SN II) energy to eject the rest of the gas, f ,and an adopted (double) power law ﬁts to the AGN luminosity function ∗ ej eff upper maximum threshold, f ,sothat f = min[f ,f ]where at different redshifts (Kulkarni, Worseck & Hennawi 2019). Yet, if ∗ ∗ ∗ a fraction f of the SN II energy can couple to the gas. The gas the high comoving emissivity claimed by Giallongo et al. (2015) w mass left after including the effects of star formation and supernova persists up to z 10, then AGN alone could drive reionization feedback is then given by: with little/no contribution from starlight (Madau & Haardt 2015). A similar scenario, where more than 50 per cent of the ionizing eff gf ∗ M (z) = [M (z) − M (z)] 1 − . (1) photons are emitted by rare and bright sources, such as quasars, gi ∗ ∗ ej has been proposed by Chardin et al. (2015), Chardin, Puchwein & Haehnelt (2017) as a possible explanation of the large ﬂuctuations in Our model also includes two types of black hole seeds that can −1 the Ly α effective optical depth on scales of 50 h cMpc measured be assigned to the ﬁrst progenitors of any halo. These include at the end stages of reionization (4 <z < 6) by Becker et al. (i) massive direct-collapse black hole (DCBH) seeds with masses 3−4 (2015). These AGN-dominated or AGN-assisted models, however, between M = 10 M and, (ii) Pop III stellar black hole seeds of bh are found to reionize helium (He II) too early (Puchwein et al. 2019) 150 M masses. As detailed in Dayal et al. (2017b), we calculate the and result in an IGM temperature evolution that is inconsistent with strength of the Lyman–Werner (LW) background irradiating each the observational constraints (Becker et al. 2011). such starting halo. Haloes with an LW background strength J LW −21 −1 −1 −2 −1 In this work, we use a semi-analytic model (Delphi)thathas > J = αJ (where J = 10 ergs Hz cm sr and α is a crit 21 21 been shown to reproduce all key observables for galaxies and AGN free parameter) are assigned DCBH seeds while haloes not meeting at z 5 to revisit the AGN contribution to reionization, specially this criterion are assigned the lighter Pop III seeds. We note that, as a function of the host galaxy stellar mass. The key strengths of given that the number densities of DCBH seeds are ∼−2(−3.8) MNRAS 495, 3065–3078 (2020) Downloaded from https://academic.oup.com/mnras/article/495/3/3065/5843276 by DeepDyve user on 13 July 2022 Reionization with galaxies and AGN 3067 Table 1. Free parameters, their symbols and values used for the ﬁducial model (ins1 in Dayal et al. 2019). As noted, using these parameter values our model reproduces all key observables for galaxies and AGN at z 5 (including their UV luminosity functions, stellar mass/black hole mass densities, star formation rate densities, the stellar/black hole mass function) as well as the key reionization observables (the integrated electron scattering optical depth and the redshift evolution of the ionizing photon emissivity). Simultaneously ﬁtting the optical depth and the emissivity constraints, we obtain f = 0.02 (0.0185) and β = 2.8(2.8) if we consider the ionizing photons provided by star formation (star formation and AGN). Parameter Symbol Value Maximum star formation efﬁciency f 0.02 Fraction of SN II energy coupling to gas f 0.1 Radiative efﬁciency of black hole accretion 0.1 Fraction of AGN energy coupling to gas f 0.003 bh ac −4 Fraction of gas mass AGN can accrete f 5.5 × 10 bh crit −5 Fraction of Eddington rate for BH accretion f (M < M )7.5 × 10 Edd h crit Fraction of Eddington rate for BH accretion f (M ≥ M)1 Edd h LW BG threshold for DCBH formation α 30 sf β Escape fraction of H I ionizing photons from star formation f f [(1 + z)/7] . esc bh Escape fraction of H I ionizing photons from AGN f Ueda et al. (2014) esc Stellar population synthesis model – Starburst99 Reionization (UVB) feedback – No orders of magnitude below that of stellar seeds for α = 30 (300), scale up (down) the UV luminosity function which could be the exact value of α (as well as the DCBH seed mass) have no accommodated by varying the free-parameters for star formation eff sensible bearing on our results, since we only consider models that (f and f ). Under these assumptions, the Starburst99 (SB99) reproduce the AGN luminosity function. In this paper we do not aim stellar population synthesis (SPS) model yields the time-evolution at investigating which type of black hole seed can contribute most of the star-formation powered production rate of H I ionizing sf to reionization, but how a population of AGN reproducing available photons (n ˙ ) and the UV luminosity (L )tobe: UV int observational constraints can contribute to reionization. sf 46.6255 −1 Once seeded, the black holes (as the baryonic and dark matter n ˙ (t ) = 10 − 3.92 log10 + 0.7[s ], (4) int 2Myr components) grow in mass through mergers and accretion in successive time-steps. A fraction of the gas mass left after star and formation and SN II ejection (see equation 1) can be accreted on to 33.077 −1 −1 the black hole. This accretion rate depends on both the host halo L (t ) = 10 − 1.33 log10 + 0.462 [erg s Å ]. UV 2Myr mass and redshift through a critical halo mass (Bower et al. 2017): (5) crit 11.25 3 0.125 M (z) = 10 M [ (1 + z) + ] , (2) m λ Inspired by the Shakura–Sunyaev solution (Shakura & Sunyaev such that the mass accreted by the black hole (of mass M )atany 1973), AGNs are assigned a spectral energy distribution (SED) that bh given time-step is: depends on the key black hole physical parameters, namely the black hole mass and Eddington ratio (Volonteri et al. 2017). We ac ac gf M (z) = min f M (z), (1 − )f M (z) , (3) Edd Edd r bh bh ∗ follow here a variant based on the physical models developed by −1 Done et al. (2012). Speciﬁcally, we calculate the energy of the peak where M (z) = (1 − )[4πGM (z)m ][σ c] t is the total Edd r bh p T r of the SED as described in Thomas et al. (2016), but adopt the mass that can be accreted in a time-step assuming Eddington default functional form of the spectrum used in Cloudy (Ferland luminosity. Here, G is the gravitational constant, m is the proton et al. 2013). mass, σ is the Thomson scattering optical depth, is the BH T r Once an AGN is assigned a luminosity and an SED, the UV radiative efﬁciency, c is the speed of light, and t = 20 Myr is the luminosity is calculated as detailed in Dayal et al. (2019). Further, merger tree time-step. Further, the value of f is assigned based Edd ac we integrate above 13.6 eV to obtain the H I ionizing luminosity and on the critical halo mass (equation 2) as detailed in Table 1 and f bh mean energy of ionizing photons (see Fig. A1 in the Appendix). represents a ﬁxed fraction of the total gas mass present in the host For AGN, this provides an upper limit, as photons above 24.59 eV galaxy that can be accreted by the black hole. A ﬁxed fraction f bh and 54.4 eV can ionize He I and He II. We further include a of the total energy emitted by the accreting black hole is allowed correction for secondary ionizations from the hard AGN photons, to couple to the gas content. The values used for each of these by taking the upper limit to their contribution, i.e. assuming fully parameters in our ﬁducial model are detailed in Table 1. Finally, neutral hydrogen and that 39 per cent of their energy goes into sec- reionization feedback is included by suppressing the gas content, ondary ionizations (Shull & van Steenberg 1985; Madau & Fragos and hence star formation and black hole accretion, of haloes with −1 2017; Kakiichi et al. 2017; Eide et al. 2018). a virial velocity V 40 km s at all redshifts, as detailed in vir Section 2.3. In the interest of simplicity, every newly formed stellar population 2.2 The escape fraction of H I ionizing photons is assumed to follow a Salpeter initial mass function (IMF; Salpeter 1955) with masses in the range 0.1 − 100 M , with a metallicity In what follows, we discuss our calculations of f for both AGN esc Z = 0.05Z and an age of 2 Myr; a lower (higher) metallicity and stellar radiation from galaxies. In addition to the ﬁducial model, or a younger (older) stellar population across all galaxies would we study ﬁve combinations of f from star formation and AGN in esc MNRAS 495, 3065–3078 (2020) Downloaded from https://academic.oup.com/mnras/article/495/3/3065/5843276 by DeepDyve user on 13 July 2022 3068 P. Dayal et al. order to explore the available parameter space and its impact on our reproducing the values of electron scattering optical depth (τ )and es results as detailed in Section 4. the redshift evolution of the emissivity require such a decrease in the global value of the escape fraction of ionizing photons from star formation. The values of f and β required to simultaneously bh 2.2.1 The escape fraction for AGN f esc ﬁt the above-noted data sets (with and without AGN contribution) are shown in Table 1. For the ionizing radiation emitted from the AGN, we consider four Secondly, whilst maintaining the same functional form, we ﬁnd different models. We start by taking an approach similar to Ricci the values of the two coefﬁcients (f and β ) required to ﬁt the optical et al. (2017) for the ﬁducial model. Essentially, we assume that 0 depth and emissivity constraints using the same escape fraction from the unobscured fraction, i.e. the fraction of AGN with column 22 −2 bh AGN and star formation. density < 10 cm is a proxy for the escape fraction, f . esc Thirdly, following recent results (e.g. Borthakur et al. 2014; The argument is that by applying a column-density dependent Naidu et al. 2019), we use a model wherein the escape fraction for correction to the X-ray LF, one recovers the UV luminosity star formation scales positively with the stellar mass. In this case, function. As in Dayal et al. (2019), we adopt the luminosity- sf bh for galaxies that have black holes, we assume f = f using dependent formalism of Ueda et al. (2014), taking as unobscured esc esc bh sf the ﬁducial model for f ; f = 0 for galaxies without a black fraction f ≡ f , which varies from 10 per cent for faint esc esc unabs logNH < 22 43 −1 hole. This accounts for the possibility that AGN feedback enhances AGN (L < 10 erg s )to 67 per cent for bright AGN 2-10keV 46 −1 the effect of SN feedback in carving ‘holes’ in the interstellar (L > 10 erg s ). The unobscured fraction can be written 2-10keV medium, facilitating the escape of ionizing radiation. This is a as: very optimistic assumption, as dedicated simulations show that 1 − ψ f = , (6) unabs AGN struggle to shine and amplify the escape fraction in low-mass 1 + ψ galaxies (Trebitsch et al. 2018). sf where ψ = ψ − 0.24(L − 43.75), ψ = 0.43[1 + min (z, z x z Fourthly, we explore a model with a constant f = 0.035. esc 0.48 2)] and L is the log of the intrinsic 2–10 keV X-ray luminosity x Although a constant escape fraction for stellar radiation from all −1 in erg s ; given our model is for z 5, this implies ψ = z galaxies can reproduce the τ value, it overshoots the value of the es 0.73. We do not extrapolate the evolution beyond z = 2, the observed emissivity (see e.g. ﬁg. 3, Dayal et al. 2017a). sf range for which the dependence has been studied using data. Finally, we explore a model wherein f increases with de- esc As in Ricci et al. (2017), we assume that unobscured quasars creasing stellar mass, as has been shown by a number of theoretical have f = 1 and zero otherwise (see their Section 4.1 for esc works (e.g. Yajima et al. 2011;Wiseetal. 2014; Paardekooper sf a discussion and alternative models and Volonteri et al. 2017, et al. 2015). Essentially, we assume f scales with the ejected esc ej sf eff for a discussion on the redshift evolution of the obscured frac- gas fraction such that f = f (f /f ). This naturally results 0 ∗ esc ∗ ej tion). sf eff sf in a high f value for low mass galaxies where f = f ; f esc ∗ esc Secondly, Merloni et al. (2014) ﬁnd that X–ray and optical ej eff drops with increasing mass where f ∼ f <f . The results from ∗ ∗ obscuration are not necessarily the same for AGN, although the these last four cases are discussed in detail in Section 4. trend of optically obscured AGN with luminosity is consistent with sf We clarify that while we assume the same f value for each esc bh the scaling we adopt. Our second model for f considers the esc galaxy, in principle, this should be thought of as an ensemble average fraction of optically unobscured AGN as a function of luminosity that depends on, and evolves with, the underlying galaxy properties, from Merloni et al. (2014), where this fraction is found to be such as mass or star formation or a combination of both. independent of redshift. It takes the functional form: 1 43.89 − log L bh 2.3 Modelling reionization f = 1 − 0.56 + arctan , (7) esc π 0.46 The reionization history, expressed through the evolution of the where log L is the logarithm of the intrinsic 2–10 keV X-ray volume ﬁlling fraction (Q ) for ionized hydrogen (H II), can be II −1 luminosity in erg s . written as (Shapiro & Giroux 1987; Madau, Haardt & Rees 1999): Thirdly, we can maximize the contribution of AGN to reionization bh by assuming f = 1, although Micheva, Iwata & Inoue (2017)ﬁnd dQ dn 1 Q dt II ion II esc = − , (8) bh that even for unobscured AGN f is not necessarily unity. dz dz n t dz esc H rec Finally, we explore a model wherein we use the same (redshift- where the ﬁrst term on the right-hand side is the source term while dependent) escape fraction for the ionizing radiation from both star the second term accounts for the decrease in Q due to recom- II formation and AGN. The results from these last three cases are binations. Here, dn /dz = n represents the hydrogen ionizing ion ion discussed in detail in Section 4. photon rate density contributing to reionization. Further, n is the comoving hydrogen number density and t is the recombination rec sf time-scale that can be expressed as (e.g. Madau et al. 1999): 2.2.2 The escape fraction for star formation (f ) esc Both the value of the escape fraction of H I ionizing radiation emitted t = . (9) rec sf 3 from the stellar population (f ) as well as its trend with the χn (1 + z) α C H B esc galaxy mass or even redshift remain extremely poorly understood Here α is the hydrogen case-B recombination coefﬁcient, χ = 1.08 sf (Section 7.1, Dayal & Ferrara 2018). We study four cases for f esc accounts for the excess free electrons arising from singly ionized in this work: ﬁrst, in our ﬁducial model, we use an escape fraction helium and C is the IGM clumping factor. We use a value of C that sf β that scales down with decreasing redshift as f = f [(1 + z)/7] esc evolves with redshift as where β> 1and f is a constant at a given redshift. This is in accord with a number of studies (Robertson et al. 2015; Dayal et al. H II −1.71 C = = 1 + 43 z (10) 2017a; Puchwein et al. 2019) that have shown that simultaneously H II MNRAS 495, 3065–3078 (2020) Downloaded from https://academic.oup.com/mnras/article/495/3/3065/5843276 by DeepDyve user on 13 July 2022 Reionization with galaxies and AGN 3069 using the results of Pawlik, Schaye & van Scherpenzeel (2009) the stellar emissivity at all redshifts and the total (star forma- and Haardt & Madau (2012) who show that the UVB generated tion + AGN) emissivity down to z ∼ 5; although sub-dominant, the by reionization can act as an effective pressure term, reducing the importance of stars in massive (M 10 M ) galaxies increases clumping factor. with decreasing redshift and they contribute as much as 40 per cent While reionization is driven by the hydrogen ionizing photons (∼ 15 per cent) to the stellar (total) emissivity at z ∼ 4. produced by stars in early galaxies, the UVB built up during On the other hand, driven by the growth of black holes and the bh reionization suppresses the baryonic content of galaxies by photo- constancy of f with redshift, the AGN emissivity shows a steep esc heating/evaporating gas at their outskirts (Klypin et al. 1999; Moore (six-fold) increase in the 370 Myr between z ∼ 6 and 4. A turning et al. 1999; Somerville 2002), suppressing further star formation and point is reached at z ∼ 5 where AGN and star formation contribute slowing down the reionization process. In order to account for the equally to the total emissivity, with the AGN contribution (dom- 6 9 effect of UVB feedback on n ˙ , we assume total photoevaporation inated by M 10 M black holes in M 10 M galaxies) ion bh ∗ −1 of gas from haloes with a virial velocity below V = 40 km s overtaking that from star formation at lower-z. Indeed, the AGN vir embedded in ionized regions at any z. In this ‘maximal external emissivity is almost twice of that provided by stars by z ∼ 4 leading feedback’ scenario, haloes below V in ionized regions neither to an increase in the total value. vir form stars nor contribute any gas in mergers. To summarize, while the trend of the total emissivity is driven The globally averaged n ˙ can then be expressed as: by star formation in low-mass galaxies down to z = 5, AGN take ion over as the dominant contributors at lower redshifts. This result is sf bh n ˙ (z) = n ˙ (z) + n ˙ (z), (11) ion esc esc in agreement with synthesis models for the UVB (Faucher-Giguere ` et al. 2008;Haardt&Madau 2012) as shown in the same ﬁgure. where The above trends can also be used to interpret the latest results on sf sf sf sf n ˙ (z) = f Q (z)n ˙ (z) + Q (z)n ˙ (z) , (12) II I esc esc int,II int,I the integrated electron scattering optical depth (τ = 0.054 ± 0.007; es Planck Collaboration VI 2018), shown in the right-hand panel bh bh bh bh n ˙ (z) = f Q (z)n ˙ (z) + Q (z)n ˙ (z) , (13) II I esc esc int,II int,I of Fig. 1. We start by noting that ﬁtting to this data requires sf 2.8 sf bh sf bh where Q (z) = 1 − Q (z). Further, n ˙ (n ˙ )and n ˙ (n ˙ ) f = 0.02[(1 + z)/7] if stars in galaxies are considered to be I II int,II int,II int,I int,I esc account for the intrinsic hydrogen ionizing photon production rate the only reionization sources; as shown in Table 3 considering the density from star formation (black hole accretion) in case of full UV- contribution of both stars and AGN leads to a marginal decrease sf in the co-efﬁcient of f to 0.0185 whilst leaving the redshift- suppression of the gas mass and no UV suppression, respectively. esc sf bh 9 relation unchanged. Stellar radiation in low-mass (M 10 M ) The term n ˙ (n ˙ ) weights these two contributions over the volume esc esc galaxies dominate the contribution to τ for most of reionization ﬁlling fraction of ionized and neutral regions – i.e. while n ˙ int,I es history. AGN only start making a noticeable contribution at z represents the contribution from all sources, stars, and black holes −1 5, where they can generate an optical depth of τ ∼ 0.22, in haloes with V < 40 kms do not contribute to n ˙ .Atthe vir int,II es comparable to stars, which generate a total value of τ ∼ 0.24. beginning of the reionization process, the volume ﬁlled by ionized es Stellar radiation from high-mass (M 10 M ) galaxies has a hydrogen is very small (Q <<1) and most galaxies are not affected II ∗ sf sf bh bh by UVB-feedback, so that n ˙ (z) ≈ n ˙ (z)f + n ˙ (z)f . sub-dominant contribution to τ at all redshifts. ion es int,I esc int,I esc As Q increases and reaches a value 1, all galaxies in haloes II −1 with circular velocity less than V = 40 km s are feedback- vir 3.2 AGN contribution to reionization as a function of stellar sf sf bh bh suppressed, so that n ˙ (z) ≈ n ˙ (z)f + n ˙ (z)f . ion int,II esc int,II esc mass To understand the AGN contribution to reionization in the ﬁducial 3 RESULTS model, we start by looking at the (intrinsic) production rate of H I Given that n ˙ (z) is an output of the model, t is calculated as a ionizing photons as a function of M for z ∼ 4 − 9 (panel a; ion rec ∗ bh sf function of z and f is obtained from the AGN obscuration fraction, Fig. 2). As expected, n ˙ scales with M since higher mass galaxies esc int sf f is the only free parameter in our reionization calculations. As typically have larger associated star formation rates. Further, given esc sf bh explained above, in the ﬁducial model, f is composed of two their larger gas and black hole masses, n ˙ too scales with M .As esc int free parameters (f and β ) that are ﬁt by jointly reproducing the seen, stars dominate the intrinsic H I ionizing radiation production observed values of τ and the emissivity as discussed in Section 3.1 rate for all stellar masses at z 7. However, moving to lower es sf that follows. We use this f value to study the AGN contribution redshifts, black holes can contribute as much as stars in galaxies esc 10.2−10.9 to reionization in Section 3.2. In order to test the robustness of our with M ∼ 10 M at z ∼ 6. This mass range decreases to 9.6−10 results to assumptions, we also explore alternative models for the M ∼ 10 M at z ∼ 4 where intermediate-mass galaxies host escape fraction from AGN and star formation and the impact of black holes that can accrete at the Eddington rate. different stellar population synthesis models in Section 4. The second factor that needs to be considered is the escape fraction of ionizing photons which is shown in panel (b) of the same sf ﬁgure. As noted above, f is independent of galaxy properties esc 3.1 The electron scattering optical depth and the ionizing and decreases with decreasing z, going from a value of about photon emissivity 5.4 per cent at z ∼ 9to0.77 per cent at z ∼ 4. bh We start by discussing the redshift evolution of the ionizing photon However, f scales with M , and this is the result of the esc emissivity (equation 11) from the ﬁducial model shown in the left- dependence of the unabsorbed AGN fraction with luminosity: at hand panel of Fig. 1. For star formation, the ‘escaping’ emissivity higher AGN luminosity a higher fraction of AGN are unabsorbed. sf bh 9.7 includes the effect of f that decreases with redshift as ∝ [(1 Quantitatively, while f ∼ 10 per cent for M 10 M ,itcan esc esc 2.8 10.9 + z)/7] . As a result, whilst increasing from z ∼ 19 to z ∼ 8the have a value as high as 30 per cent for M 10 M at z ∼ 6–9. emissivity from stellar sources in galaxies thereafter shows a drop We can now combine the intrinsic production rate of H I ionizing at lower redshifts. Low-mass (M 10 M ) galaxies dominate photons and the escape fraction to look at the rate of ‘escaping’ MNRAS 495, 3065–3078 (2020) Downloaded from https://academic.oup.com/mnras/article/495/3/3065/5843276 by DeepDyve user on 13 July 2022 3070 P. Dayal et al. Figure 1. Redshift evolution of the H I ionizing photon emissivity (left-hand panel) and the CMB electron scattering optical depth (τ ) as a function of es redshift (right-hand panel)for the ﬁducial model. In the left-hand panel, the open squares show observational results (and associated error bars) calculated following the approach of Kuhlen & Faucher-Giguere (2012). In the right-hand panel, the dot-dashed horizontal line shows the central value for τ inferred by es the latest Planck results (Planck Collaboration, Aghanim & Akrami 2018) with the grey striped region showing the 1–σ errors. Overplotted are the escaping emissivities (left-hand panel) and the optical depths (right-hand panel) contributed by: star formation only (SF; dot-long-dashed line), AGN + star formation sf bh sf (solid line), and AGN only (short-long-dashed line) using the f and f values for the ﬁducial model reported in Table 1; note that f is lower in the esc esc esc AGN + SF case (f = 0.0185) as compared to the SF only case (f = 0.02). We deconstruct the contribution from star formation in galaxies into those with 0 0 9 9 6 stellar masses M 10 M (short-dashed line) and M 10 M (long-dashed line) and show the contribution of black holes of masses 10 M using ∗ ∗ the dotted line, as marked. ionizing radiation for star formation and AGN in panel (c) of properties (mass, accretion rate). In general, models that reproduce sf sf bh Fig. 2. As expected, n ˙ ∝ M and n ˙ > n ˙ at z> 7. However the generally accepted UV luminosity functions of galaxies and esc esc esc at z< 7 the situation is quite different: the most massive black AGN will all converge to a similar fractional contribution of AGN holes and therefore the most luminous AGN are hosted in massive to reionization. The main reason for the agreement between our galaxies. Additionally, the presence of a critical halo mass below results and those of Qin et al. (2017) is that in both models black which black hole growth is suppressed (see Section 2.1) translates hole growth is retarded with respect to galaxies, although in different into a critical stellar mass (ﬁg. 6; Dayal et al. 2019), below which ways. In our model suppression of black hole growth leads to a black bh only low-luminosity AGN exist and f is very low. The fact that hole mass function with a step-like appearance, in their case it is esc bh both the intrinsic photon production from AGN and f are very the overall normalization of the mass function that decreases with esc low in low-mass galaxies suppresses the AGN contribution from increasing redshift. In principle, this can be tested observationally such galaxies to the escaping photon budget. However, the fact through measurements of the relation between black hole and stellar sf bh that n ˙ n ˙ for high-mass galaxies coupled with an increasing masses in high redshift galaxies. int int bh f value results in black holes dominating the escaping ionizing As expected from the above discussion, star formation in galaxies esc radiation rate for galaxies with mass above a ‘transition stellar mass’ dominate n ˙ for all stellar masses at z> 7 although the AGN esc 9.6 9.2 of M 10 (10 )M at z ∼ 6(4). contribution increases with M as showninpanel (d)ofFig. 2. ∗ ∗ The suppression of black hole growth in low-mass galaxies, At z< 7, however, AGN can start dominating n ˙ by as much as esc advocated from either trying to reconcile seemingly contradictory one order of magnitude for M ∼ 10 M galaxies at z ∼ 6where observational results (Volonteri & Stark 2011) or from the results black holes can accrete at the Eddington rate. This peak mass shifts of cosmological hydrodynamical simulations (Dubois et al. 2015; to lower M values with decreasing redshift – at z ∼ 4AGN in 9.6 Bower et al. 2017), modiﬁes the picture compared to early papers galaxies with masses as low as M ∼ 10 M , which can accrete that assumed unimpeded growth of massive black holes in small at the Eddington limit, dominate n ˙ by a factor of 10. esc galaxies/haloes (Volonteri & Gnedin 2009). As noted above, the The redshift evolution of the ‘transition mass’, at which AGN suppression of black hole contribution from small galaxies/haloes, start dominating n ˙ , is shown in panel (e) of the same ﬁgure which esc which dominate the mass function at the highest redshifts, is shows two key trends: ﬁrst, as expected, the transition mass only bh further strengthened by the assumption that f increases with AGN exists at z< 7 with stellar radiation dominating n ˙ at higher-z. esc esc luminosity. Secondly, as black holes in galaxies of increasingly lower stellar The contribution of AGN to reionization was studied using a mass can accrete at the Eddington limit with decreasing redshift semi-analytical model also by Qin et al. (2017). Qualitatively, our (Piana et al., in preparation), the transition mass too decreases with 10.7 9.3 results agree with theirs, in the sense that only relatively high-mass z from ∼ 10 M at z ∼ 6.8 to ∼ 10 M by z ∼ 4. In the same black holes are important thus limiting the contribution of AGN panel, we also show a comparison of this transition mass to the knee to low redshift, and that the AGN contribution to reionization is observationally inferred knee of the stellar mass function (M ) 10.5 11 sub-dominant, of the order of 10–15 per cent at z< 6. The speciﬁc which ranges between 10 and 10 M at z ∼ 4–7. While the assumptions of the models differ, though: Qin et al. (2017) assume a transition mass is comparable to the knee stellar mass at z ∼ 6.8, luminosity-independent obscured fraction, and they do not include it shows a very rapid decline with decreasing redshift. Indeed, by z a spectral energy distribution that depends on intrinsic black hole ∼ 4, AGN start dominating n ˙ from galaxies that are (at least) an esc MNRAS 495, 3065–3078 (2020) Downloaded from https://academic.oup.com/mnras/article/495/3/3065/5843276 by DeepDyve user on 13 July 2022 Reionization with galaxies and AGN 3071 Figure 2. As a function of stellar mass, the panels (top to bottom) show the results for star formation (solid lines) and AGN (light shaded regions) for the ﬁducial model for: (a): the intrinsic H I ionizing photon rate; (b): the escape fraction of H I ionizing photons; (c): the escaping H I ionizing photon rate; (d): the ratio between the escaping H I ionizing photon rate for AGN and star formation with the horizontal line showing a ratio of unity; and (e): the transition stellar mass at which AGN start dominating the escaping ionizing photon production rate. In this panel, the solid circles and empty triangles show the knee value of the stellar mass function (and the associated error bars) observationally inferred by Grazian et al. (2015) and Song et al. (2016), respectively. Finally, the different colours in panels (a)–(c) are for the redshifts marked in panel (a) while the different lines in panel (d) are for the redshifts marked in that panel. order of magnitude less massive compared to the knee mass and in range is weighted by its cosmic abundance, via the mass of the host fact the ratio between the escaping H I ionizing photon rate for AGN halo – therefore this ﬁgure represents the effective contribution of and star formation peaks at intermediate galaxy masses. Finally, we that mass range to the global photon budget. We note that, at any sf sf bh note that such a transition mass only exists in the case that the stellar z, while n ˙ is just a scaled version of n ˙ , n ˙ instead evolves esc int esc sf bh mass dependence of f is shallower than f (see Section 4). based on the luminosity/mass evolution. The key trends emerging esc esc We summarize the impact of the above-noted trends on the are: ﬁrst, at any z, whilst the contribution of stars (weighted by production/escape rates of H I ionizing photons per baryon over the number density) is the highest at intermediate stellar mass 7−9 a Hubble time in Fig. 3. Here the contribution in each galaxy mass galaxies (10 M )at z ∼ 6, the contribution is essentially mass MNRAS 495, 3065–3078 (2020) Downloaded from https://academic.oup.com/mnras/article/495/3/3065/5843276 by DeepDyve user on 13 July 2022 3072 P. Dayal et al. Figure 3. The ionizing photon per baryon value as a function of stellar mass for the ﬁducial model for star formation and AGN at z ∼ 6 and 9, as marked. The dot-dashed and solid lines show the intrinsic and escaping H I ionizing photon rates, respectively. 5−8 hydrogen reionization. One could argue that this is a consequence of independent between a stellar mass of 10 M at z ∼ 9. Although sf 9 10 the steep increase of f at high redshifts, which artiﬁcially boosts massive galaxies, M ∼ 10 − 10 M , have higher production ∗ esc the contribution of stars in low-mass galaxies and correspondingly rates of ionizing radiation from both stars and black holes in addition bh reduces the contribution of AGN. In this section we examine the to higher f values, they are rarer than their low-mass counterparts, esc robustness of our results by exploring six different combinations of which therefore dominate the total emissivity as also shown in the bh sf f and f in Section 4.1 and two different stellar population left-hand panel of Fig. 1. Secondly, AGN only have a contribution esc esc 9−10 synthesis models in Section 4.2 in order to explore the physically at the high stellar mass end (M ∼ 10 M )at z 9. Thirdly, as plausible parameter space. expected from the above discussions, given both the higher values of the intrinsic H I ionizing photon production rate and f , AGN dom- esc inate the emissivity at the high-mass end (M 10 M )at z ∼ 6. II ionizing photons, 4.1 Alternative models for AGN and star formation escape Since AGNs are efﬁcient producers of He useful constraints can be obtained on their contribution from the fractions corresponding observations, e.g. He II Ly α optical depth at z ∼ 3 sf bh Given that the trends of f and f with galaxy properties are esc esc (Worseck et al. 2016) and the heating of the IGM at z 5 (Becker still uncertain, both theoretically and observationally, Fig. 4 shows et al. 2011). A detailed modelling of the He II reionization history the optical depth and emissivity predicted by the alternative models is beyond the scope of this work. However, we have computed summarized in Table 2: the He III volume ﬁlling fraction, Q , and found that Q ∼ He III HeIII bh 0.4(0.2) at z = 4 (5), assuming that the escape fraction of He II (i) In the ﬁrst model (Alt1, panels a1 and a2), f is obtained esc ionizing photons is the same as that of the H I ionizing photons. from the results of Merloni et al. (2014). We ﬁt to the optical depth sf 3.8 While this implies a He II reionization earlier than the model of and emissivity observations to derive f = 0.017[(1 + z)/7] . esc Haardt & Madau (2012), it is still within the 2–σ bounds as allowed This steep redshift-dependence for the escaping stellar radiation by the observations (see e.g. Mitra et al. 2018). from galaxies (left-most column of Fig. 5) is required to off-set the increasing AGN contribution at z 5 which is driven by the higher bh f values (compared to the ﬁducial model) as shown in the middle esc 4 ALTERNATIVE MODELS bh sf column of Fig. 5. This enhances the ratio n ˙ /n ˙ by more than esc esc Our key result is that the AGN contribution of ionizing photons one order of magnitude compared to the ﬁducial model at z< 7 is subdominant at all galaxy masses at z> 7. At z ∼ 6–7 their (right-most column of Fig. 5). As seen from the same panel, we ﬁnd contribution increases with stellar mass, and at lower redshift it that the transition mass remains almost unchanged compared to the is AGN in intermediate-mass galaxies that produce most ionizing ﬁducial case. sf photons (Fig. 2). This results in a ‘transition’ stellar mass at which (ii) In the second model (Alt2, panels b1 and b2) we keep f esc AGN overtake the stellar contribution to the escaping ionizing equal to the ﬁducial value and maximize the escape fraction from bh radiation; for stars in galaxies to dominate all the way through in the AGN by assuming f = 1. Driven by such maximal AGN contribu- esc mass function, either the escape fraction of stellar radiation from tion, this model severely overpredicts the emissivity at z 5; the op- galaxies should increase with galaxy mass or that from AGN should tical depth, being dominated by star formation in galaxies for most decrease, especially at high masses. In our ﬁducial model, this of the reionization history, can still be ﬁt within the 1–σ error bars. bh sf transition stellar mass decreases with decreasing redshift. Further, As seen from the right-most panel of Fig. 5, n ˙ /n ˙ is higher by esc esc star formation in galaxies with mass < 10 M is the main driver of more than one order of magnitude compared to the ﬁducial model. MNRAS 495, 3065–3078 (2020) Downloaded from https://academic.oup.com/mnras/article/495/3/3065/5843276 by DeepDyve user on 13 July 2022 Reionization with galaxies and AGN 3073 Figure 4. The redshift evolution of the electron scattering optical depth (left-hand column) and the associated escaping ionizing emissivity (right-hand column). In the left-hand column, the dot-dashed horizontal line shows the central value for τ inferred by the latest Planck results (Planck Collaboration es VI 2018) with the grey striped region showing the 1–σ errors. In the right column, open squares show the observational results (and associated error bars) calculated following the approach of Kuhlen & Faucher-Giguere (2012). In each panel, we show results for star formation + AGN (solid line), star formation (dot-dashed line), and AGN (short-long-dashed line) for the different alternative escape fraction models (Alt1-Alt6) discussed in Section 4.1 and summarized in Table 2. The model name and the f values used for star formation and AGN are noted in each panel of the right column. esc Again, a transition stellar mass exists at z< 7 and is only slightly seen from the left and middle columns of Fig. 5. This model lower (by about 0.2–0.4 dex) compared to the ﬁducial model. naturally results in a lower AGN contribution to the escap- (iii) In the third model (Alt3, panels c1 and c2) we con- ing ionizing radiation at all masses and redshifts as com- sider the same redshift-dependent escape fraction for the ion- pared to the ﬁducial model (right most panel of the same izing radiation from both stellar radiation and AGN. Here, si- ﬁgure). Similar to the results of model Alt4 that follows, multaneously ﬁtting to the optical depth and emissivity val- in this model the AGN ionizing radiation contribution is sf bh ues yields an escape fraction that evolves as f = f = minimized and only slightly exceeds that from galaxies at esc esc 3.2 sf bh 9.5−9.8 0.017[(1 + z)/7] . The evolution of f and f can be M ∼ 10 M by z ∼ 4, i.e. stellar radiation dominates esc esc MNRAS 495, 3065–3078 (2020) Downloaded from https://academic.oup.com/mnras/article/495/3/3065/5843276 by DeepDyve user on 13 July 2022 3074 P. Dayal et al. sf bh Table 2. For the alternative models studied in Section 4.1, we summarize the model name (column 1), the parameter values for f (column 2) and f esc esc bh sf (column 3), the impact on the ratio n ˙ /n ˙ compared to the ﬁducial model (column 4) and the impact on the transition mass at which AGN start dominating esc esc the escaping H I ionizing photon production rate compared to the ﬁducial model (column 5). We note that of models Alt1 – Alt6, only Alt1, Alt3 and Alt6 simultaneously ﬁt τ (Planck Collaboration VI 2018) and the redshift evolution of the H I ionizing photon emissivity. We use the ﬁducial values of the free es parameters for galaxy formation as in Table 1. sf bh bh sf Model f f n ˙ /n ˙ Transition M esc esc esc esc 3.8 Alt1 0.017[(1 + z)/7] Merloni et al. (2014) Increases at all M Almost unchanged Alt2 ﬁducial 1 Increases at all M Decreases by 0.2 (0.4 dex) at z ∼ 6(4) 3.2 3.2 Alt3 0.017[(1 + z)/7] 0.017[(1 + z)/7] Decreases at all M – bh Alt4 ﬁducial f ∝ M ﬁducial Decreases at all M – ∗ ∗ esc Alt5 0.035 Ueda et al. (2014) Decreases at all M for z 7.5 Increases by 0.1 dex at z ∼ 6–4 ej −ζ eff 9.2 Alt6 0.1(f /f ) ∝ M ﬁducial Increases for M 10 M Decreases by 0.3 dex (unchanged) at z ∼ 6(4) ∗ ∗ ∗ sf bh Figure 5. As a function of stellar mass, we show f (left-hand column), f (middle column) and the ratio between the escaping H I ionizing photon rate esc esc for AGN and stars (right-hand column)for z ∼ 4.1 (top row) and z ∼ 6 (bottom row). We show results for the six different alternative escape fraction models (Alt1- Alt6) discussed in Section 4.1 and summarized in Table 2 and also plot the ﬁducial model for comparison. In the right-most column, the horizontal line shows a ratio of unity. the ionizing budget at effectively all masses and redshifts As seen from the bottom panels of Fig. 4, this model is unable to although the AGN contribution still increases with increasing stellar simultaneously reproduce both the values of τ and the emissivity. es sf mass. In this model, the value of f is decreased (increased) at z esc (iv) In the fourth model (Alt4, panels d1 and d2) we assume 7.5( 7.5) compared to the ﬁducial case as shown in the left-hand sf bh bh f = f using the ﬁducial f value from Ueda et al. (2014) panel of Fig. 5. Compared to the ﬁducial model, this results in esc esc esc sf bh sf for galaxies that have a black hole; we use f = 0 for galaxies a lower value of n ˙ /n ˙ by about 0.3 (0.8 dex) at z ∼ 6(z ∼ esc esc esc sf bh that do not host a black hole. This results in both f and f 4.1) and the transition mass increases negligibly (by ∼0.1 dex) at esc esc scaling positively with the stellar mass as shown in the left-most z = 4−6. and middle panels of Fig. 5. As in the previous model, this identical (vi) In the sixth model (Alt6, panels f1 and f2), while we use the bh sf escape fraction for both stellar radiation and AGN results in stellar ﬁducial value for f , we assume that f scales with the ejected esc esc ej sf eff radiation dominating the ionizing budget at almost all masses and gas fraction such that f = f (f /f ). This naturally results in 0 ∗ esc ∗ sf redshifts; the AGN ionizing radiation contribution only slightly f decreasing with an increasing halo (and stellar) mass. A value esc exceeds that from galaxies at M ∼ 10 M by z ∼ 4. However, ∗ of f = 0.1 is required to simultaneously ﬁt both the optical depth and we note that this model overpredicts the emissivity from stellar emissivity constraints as shown in the same ﬁgure. In this model, sources at all redshifts and is unable to simultaneously reproduce the increasing suppression of the star formation rate in low-mass both the values of τ the the emissivity. es haloes due to both supernova and reionization feedback naturally (v) In the ﬁfth model (Alt5, panels e1 and e2) we assume a leads to a downturn in the stellar emissivity with decreasing redshift. sf bh sf constant f = 3.5 per cent and use the ﬁducial value for f . As showninFig. 5, in this model the f values lie below the esc esc esc MNRAS 495, 3065–3078 (2020) Downloaded from https://academic.oup.com/mnras/article/495/3/3065/5843276 by DeepDyve user on 13 July 2022 Reionization with galaxies and AGN 3075 8.4 Table 3. The parameter values for the z-evolution of the escape fraction, ﬁducial one for all M 10 M at z ∼ 6. However, by z ∼ 4, sf β sf 8.6 f = f [(1 + z)/7] for different models constrained to simultaneously esc the f values for the lowest mass haloes (∼ 10 M ) approach esc ﬁt τ (Planck Collaboration VI 2018) that combines polarization, lensing, es the values for the ﬁducial model. Compared to the ﬁducial model, and temperature data, and the redshift evolution of the H I ionizing photon bh sf this results in an increasing n ˙ /n ˙ with increasing stellar mass, esc esc bh emissivity (see the text). We use the ﬁducial value for f and the same 9.2 esc specially for M 10 M . This naturally leads a transition mass values of the free parameters for galaxy formation as in Table 1. that is lower than that in the ﬁducial model by about 0.3 dex at z ∼ 6, whilst being almost identical at z ∼ 4. SPS Model Sources f × 100 β sf bh To summarize, the possible range of f and f combinations SB99 SF 2.0 2.8 esc esc (ranging from redshift-dependent to constant to scaling both pos- SB99 SF + AGN 1.85 2.8 BPB SF 0.46 2.8 itively and negatively with stellar mass) have conﬁrmed our key BPB SF + AGN 0.43 2.8 results: the AGN contribution of ionizing photons is subdominant SB99 + sb SF 1.7 2.8 at all galaxy masses at z> 7 and increases with stellar mass at z< SB99 + sb SF + AGN 1.6 2.8 7. Additionally, we have conﬁrmed the existence of a ‘transition’ stellar mass (at which AGN overtake the stellar contribution to the escaping ionizing radiation) which decreases with decreasing redshift. Stars dominate all the way through the mass function only sf bh 5 REIONIZATION HISTORY AND THE when the stellar mass dependence of f is steeper than f or esc esc CUMULATIVE AGN CONTRIBUTION if we assume the same f values for both star formation and AGN esc (i.e. the Alt3 and Alt4 models); in this case, naturally, the transition We start with a recap of the total (star formation + AGN) ionizing mass no longer exists. emissivity for all the different models considered in this work in the left-hand panel of Fig. 6. In all models, the ionizing emissivity from star formation dominates at z> 6 and is virtually indistinguishable for all the models (ﬁducial, Alt1, Alt2, and Alt3) that use a redshift 4.2 Alternative stellar population synthesis models sf dependent f value. The redshift evolution of the emissivity esc In addition to the ﬁducial SB99 model, we have considered two sf is the steepest for the Alt4 model where f ∝ M . With its esc other population synthesis models: BPASS binaries (BPB; Eldridge sf constant value of f = 0.035, model Alt5 shows the shallowest esc et al. 2017) and Starburst99 including stripped binaries (SB99 + sb; sf slope. Given its lower f values for all stellar masses at high esc Gotberg et al. 2019). The time evolution of the intrinsic ionizing redshifts, the Alt6 model naturally shows a lower ionizing emissivity and UV photons from star formation in the BPB model can be compared to ﬁducial; the stellar emissivity from the Alt6 model expressed as: converges to the ﬁducial one by z ∼ 9 as a result of the decreasing sf f values for the latter. As expected, the AGN contribution is t esc sf 47.25 −1 sf bh n ˙ (t ) = 10 − 2.28 log + 0.6[s ], (14) int the lowest for the model Alt3 where f = f = a decreasing esc esc 2Myr function of redshift (as shown in the same panel). It then increases by a factor of 3 from the ﬁducial case to the Alt1 case and reaches bh its maximum for the Alt2 case where f = 1. esc 33.0 −1 −1 We then discuss reionization history, expressed through the L (t ) = 10 − 1.2log + 0.5[ergs Å ]. (15) UV 2Myr redshift evolution of the volume ﬁlling fraction of ionized hydrogen (Q ), as shown in the right-panel of Fig. 6. Interestingly, despite the II In the SB99 + sb model, these quantities evolve as: sf bh range and trends used for f and f , reionization is 50 per cent esc esc complete in all cases in the very narrow redshift range of z ∼ 6.6– sf 46.7 −1 n ˙ (t ) = 10 − 2.3log [s ], (16) int 7.6. Further, we ﬁnd an end redshift of reionization value of z ∼ 2Myr re 5–6.5 in all the models studied here except Alt 3. In this model, the 33.01 −1 −1 L (t ) = 10 − 1.3log + 0.49 [erg s Å ]. (17) UV decrease in the star formation emissivity (driven by the decrease of 2Myr sf f ) with decreasing redshift is not compensated by an increasing esc The rest-frame UV luminosity has almost the same normalization AGN contribution as in the other models; as a result, reionization and time-evolution in all three models (SB99, BPB, SB99 + sb) does not ﬁnish even by z ∼ 4. Given that star formation in low-mass resulting in the same UV LFs. However, as seen from equations haloes is the key driver of reionization, it is not surprising to see that (5), (15). and (17), the slope of the time evolution of n ˙ is much reionization ﬁnishes ﬁrst (z ∼ 6.5) in the Alt4 model that has the int re sf shallower in the BPB and SB99 + sb models compared to the largest value of f . Models Alt2 and Alt5 show a similar z ∼ 5.8 re esc sf ﬁducial (SB99) model. We re-tune f for each of these models driven by an increasing contribution from star formation and AGN, esc to match to the reionization data (τ and the emissivity) using the respectively. Finally, given their lower values of the total ionizing es bh ﬁducial f values, the results of which are summarized in Table 3. emissivity at z 7, reionization ends at z ∼ 5 in the ﬁducial, Alt1 re esc sf As seen, while the slope of the redshift dependence of f remains and Alt6 models. esc unchanged (β = 2.8), the normalization (f ) is the lowest for the Finally, we show the AGN contribution to the cumulative ionizing BPB model as compared to SB99 by a factor 4.6; the SB99 and emissivity as a function of redshift in Fig. 7.Asseen, AGN SB99 + sb models on the other hand only differ by a factor 1.17. contribute at most 1 per cent of the total escaping ionizing photon sf Finally, the lower f values compensate for a higher intrinsic rate by z ∼ 4inthe Alt3 model. This increases to ∼ 10 per cent esc sf production rate to result in the same n ˙ value as a function of M . of the total ionizing emissivity for the ﬁducial and Alt4-Alt6 cases. esc bh These different stellar populations, therefore, have no bearing on Compared to the ﬁducial case, the higher f in the Alt1 case esc our result regarding the relative AGN/starlight contribution to the results in an AGN contribution as high as 25 per cent by z ∼ 4. bh ionizing radiation for different galaxy stellar masses. Finally, the Alt2 case (f = 1) provides the upper limit to the esc MNRAS 495, 3065–3078 (2020) Downloaded from https://academic.oup.com/mnras/article/495/3/3065/5843276 by DeepDyve user on 13 July 2022 3076 P. Dayal et al. Figure 6. Left-hand panel: As a function of redshift, we show the escaping H I ionizing photon emissivity. The different lines show the emissivity from star formation + AGN while the shaded regions (of the same lighter colour) show the contribution from AGN only. Right-hand panel: The reionization history, expressed through the redshift evolution of the volume ﬁlling fraction of H II. The horizontal dashed line shows Log(Q ) =−0.301, i.e. when reionization II is 50 per cent complete. The different colours in both panels show results for the ﬁducial and alternative escape fraction models (discussed in Section4.1)as marked in the right-hand panel. Figure 7. The cumulative fraction of ionizing photons contributed by AGN as a function of redshift; the horizontal short-dashed line shows the 50 per cent contribution to the cumulative ionizing emissivity for the various models discussed in this work (see Section 4.1 for details), as marked. AGN contribution. Here, AGN contribute as much as galaxies to in galaxies via suppression of black hole accretion in low-mass the cumulative emissivity by z ∼ 4.4. galaxies, caused by supernova feedback. Furthermore, in our model In addition to the ﬁducial model, only Alt1, Alt3,and Alt6 are each accreting black hole has a spectral energy distribution that able to simultaneously reproduce the emissivity and optical depth depends on the black hole mass and accretion rate. Given that the constraints. However, as seen above, the Alt3 model does not have escape fractions for both star formation and AGN remain poorly enough ionizing photons to ﬁnish the process of reionization. This understood, we have explored a wide range of combinations for leaves us with three physically plausible models – the ﬁducial one, these (ranging from redshift-dependent to constant to scaling both Alt1,and Alt6. In these, the AGN contribution to the total emissivity positively and negatively with stellar mass). Using these models, is sub-dominant at all z; AGN contribute about 0.5 − 1 per cent we ﬁnd the following key results: to the cumulative ionizing emissivity by z ∼ 6 that increases to 10 − 25 per cent by z = 4. (i) The intrinsic production rate of ionizing photons for both star formation and AGN scales positively with stellar mass with star formation dominating at all masses and redshifts. 6 CONCLUSIONS (ii) Irrespective of the escape fraction values used, the AGN In this paper, we have studied the contribution of AGN to hydrogen contribution to the escaping ionizing photons is always sub- reionization. Our model includes a delayed growth of black holes dominant at all galaxy masses at z> 7. In the case that the MNRAS 495, 3065–3078 (2020) Downloaded from https://academic.oup.com/mnras/article/495/3/3065/5843276 by DeepDyve user on 13 July 2022 Reionization with galaxies and AGN 3077 sf bh Dayal P., Ferrara A., 2018, Phys. Rep., 780, 1 stellar mass dependence of f is shallower than f ,at z< esc esc Dayal P., Ferrara A., Dunlop J. S., Pacucci F., 2014, MNRAS, 445, 2545 7 a ‘transition’ stellar mass exists above which AGN dominate the Dayal P., Choudhury T. R., Bromm V., Pacucci F., 2017a, ApJ, 836, 16 escaping ionizing photon production rate. This transition stellar Dayal P., Choudhury T. R., Pacucci F., Bromm V., 2017b, MNRAS, 472, mass decreases with redshift from being equal to the knee of the stellar mass function at z ∼ 6.8 to being an order of magnitude less Dayal P., Rossi E. M., Shiralilou B., Piana O., Choudhury T. R., Volonteri than the knee by z = 4. M., 2019, MNRAS, 486, 2336 (iii) Overall, the ionizing budget is dominated by stellar radiation Done C., Davis S. W., Jin C., Blaes O., Ward M., 2012, MNRAS, 420, 1848 from low-mass (M < 10 M ) galaxies down to z 6inall ∗ Dubois Y., Volonteri M., Silk J., Devriendt J., Slyz A., Teyssier R., 2015, models. In the ﬁducial model, at z = 6 AGN and stars in M > MNRAS, 452, 1502 10 M contribute equally to the ionizing budget (∼ 15 per cent Duncan K., Conselice C. J., 2015, MNRAS, 451, 2030 Eldridge J. J., Stanway E. R., Xiao L., McClelland L. A. S., Taylor G., Ng of the total). However at z< 5.5, the AGN contribution (driven 6 9 M., Greis S. M. L., Bray J. C., 2017, Publ. Astron. Soc. Aust., 34, e058 by M > 10 M black holes in M 10 M galaxies) overtakes bh ∗ 9 Eide M. B., Graziani L., Ciardi B., Feng Y., Kakiichi K., Di Matteo T., 2018, that from star formation in M < 10 M galaxies. The contribution MNRAS, 476, 1174 from star formation in high-mass (M > 10 M ) galaxies is sub- Faucher-Giguere ` C.-A., Lidz A., Hernquist L., Zaldarriaga M., 2008, ApJ, dominant at all redshifts, reaching a maximum value of 20 per cent 688, 85 of the total ionizing budget at z 6. Ferland G. J. et al., 2013, Rev. Mex. Astron. Astroﬁs., 49, 137 (iv) Different stellar population synthesis models (SB99, BPB, Finkelstein S. L. et al., 2012, ApJ, 758, 93 SB99 + sb) have no bearing on our result regarding the relative Finkelstein S. L. et al., 2019, ApJ, 879, 36 AGN/starlight contribution to the ionizing radiation for different Giallongo E. et al., 2015, A&A, 578, A83 galaxy stellar masses. Giallongo E. et al., 2019, ApJ, 884, 19 Gotber ¨ g Y., de Mink S. E., Groh J. H., Leitherer C., Norman C., 2019, A&A, (v) For all models that match the observed reionization con- 629, A134 straints (electron scattering optical depth and the ionizing emissiv- Grazian A. et al., 2015, A&A, 575, A96 ity) and where reionization ﬁnishes by z ∼ 5, AGN can contribute Grazian A. et al., 2018, A&A, 613, A44 as much as 50 − 83 per cent of the emissivity at z = 5. However, Haardt F., Madau P., 2012, ApJ, 746, 125 AGN only contribute 0.5 − 1 per cent to the cumulative ionizing Hassan S., Dave R., Mitra S., Finlator K., Ciardi B., Santos M. G., 2018, emissivity by z ∼ 6 that increases to 10 − 25 per cent by z = 4. MNRAS, 473, 227 Kakiichi K., Graziani L., Ciardi B., Meiksin A., Compostella M., Eide M. B., Zaroubi S., 2017, MNRAS, 468, 3718 ACKNOWLEDGEMENTS Klypin A., Kravtsov A. V., Valenzuela O., Prada F., 1999, ApJ, 522, 82 PD acknowledges support from the European Research Council’s Kuhlen M., Faucher-Giguere C.-A., 2012, MNRAS, 423, 862 Kulkarni G., Worseck G., Hennawi J. F., 2019, MNRAS, 488, 1035 starting grant ERC StG-717001 (‘DELPHI’), from the NWO grant Liu C., Mutch S. J., Angel P. W., Duffy A. R., Geil P. M., Poole G. B., 016.VIDI.189.162 (‘ODIN’) and the European Commission’s and Mesinger A., Wyithe J. S. B., 2016, MNRAS, 462, 235 University of Groningen’s CO-FUND Rosalind Franklin program. Madau P., Fragos T., 2017, ApJ, 840, 39 MV and MT acknowledge funding from the European Research Madau P., Haardt F., 2015, ApJ, 813, L8 Council under the European Community’s Seventh Framework Madau P., Haardt F., Rees M. J., 1999, ApJ, 514, 648 Programme (FP7/2007-2013 Grant Agreement no. 614199, project McGreer I. D., Fan X., Jiang L., Cai Z., 2018, AJ, 155, 131 ‘BLACK’). TRC acknowledges support from the Associateship Merloni A. et al., 2014, MNRAS, 437, 3550 Scheme of ICTP. MV and RS acknowledge support from the Amaldi Micheva G., Iwata I., Inoue A. K., 2017, MNRAS, 465, 302 Research Center funded by the MIUR program ‘Dipartimento di Ec- Mitra S., Choudhury T. R., Ferrara A., 2015, MNRAS, 454, L76 cellenza’ (CUP:B81I18001170001). MT is supported by Deutsche Mitra S., Choudhury T. R., Ferrara A., 2018, MNRAS, 473, 1416 Forschungsgemeinschaft (DFG, German Research Foundation) un- Moore B., Quinn T., Governato F., Stadel J., Lake G., 1999, MNRAS, 310, der Germany’s Excellence Strategy EXC-2181/1 – 390900948 (the Naidu R. P., Tacchella S., Mason C. A., Bose S., Oesch P. A., Conroy C., Heidelberg STRUCTURES Cluster of Excellence). HA is supported 2020, ApJ, 892, 109 by the Centre National d’Etudes Spatiales (CNES). MH acknowl- Oke J. B., Gunn J. E., 1983, ApJ, 266, 713 edges ﬁnancial support from the Carlsberg Foundation via a Semper Onoue M. et al., 2017, ApJ, 847, L15 Ardens grant (CF15-0384). Finally, PD and MV thank La Sapienza Paardekooper J.-P., Khochfar S., Dalla Vecchia C., 2015, MNRAS, 451, for their hospitality, where the bulk of this work was carried out and thank D. Stark and P. Oesch for their insightful comments. Pawlik A. 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Abstract

Downloaded from https://academic.oup.com/mnras/article/495/3/3065/5843276 by DeepDyve user on 13 July 2022 MNRAS 495, 3065–3078 (2020) doi:10.1093/mnras/staa1138 Advance Access publication 2020 May 23 1‹ 2 3 Pratika Dayal , Marta Volonteri, Tirthankar Roy Choudhury , 4,5,6 2,7,8 9,10,11 Raffaella Schneider, Maxime Trebitsch , Nickolay Y. Gnedin, 2 12 13 Hakim Atek, Michaela Hirschmann and Amy Reines Kapteyn Astronomical Institute, University of Groningen, PO Box 800, NL-9700 AV Groningen, the Netherlands Institut d’Astrophysique de Paris, Sorbonne Universite, CNRS, UMR 7095, 98 bis bd Arago, F-75014 Paris, France National Centre for Radio Astrophysics, Tata Institute of Fundamental Research, Pune 411007, India Dipartimento di Fisica, ‘Sapienza’ Universita ` di Roma, Piazzale Aldo Moro 5, I-00185 Roma, Italy INAF/Osservatorio Astronomico di Roma, Via di Frascati 33, I-00040 Monte Porzio Catone, Italy INFN, Sezione Roma 1, Dipartimento di Fisica, ‘Sapienza’ Universita ` di Roma, Piazzale Aldo Moro 5, I-00185 Roma, Italy Max-Planck-Institut fur ¨ Astronomie, Konigstuhl ¨ 17, D-69117 Heidelberg, Germany Zentrum fur ¨ Astronomie der Universitat ¨ Heidelberg, Institut fur ¨ Theoretische Astrophysik, Albert-Ueberle-Str. 2, D-69120 Heidelberg, Germany Particle Astrophysics Center, Fermi National Accelerator Laboratory, Batavia, IL 60510, USA Kavli Institute for Cosmological Physics, The University of Chicago, Chicago, IL 60637, USA Department of Astronomy and Astrophysics, The University of Chicago, Chicago, IL 60637, USA DARK, Niels Bohr Institute, University of Copenhagen, Lyngbyvej 2, DK-2100 Copenhagen, Denmark eXtreme Gravity Institute, Department of Physics, Montana State University, Bozeman, MT 59717, USA Accepted 2020 April 21. Received 2020 April 16; in original form 2020 January 16 ABSTRACT In this work we investigate the properties of the sources that reionized the intergalactic medium (IGM) in the high-redshift Universe. Using a semi-analytical model aimed at reproducing galaxies and black holes in the ﬁrst ∼1.5 Gyr of the Universe, we revisit the relative role of star formation and black hole accretion in producing ionizing photons that can escape into the IGM. Both star formation and black hole accretion are regulated by supernova feedback, resulting in black hole accretion being stunted in low-mass haloes. We explore a wide range of combinations for the escape fraction of ionizing photons (redshift-dependent, constant, sf bh and scaling with stellar mass) from both star formation (f ) and AGN (f ) to ﬁnd: (i) the esc esc ionizing budget is dominated by stellar radiation from low stellar mass (M < 10 M ) galaxies 6 9 at z> 6 with the AGN contribution (driven by M > 10 M black holes in M 10 M bh ∗ galaxies) dominating at lower redshifts; (ii) AGN only contribute 10 − 25 per cent to the cumulative ionizing emissivity by z = 4 for the models that match the observed reionization sf bh constraints; (iii) if the stellar mass dependence of f is shallower than f ,at z< 7 esc esc a transition stellar mass exists above which AGN dominate the escaping ionizing photon production rate; (iv) the transition stellar mass decreases with decreasing redshift. While AGN dominate the escaping emissivity above the knee of the stellar mass function at z ∼ 6.8, they take-over at stellar masses that are a tenth of the knee mass by z = 4. Key words: galaxies: evolution – galaxies: high-redshift – intergalactic medium – quasars: general – reionization. ﬁrst metal-free (population III; PopIII) stars at z 30, with the 1 INTRODUCTION key sources gradually shifting to larger metal-enriched haloes, The epoch of (hydrogen) reionization (EoR) begins when the ﬁrst powered by population II (PopII) stars and accreting black holes. stars start producing neutral hydrogen (H I) ionizing photons and However, this picture is complicated by the fact that the progress and carving out ionized regions in the intergalactic medium (IGM). sources of reionization depend on a number of (poorly constrained) In the simplest picture, the EoR starts with the formation of the parameters including the minimum halo mass of star-forming galaxies, the star formation/black hole accretion rates, the escape fraction (f )ofH I ionizing photons from the galactic environment, esc E-mail: [email protected] the impact of the reionization ultraviolet background (UVB) on the The Author(s) 2020. Published by Oxford University Press on behalf of The Royal Astronomical Society. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited. Downloaded from https://academic.oup.com/mnras/article/495/3/3065/5843276 by DeepDyve user on 13 July 2022 3066 P. Dayal et al. gas content of low-mass haloes and the clumping factor of the IGM this model lie in that: (i) it is seeded with two types of black hole (see e.g. Dayal & Ferrara 2018). seeds (stellar and direct collapse); (ii) the black hole accretion rate Observationally, a number of works have used a variety of data is primarily regulated by the host halo mass; (iii) it uses a minimal sets and trends – e.g. the UV luminosity density, the faint-end set of free parameters for star formation and black holes and their slope of the Lyman Break Galaxy (LBG) luminosity function, f associated feedback. esc increasing with bluer UV slopes, and the abundance and luminosity The cosmological parameters used in this work correspond to , distribution of galaxies – to conclude that star formation in low- , , h, n , σ = 0.3089, 0.6911, 0.049, 0.67, 0.96, 0.81 (Planck b s 8 mass galaxies with an absolute magnitude M −10 to −15 Collaboration XIII 2016). We quote all quantities in comoving units UV alone can reionize the IGM (Bouwens et al. 2012; Finkelstein et al. unless stated otherwise and express all magnitudes in the standard 2012; Duncan & Conselice 2015; Robertson et al. 2015), although AB system (Oke & Gunn 1983). Naidu et al. (2019) assume f ∝ the star formation rate surface The paper is organized as follows. In Section 2, we detail our esc density and infer that high stellar mass (M 10 M ) galaxies code for the galaxy-BH (co)-evolution, our calculation of f and the ∗ esc dominate the reionization budget (see also Sharma et al. 2016). The progress of reionization. The results of the ﬁducial and of alternative bulk of the observational results are in agreement with theoretical models are presented in Sections 3 and 4. Finally, we discuss our 9.5 results that converge on stars in low-mass haloes (M 10 M results and present our main conclusions in Section 6. and M −17) providing the bulk of H I ionizing photons at z 7 UV (e.g. Choudhury & Ferrara 2007; Salvaterra, Ferrara & Dayal 2011; 2 THEORETICAL MODEL Yajima, Choi & Nagamine 2011;Wiseetal. 2014; Paardekooper, We start by introducing the galaxy formation model in Section 2.1 Khochfar & Dalla Vecchia 2015; Liu et al. 2016; Dayal et al. 2017a). before discussing the escape fraction of ionizing radiation from A key caveat in the results, however, is that the redshift-dependent galaxies and AGN in the ﬁducial model in Section 2.2. These are reionization contribution from star formation in galaxies of different used to calculate the reionization history and electron scattering masses crucially depends on the strength of UVB feedback, the trend optical depth in Section 2.3. Our ﬁducial model parameters are of f with mass and redshift and the evolution of the clumping esc described in Table 1. factor (for details see Section 7, Dayal & Ferrara 2018). In addition, the contribution of Active Galactic Nuclei (AGNs) to reionization and its dependence on redshift and on the host 2.1 Galaxy formation at high-z galaxy stellar mass still remain key open questions. A number of works show AGN can only have a minor reionization contribution In this work, we use the semi-analytic code Delphi (Dark matter and (Onoue et al. 2017; Yoshiura et al. 2017;Hassanetal. 2018). the emergence of galaxies in the epoch of reionization) that aims at Contrary to these studies, a number of results show that radiation simulating the assembly of the dark matter, baryonic and black hole from AGN/quasars might contribute signiﬁcantly to reionization components of high-redshift (z 5) galaxies (Dayal et al. 2014, (Volonteri & Gnedin 2009; Madau & Haardt 2015; Mitra, Choud- 2019). In brief, starting at z = 4 we build analytic merger trees up hury & Ferrara 2015, 2018; Grazian et al. 2018; Finkelstein et al. to z = 20, in time-steps of 20 Myr, for 550 haloes equally separated 8 13.5 in log space between 10 and 10 M . Each halo is assigned a 2019), especially at z 8 if ionizations by secondary electrons are number density according to the Sheth–Tormen halo mass function accounted for, with stars taking over as the dominant reionization (HMF) which is propagated throughout its merger tree; the resulting sources at z 6 (Volonteri & Gnedin 2009). The question of the HMFs have been conﬁrmed to be in accord with the Sheth–Tormen contribution of AGN to reionization has witnessed a resurgence HMF at all z ∼ 5–20. after recent claims of extremely high number densities of faint The very ﬁrst progenitors of any galaxy are assigned an initial AGN measured by Giallongo et al. (2015, 2019)at z 4. While gas mass as per the cosmological baryon-to-dark matter ratio such other direct searches for high-redshift AGN have found lower that M = ( / )M ,where M is the halo mass. The effective number densities (Weigel et al. 2015; McGreer et al. 2018), the gi b m h h eff star formation efﬁciency, f ,for anyhaloiscalculatedasthe integrated H I ionizing emissivities can be signiﬁcantly affected by minimum between the efﬁciency that produces enough type II the inhomogeneous selection and analysis of the data and by the ej supernova (SN II) energy to eject the rest of the gas, f ,and an adopted (double) power law ﬁts to the AGN luminosity function ∗ ej eff upper maximum threshold, f ,sothat f = min[f ,f ]where at different redshifts (Kulkarni, Worseck & Hennawi 2019). Yet, if ∗ ∗ ∗ a fraction f of the SN II energy can couple to the gas. The gas the high comoving emissivity claimed by Giallongo et al. (2015) w mass left after including the effects of star formation and supernova persists up to z 10, then AGN alone could drive reionization feedback is then given by: with little/no contribution from starlight (Madau & Haardt 2015). A similar scenario, where more than 50 per cent of the ionizing eff gf ∗ M (z) = [M (z) − M (z)] 1 − . (1) photons are emitted by rare and bright sources, such as quasars, gi ∗ ∗ ej has been proposed by Chardin et al. (2015), Chardin, Puchwein & Haehnelt (2017) as a possible explanation of the large ﬂuctuations in Our model also includes two types of black hole seeds that can −1 the Ly α effective optical depth on scales of 50 h cMpc measured be assigned to the ﬁrst progenitors of any halo. These include at the end stages of reionization (4 <z < 6) by Becker et al. (i) massive direct-collapse black hole (DCBH) seeds with masses 3−4 (2015). These AGN-dominated or AGN-assisted models, however, between M = 10 M and, (ii) Pop III stellar black hole seeds of bh are found to reionize helium (He II) too early (Puchwein et al. 2019) 150 M masses. As detailed in Dayal et al. (2017b), we calculate the and result in an IGM temperature evolution that is inconsistent with strength of the Lyman–Werner (LW) background irradiating each the observational constraints (Becker et al. 2011). such starting halo. Haloes with an LW background strength J LW −21 −1 −1 −2 −1 In this work, we use a semi-analytic model (Delphi)thathas > J = αJ (where J = 10 ergs Hz cm sr and α is a crit 21 21 been shown to reproduce all key observables for galaxies and AGN free parameter) are assigned DCBH seeds while haloes not meeting at z 5 to revisit the AGN contribution to reionization, specially this criterion are assigned the lighter Pop III seeds. We note that, as a function of the host galaxy stellar mass. The key strengths of given that the number densities of DCBH seeds are ∼−2(−3.8) MNRAS 495, 3065–3078 (2020) Downloaded from https://academic.oup.com/mnras/article/495/3/3065/5843276 by DeepDyve user on 13 July 2022 Reionization with galaxies and AGN 3067 Table 1. Free parameters, their symbols and values used for the ﬁducial model (ins1 in Dayal et al. 2019). As noted, using these parameter values our model reproduces all key observables for galaxies and AGN at z 5 (including their UV luminosity functions, stellar mass/black hole mass densities, star formation rate densities, the stellar/black hole mass function) as well as the key reionization observables (the integrated electron scattering optical depth and the redshift evolution of the ionizing photon emissivity). Simultaneously ﬁtting the optical depth and the emissivity constraints, we obtain f = 0.02 (0.0185) and β = 2.8(2.8) if we consider the ionizing photons provided by star formation (star formation and AGN). Parameter Symbol Value Maximum star formation efﬁciency f 0.02 Fraction of SN II energy coupling to gas f 0.1 Radiative efﬁciency of black hole accretion 0.1 Fraction of AGN energy coupling to gas f 0.003 bh ac −4 Fraction of gas mass AGN can accrete f 5.5 × 10 bh crit −5 Fraction of Eddington rate for BH accretion f (M < M )7.5 × 10 Edd h crit Fraction of Eddington rate for BH accretion f (M ≥ M)1 Edd h LW BG threshold for DCBH formation α 30 sf β Escape fraction of H I ionizing photons from star formation f f [(1 + z)/7] . esc bh Escape fraction of H I ionizing photons from AGN f Ueda et al. (2014) esc Stellar population synthesis model – Starburst99 Reionization (UVB) feedback – No orders of magnitude below that of stellar seeds for α = 30 (300), scale up (down) the UV luminosity function which could be the exact value of α (as well as the DCBH seed mass) have no accommodated by varying the free-parameters for star formation eff sensible bearing on our results, since we only consider models that (f and f ). Under these assumptions, the Starburst99 (SB99) reproduce the AGN luminosity function. In this paper we do not aim stellar population synthesis (SPS) model yields the time-evolution at investigating which type of black hole seed can contribute most of the star-formation powered production rate of H I ionizing sf to reionization, but how a population of AGN reproducing available photons (n ˙ ) and the UV luminosity (L )tobe: UV int observational constraints can contribute to reionization. sf 46.6255 −1 Once seeded, the black holes (as the baryonic and dark matter n ˙ (t ) = 10 − 3.92 log10 + 0.7[s ], (4) int 2Myr components) grow in mass through mergers and accretion in successive time-steps. A fraction of the gas mass left after star and formation and SN II ejection (see equation 1) can be accreted on to 33.077 −1 −1 the black hole. This accretion rate depends on both the host halo L (t ) = 10 − 1.33 log10 + 0.462 [erg s Å ]. UV 2Myr mass and redshift through a critical halo mass (Bower et al. 2017): (5) crit 11.25 3 0.125 M (z) = 10 M [ (1 + z) + ] , (2) m λ Inspired by the Shakura–Sunyaev solution (Shakura & Sunyaev such that the mass accreted by the black hole (of mass M )atany 1973), AGNs are assigned a spectral energy distribution (SED) that bh given time-step is: depends on the key black hole physical parameters, namely the black hole mass and Eddington ratio (Volonteri et al. 2017). We ac ac gf M (z) = min f M (z), (1 − )f M (z) , (3) Edd Edd r bh bh ∗ follow here a variant based on the physical models developed by −1 Done et al. (2012). Speciﬁcally, we calculate the energy of the peak where M (z) = (1 − )[4πGM (z)m ][σ c] t is the total Edd r bh p T r of the SED as described in Thomas et al. (2016), but adopt the mass that can be accreted in a time-step assuming Eddington default functional form of the spectrum used in Cloudy (Ferland luminosity. Here, G is the gravitational constant, m is the proton et al. 2013). mass, σ is the Thomson scattering optical depth, is the BH T r Once an AGN is assigned a luminosity and an SED, the UV radiative efﬁciency, c is the speed of light, and t = 20 Myr is the luminosity is calculated as detailed in Dayal et al. (2019). Further, merger tree time-step. Further, the value of f is assigned based Edd ac we integrate above 13.6 eV to obtain the H I ionizing luminosity and on the critical halo mass (equation 2) as detailed in Table 1 and f bh mean energy of ionizing photons (see Fig. A1 in the Appendix). represents a ﬁxed fraction of the total gas mass present in the host For AGN, this provides an upper limit, as photons above 24.59 eV galaxy that can be accreted by the black hole. A ﬁxed fraction f bh and 54.4 eV can ionize He I and He II. We further include a of the total energy emitted by the accreting black hole is allowed correction for secondary ionizations from the hard AGN photons, to couple to the gas content. The values used for each of these by taking the upper limit to their contribution, i.e. assuming fully parameters in our ﬁducial model are detailed in Table 1. Finally, neutral hydrogen and that 39 per cent of their energy goes into sec- reionization feedback is included by suppressing the gas content, ondary ionizations (Shull & van Steenberg 1985; Madau & Fragos and hence star formation and black hole accretion, of haloes with −1 2017; Kakiichi et al. 2017; Eide et al. 2018). a virial velocity V 40 km s at all redshifts, as detailed in vir Section 2.3. In the interest of simplicity, every newly formed stellar population 2.2 The escape fraction of H I ionizing photons is assumed to follow a Salpeter initial mass function (IMF; Salpeter 1955) with masses in the range 0.1 − 100 M , with a metallicity In what follows, we discuss our calculations of f for both AGN esc Z = 0.05Z and an age of 2 Myr; a lower (higher) metallicity and stellar radiation from galaxies. In addition to the ﬁducial model, or a younger (older) stellar population across all galaxies would we study ﬁve combinations of f from star formation and AGN in esc MNRAS 495, 3065–3078 (2020) Downloaded from https://academic.oup.com/mnras/article/495/3/3065/5843276 by DeepDyve user on 13 July 2022 3068 P. Dayal et al. order to explore the available parameter space and its impact on our reproducing the values of electron scattering optical depth (τ )and es results as detailed in Section 4. the redshift evolution of the emissivity require such a decrease in the global value of the escape fraction of ionizing photons from star formation. The values of f and β required to simultaneously bh 2.2.1 The escape fraction for AGN f esc ﬁt the above-noted data sets (with and without AGN contribution) are shown in Table 1. For the ionizing radiation emitted from the AGN, we consider four Secondly, whilst maintaining the same functional form, we ﬁnd different models. We start by taking an approach similar to Ricci the values of the two coefﬁcients (f and β ) required to ﬁt the optical et al. (2017) for the ﬁducial model. Essentially, we assume that 0 depth and emissivity constraints using the same escape fraction from the unobscured fraction, i.e. the fraction of AGN with column 22 −2 bh AGN and star formation. density < 10 cm is a proxy for the escape fraction, f . esc Thirdly, following recent results (e.g. Borthakur et al. 2014; The argument is that by applying a column-density dependent Naidu et al. 2019), we use a model wherein the escape fraction for correction to the X-ray LF, one recovers the UV luminosity star formation scales positively with the stellar mass. In this case, function. As in Dayal et al. (2019), we adopt the luminosity- sf bh for galaxies that have black holes, we assume f = f using dependent formalism of Ueda et al. (2014), taking as unobscured esc esc bh sf the ﬁducial model for f ; f = 0 for galaxies without a black fraction f ≡ f , which varies from 10 per cent for faint esc esc unabs logNH < 22 43 −1 hole. This accounts for the possibility that AGN feedback enhances AGN (L < 10 erg s )to 67 per cent for bright AGN 2-10keV 46 −1 the effect of SN feedback in carving ‘holes’ in the interstellar (L > 10 erg s ). The unobscured fraction can be written 2-10keV medium, facilitating the escape of ionizing radiation. This is a as: very optimistic assumption, as dedicated simulations show that 1 − ψ f = , (6) unabs AGN struggle to shine and amplify the escape fraction in low-mass 1 + ψ galaxies (Trebitsch et al. 2018). sf where ψ = ψ − 0.24(L − 43.75), ψ = 0.43[1 + min (z, z x z Fourthly, we explore a model with a constant f = 0.035. esc 0.48 2)] and L is the log of the intrinsic 2–10 keV X-ray luminosity x Although a constant escape fraction for stellar radiation from all −1 in erg s ; given our model is for z 5, this implies ψ = z galaxies can reproduce the τ value, it overshoots the value of the es 0.73. We do not extrapolate the evolution beyond z = 2, the observed emissivity (see e.g. ﬁg. 3, Dayal et al. 2017a). sf range for which the dependence has been studied using data. Finally, we explore a model wherein f increases with de- esc As in Ricci et al. (2017), we assume that unobscured quasars creasing stellar mass, as has been shown by a number of theoretical have f = 1 and zero otherwise (see their Section 4.1 for esc works (e.g. Yajima et al. 2011;Wiseetal. 2014; Paardekooper sf a discussion and alternative models and Volonteri et al. 2017, et al. 2015). Essentially, we assume f scales with the ejected esc ej sf eff for a discussion on the redshift evolution of the obscured frac- gas fraction such that f = f (f /f ). This naturally results 0 ∗ esc ∗ ej tion). sf eff sf in a high f value for low mass galaxies where f = f ; f esc ∗ esc Secondly, Merloni et al. (2014) ﬁnd that X–ray and optical ej eff drops with increasing mass where f ∼ f <f . The results from ∗ ∗ obscuration are not necessarily the same for AGN, although the these last four cases are discussed in detail in Section 4. trend of optically obscured AGN with luminosity is consistent with sf We clarify that while we assume the same f value for each esc bh the scaling we adopt. Our second model for f considers the esc galaxy, in principle, this should be thought of as an ensemble average fraction of optically unobscured AGN as a function of luminosity that depends on, and evolves with, the underlying galaxy properties, from Merloni et al. (2014), where this fraction is found to be such as mass or star formation or a combination of both. independent of redshift. It takes the functional form: 1 43.89 − log L bh 2.3 Modelling reionization f = 1 − 0.56 + arctan , (7) esc π 0.46 The reionization history, expressed through the evolution of the where log L is the logarithm of the intrinsic 2–10 keV X-ray volume ﬁlling fraction (Q ) for ionized hydrogen (H II), can be II −1 luminosity in erg s . written as (Shapiro & Giroux 1987; Madau, Haardt & Rees 1999): Thirdly, we can maximize the contribution of AGN to reionization bh by assuming f = 1, although Micheva, Iwata & Inoue (2017)ﬁnd dQ dn 1 Q dt II ion II esc = − , (8) bh that even for unobscured AGN f is not necessarily unity. dz dz n t dz esc H rec Finally, we explore a model wherein we use the same (redshift- where the ﬁrst term on the right-hand side is the source term while dependent) escape fraction for the ionizing radiation from both star the second term accounts for the decrease in Q due to recom- II formation and AGN. The results from these last three cases are binations. Here, dn /dz = n represents the hydrogen ionizing ion ion discussed in detail in Section 4. photon rate density contributing to reionization. Further, n is the comoving hydrogen number density and t is the recombination rec sf time-scale that can be expressed as (e.g. Madau et al. 1999): 2.2.2 The escape fraction for star formation (f ) esc Both the value of the escape fraction of H I ionizing radiation emitted t = . (9) rec sf 3 from the stellar population (f ) as well as its trend with the χn (1 + z) α C H B esc galaxy mass or even redshift remain extremely poorly understood Here α is the hydrogen case-B recombination coefﬁcient, χ = 1.08 sf (Section 7.1, Dayal & Ferrara 2018). We study four cases for f esc accounts for the excess free electrons arising from singly ionized in this work: ﬁrst, in our ﬁducial model, we use an escape fraction helium and C is the IGM clumping factor. We use a value of C that sf β that scales down with decreasing redshift as f = f [(1 + z)/7] esc evolves with redshift as where β> 1and f is a constant at a given redshift. This is in accord with a number of studies (Robertson et al. 2015; Dayal et al. H II −1.71 C = = 1 + 43 z (10) 2017a; Puchwein et al. 2019) that have shown that simultaneously H II MNRAS 495, 3065–3078 (2020) Downloaded from https://academic.oup.com/mnras/article/495/3/3065/5843276 by DeepDyve user on 13 July 2022 Reionization with galaxies and AGN 3069 using the results of Pawlik, Schaye & van Scherpenzeel (2009) the stellar emissivity at all redshifts and the total (star forma- and Haardt & Madau (2012) who show that the UVB generated tion + AGN) emissivity down to z ∼ 5; although sub-dominant, the by reionization can act as an effective pressure term, reducing the importance of stars in massive (M 10 M ) galaxies increases clumping factor. with decreasing redshift and they contribute as much as 40 per cent While reionization is driven by the hydrogen ionizing photons (∼ 15 per cent) to the stellar (total) emissivity at z ∼ 4. produced by stars in early galaxies, the UVB built up during On the other hand, driven by the growth of black holes and the bh reionization suppresses the baryonic content of galaxies by photo- constancy of f with redshift, the AGN emissivity shows a steep esc heating/evaporating gas at their outskirts (Klypin et al. 1999; Moore (six-fold) increase in the 370 Myr between z ∼ 6 and 4. A turning et al. 1999; Somerville 2002), suppressing further star formation and point is reached at z ∼ 5 where AGN and star formation contribute slowing down the reionization process. In order to account for the equally to the total emissivity, with the AGN contribution (dom- 6 9 effect of UVB feedback on n ˙ , we assume total photoevaporation inated by M 10 M black holes in M 10 M galaxies) ion bh ∗ −1 of gas from haloes with a virial velocity below V = 40 km s overtaking that from star formation at lower-z. Indeed, the AGN vir embedded in ionized regions at any z. In this ‘maximal external emissivity is almost twice of that provided by stars by z ∼ 4 leading feedback’ scenario, haloes below V in ionized regions neither to an increase in the total value. vir form stars nor contribute any gas in mergers. To summarize, while the trend of the total emissivity is driven The globally averaged n ˙ can then be expressed as: by star formation in low-mass galaxies down to z = 5, AGN take ion over as the dominant contributors at lower redshifts. This result is sf bh n ˙ (z) = n ˙ (z) + n ˙ (z), (11) ion esc esc in agreement with synthesis models for the UVB (Faucher-Giguere ` et al. 2008;Haardt&Madau 2012) as shown in the same ﬁgure. where The above trends can also be used to interpret the latest results on sf sf sf sf n ˙ (z) = f Q (z)n ˙ (z) + Q (z)n ˙ (z) , (12) II I esc esc int,II int,I the integrated electron scattering optical depth (τ = 0.054 ± 0.007; es Planck Collaboration VI 2018), shown in the right-hand panel bh bh bh bh n ˙ (z) = f Q (z)n ˙ (z) + Q (z)n ˙ (z) , (13) II I esc esc int,II int,I of Fig. 1. We start by noting that ﬁtting to this data requires sf 2.8 sf bh sf bh where Q (z) = 1 − Q (z). Further, n ˙ (n ˙ )and n ˙ (n ˙ ) f = 0.02[(1 + z)/7] if stars in galaxies are considered to be I II int,II int,II int,I int,I esc account for the intrinsic hydrogen ionizing photon production rate the only reionization sources; as shown in Table 3 considering the density from star formation (black hole accretion) in case of full UV- contribution of both stars and AGN leads to a marginal decrease sf in the co-efﬁcient of f to 0.0185 whilst leaving the redshift- suppression of the gas mass and no UV suppression, respectively. esc sf bh 9 relation unchanged. Stellar radiation in low-mass (M 10 M ) The term n ˙ (n ˙ ) weights these two contributions over the volume esc esc galaxies dominate the contribution to τ for most of reionization ﬁlling fraction of ionized and neutral regions – i.e. while n ˙ int,I es history. AGN only start making a noticeable contribution at z represents the contribution from all sources, stars, and black holes −1 5, where they can generate an optical depth of τ ∼ 0.22, in haloes with V < 40 kms do not contribute to n ˙ .Atthe vir int,II es comparable to stars, which generate a total value of τ ∼ 0.24. beginning of the reionization process, the volume ﬁlled by ionized es Stellar radiation from high-mass (M 10 M ) galaxies has a hydrogen is very small (Q <<1) and most galaxies are not affected II ∗ sf sf bh bh by UVB-feedback, so that n ˙ (z) ≈ n ˙ (z)f + n ˙ (z)f . sub-dominant contribution to τ at all redshifts. ion es int,I esc int,I esc As Q increases and reaches a value 1, all galaxies in haloes II −1 with circular velocity less than V = 40 km s are feedback- vir 3.2 AGN contribution to reionization as a function of stellar sf sf bh bh suppressed, so that n ˙ (z) ≈ n ˙ (z)f + n ˙ (z)f . ion int,II esc int,II esc mass To understand the AGN contribution to reionization in the ﬁducial 3 RESULTS model, we start by looking at the (intrinsic) production rate of H I Given that n ˙ (z) is an output of the model, t is calculated as a ionizing photons as a function of M for z ∼ 4 − 9 (panel a; ion rec ∗ bh sf function of z and f is obtained from the AGN obscuration fraction, Fig. 2). As expected, n ˙ scales with M since higher mass galaxies esc int sf f is the only free parameter in our reionization calculations. As typically have larger associated star formation rates. Further, given esc sf bh explained above, in the ﬁducial model, f is composed of two their larger gas and black hole masses, n ˙ too scales with M .As esc int free parameters (f and β ) that are ﬁt by jointly reproducing the seen, stars dominate the intrinsic H I ionizing radiation production observed values of τ and the emissivity as discussed in Section 3.1 rate for all stellar masses at z 7. However, moving to lower es sf that follows. We use this f value to study the AGN contribution redshifts, black holes can contribute as much as stars in galaxies esc 10.2−10.9 to reionization in Section 3.2. In order to test the robustness of our with M ∼ 10 M at z ∼ 6. This mass range decreases to 9.6−10 results to assumptions, we also explore alternative models for the M ∼ 10 M at z ∼ 4 where intermediate-mass galaxies host escape fraction from AGN and star formation and the impact of black holes that can accrete at the Eddington rate. different stellar population synthesis models in Section 4. The second factor that needs to be considered is the escape fraction of ionizing photons which is shown in panel (b) of the same sf ﬁgure. As noted above, f is independent of galaxy properties esc 3.1 The electron scattering optical depth and the ionizing and decreases with decreasing z, going from a value of about photon emissivity 5.4 per cent at z ∼ 9to0.77 per cent at z ∼ 4. bh We start by discussing the redshift evolution of the ionizing photon However, f scales with M , and this is the result of the esc emissivity (equation 11) from the ﬁducial model shown in the left- dependence of the unabsorbed AGN fraction with luminosity: at hand panel of Fig. 1. For star formation, the ‘escaping’ emissivity higher AGN luminosity a higher fraction of AGN are unabsorbed. sf bh 9.7 includes the effect of f that decreases with redshift as ∝ [(1 Quantitatively, while f ∼ 10 per cent for M 10 M ,itcan esc esc 2.8 10.9 + z)/7] . As a result, whilst increasing from z ∼ 19 to z ∼ 8the have a value as high as 30 per cent for M 10 M at z ∼ 6–9. emissivity from stellar sources in galaxies thereafter shows a drop We can now combine the intrinsic production rate of H I ionizing at lower redshifts. Low-mass (M 10 M ) galaxies dominate photons and the escape fraction to look at the rate of ‘escaping’ MNRAS 495, 3065–3078 (2020) Downloaded from https://academic.oup.com/mnras/article/495/3/3065/5843276 by DeepDyve user on 13 July 2022 3070 P. Dayal et al. Figure 1. Redshift evolution of the H I ionizing photon emissivity (left-hand panel) and the CMB electron scattering optical depth (τ ) as a function of es redshift (right-hand panel)for the ﬁducial model. In the left-hand panel, the open squares show observational results (and associated error bars) calculated following the approach of Kuhlen & Faucher-Giguere (2012). In the right-hand panel, the dot-dashed horizontal line shows the central value for τ inferred by es the latest Planck results (Planck Collaboration, Aghanim & Akrami 2018) with the grey striped region showing the 1–σ errors. Overplotted are the escaping emissivities (left-hand panel) and the optical depths (right-hand panel) contributed by: star formation only (SF; dot-long-dashed line), AGN + star formation sf bh sf (solid line), and AGN only (short-long-dashed line) using the f and f values for the ﬁducial model reported in Table 1; note that f is lower in the esc esc esc AGN + SF case (f = 0.0185) as compared to the SF only case (f = 0.02). We deconstruct the contribution from star formation in galaxies into those with 0 0 9 9 6 stellar masses M 10 M (short-dashed line) and M 10 M (long-dashed line) and show the contribution of black holes of masses 10 M using ∗ ∗ the dotted line, as marked. ionizing radiation for star formation and AGN in panel (c) of properties (mass, accretion rate). In general, models that reproduce sf sf bh Fig. 2. As expected, n ˙ ∝ M and n ˙ > n ˙ at z> 7. However the generally accepted UV luminosity functions of galaxies and esc esc esc at z< 7 the situation is quite different: the most massive black AGN will all converge to a similar fractional contribution of AGN holes and therefore the most luminous AGN are hosted in massive to reionization. The main reason for the agreement between our galaxies. Additionally, the presence of a critical halo mass below results and those of Qin et al. (2017) is that in both models black which black hole growth is suppressed (see Section 2.1) translates hole growth is retarded with respect to galaxies, although in different into a critical stellar mass (ﬁg. 6; Dayal et al. 2019), below which ways. In our model suppression of black hole growth leads to a black bh only low-luminosity AGN exist and f is very low. The fact that hole mass function with a step-like appearance, in their case it is esc bh both the intrinsic photon production from AGN and f are very the overall normalization of the mass function that decreases with esc low in low-mass galaxies suppresses the AGN contribution from increasing redshift. In principle, this can be tested observationally such galaxies to the escaping photon budget. However, the fact through measurements of the relation between black hole and stellar sf bh that n ˙ n ˙ for high-mass galaxies coupled with an increasing masses in high redshift galaxies. int int bh f value results in black holes dominating the escaping ionizing As expected from the above discussion, star formation in galaxies esc radiation rate for galaxies with mass above a ‘transition stellar mass’ dominate n ˙ for all stellar masses at z> 7 although the AGN esc 9.6 9.2 of M 10 (10 )M at z ∼ 6(4). contribution increases with M as showninpanel (d)ofFig. 2. ∗ ∗ The suppression of black hole growth in low-mass galaxies, At z< 7, however, AGN can start dominating n ˙ by as much as esc advocated from either trying to reconcile seemingly contradictory one order of magnitude for M ∼ 10 M galaxies at z ∼ 6where observational results (Volonteri & Stark 2011) or from the results black holes can accrete at the Eddington rate. This peak mass shifts of cosmological hydrodynamical simulations (Dubois et al. 2015; to lower M values with decreasing redshift – at z ∼ 4AGN in 9.6 Bower et al. 2017), modiﬁes the picture compared to early papers galaxies with masses as low as M ∼ 10 M , which can accrete that assumed unimpeded growth of massive black holes in small at the Eddington limit, dominate n ˙ by a factor of 10. esc galaxies/haloes (Volonteri & Gnedin 2009). As noted above, the The redshift evolution of the ‘transition mass’, at which AGN suppression of black hole contribution from small galaxies/haloes, start dominating n ˙ , is shown in panel (e) of the same ﬁgure which esc which dominate the mass function at the highest redshifts, is shows two key trends: ﬁrst, as expected, the transition mass only bh further strengthened by the assumption that f increases with AGN exists at z< 7 with stellar radiation dominating n ˙ at higher-z. esc esc luminosity. Secondly, as black holes in galaxies of increasingly lower stellar The contribution of AGN to reionization was studied using a mass can accrete at the Eddington limit with decreasing redshift semi-analytical model also by Qin et al. (2017). Qualitatively, our (Piana et al., in preparation), the transition mass too decreases with 10.7 9.3 results agree with theirs, in the sense that only relatively high-mass z from ∼ 10 M at z ∼ 6.8 to ∼ 10 M by z ∼ 4. In the same black holes are important thus limiting the contribution of AGN panel, we also show a comparison of this transition mass to the knee to low redshift, and that the AGN contribution to reionization is observationally inferred knee of the stellar mass function (M ) 10.5 11 sub-dominant, of the order of 10–15 per cent at z< 6. The speciﬁc which ranges between 10 and 10 M at z ∼ 4–7. While the assumptions of the models differ, though: Qin et al. (2017) assume a transition mass is comparable to the knee stellar mass at z ∼ 6.8, luminosity-independent obscured fraction, and they do not include it shows a very rapid decline with decreasing redshift. Indeed, by z a spectral energy distribution that depends on intrinsic black hole ∼ 4, AGN start dominating n ˙ from galaxies that are (at least) an esc MNRAS 495, 3065–3078 (2020) Downloaded from https://academic.oup.com/mnras/article/495/3/3065/5843276 by DeepDyve user on 13 July 2022 Reionization with galaxies and AGN 3071 Figure 2. As a function of stellar mass, the panels (top to bottom) show the results for star formation (solid lines) and AGN (light shaded regions) for the ﬁducial model for: (a): the intrinsic H I ionizing photon rate; (b): the escape fraction of H I ionizing photons; (c): the escaping H I ionizing photon rate; (d): the ratio between the escaping H I ionizing photon rate for AGN and star formation with the horizontal line showing a ratio of unity; and (e): the transition stellar mass at which AGN start dominating the escaping ionizing photon production rate. In this panel, the solid circles and empty triangles show the knee value of the stellar mass function (and the associated error bars) observationally inferred by Grazian et al. (2015) and Song et al. (2016), respectively. Finally, the different colours in panels (a)–(c) are for the redshifts marked in panel (a) while the different lines in panel (d) are for the redshifts marked in that panel. order of magnitude less massive compared to the knee mass and in range is weighted by its cosmic abundance, via the mass of the host fact the ratio between the escaping H I ionizing photon rate for AGN halo – therefore this ﬁgure represents the effective contribution of and star formation peaks at intermediate galaxy masses. Finally, we that mass range to the global photon budget. We note that, at any sf sf bh note that such a transition mass only exists in the case that the stellar z, while n ˙ is just a scaled version of n ˙ , n ˙ instead evolves esc int esc sf bh mass dependence of f is shallower than f (see Section 4). based on the luminosity/mass evolution. The key trends emerging esc esc We summarize the impact of the above-noted trends on the are: ﬁrst, at any z, whilst the contribution of stars (weighted by production/escape rates of H I ionizing photons per baryon over the number density) is the highest at intermediate stellar mass 7−9 a Hubble time in Fig. 3. Here the contribution in each galaxy mass galaxies (10 M )at z ∼ 6, the contribution is essentially mass MNRAS 495, 3065–3078 (2020) Downloaded from https://academic.oup.com/mnras/article/495/3/3065/5843276 by DeepDyve user on 13 July 2022 3072 P. Dayal et al. Figure 3. The ionizing photon per baryon value as a function of stellar mass for the ﬁducial model for star formation and AGN at z ∼ 6 and 9, as marked. The dot-dashed and solid lines show the intrinsic and escaping H I ionizing photon rates, respectively. 5−8 hydrogen reionization. One could argue that this is a consequence of independent between a stellar mass of 10 M at z ∼ 9. Although sf 9 10 the steep increase of f at high redshifts, which artiﬁcially boosts massive galaxies, M ∼ 10 − 10 M , have higher production ∗ esc the contribution of stars in low-mass galaxies and correspondingly rates of ionizing radiation from both stars and black holes in addition bh reduces the contribution of AGN. In this section we examine the to higher f values, they are rarer than their low-mass counterparts, esc robustness of our results by exploring six different combinations of which therefore dominate the total emissivity as also shown in the bh sf f and f in Section 4.1 and two different stellar population left-hand panel of Fig. 1. Secondly, AGN only have a contribution esc esc 9−10 synthesis models in Section 4.2 in order to explore the physically at the high stellar mass end (M ∼ 10 M )at z 9. Thirdly, as plausible parameter space. expected from the above discussions, given both the higher values of the intrinsic H I ionizing photon production rate and f , AGN dom- esc inate the emissivity at the high-mass end (M 10 M )at z ∼ 6. II ionizing photons, 4.1 Alternative models for AGN and star formation escape Since AGNs are efﬁcient producers of He useful constraints can be obtained on their contribution from the fractions corresponding observations, e.g. He II Ly α optical depth at z ∼ 3 sf bh Given that the trends of f and f with galaxy properties are esc esc (Worseck et al. 2016) and the heating of the IGM at z 5 (Becker still uncertain, both theoretically and observationally, Fig. 4 shows et al. 2011). A detailed modelling of the He II reionization history the optical depth and emissivity predicted by the alternative models is beyond the scope of this work. However, we have computed summarized in Table 2: the He III volume ﬁlling fraction, Q , and found that Q ∼ He III HeIII bh 0.4(0.2) at z = 4 (5), assuming that the escape fraction of He II (i) In the ﬁrst model (Alt1, panels a1 and a2), f is obtained esc ionizing photons is the same as that of the H I ionizing photons. from the results of Merloni et al. (2014). We ﬁt to the optical depth sf 3.8 While this implies a He II reionization earlier than the model of and emissivity observations to derive f = 0.017[(1 + z)/7] . esc Haardt & Madau (2012), it is still within the 2–σ bounds as allowed This steep redshift-dependence for the escaping stellar radiation by the observations (see e.g. Mitra et al. 2018). from galaxies (left-most column of Fig. 5) is required to off-set the increasing AGN contribution at z 5 which is driven by the higher bh f values (compared to the ﬁducial model) as shown in the middle esc 4 ALTERNATIVE MODELS bh sf column of Fig. 5. This enhances the ratio n ˙ /n ˙ by more than esc esc Our key result is that the AGN contribution of ionizing photons one order of magnitude compared to the ﬁducial model at z< 7 is subdominant at all galaxy masses at z> 7. At z ∼ 6–7 their (right-most column of Fig. 5). As seen from the same panel, we ﬁnd contribution increases with stellar mass, and at lower redshift it that the transition mass remains almost unchanged compared to the is AGN in intermediate-mass galaxies that produce most ionizing ﬁducial case. sf photons (Fig. 2). This results in a ‘transition’ stellar mass at which (ii) In the second model (Alt2, panels b1 and b2) we keep f esc AGN overtake the stellar contribution to the escaping ionizing equal to the ﬁducial value and maximize the escape fraction from bh radiation; for stars in galaxies to dominate all the way through in the AGN by assuming f = 1. Driven by such maximal AGN contribu- esc mass function, either the escape fraction of stellar radiation from tion, this model severely overpredicts the emissivity at z 5; the op- galaxies should increase with galaxy mass or that from AGN should tical depth, being dominated by star formation in galaxies for most decrease, especially at high masses. In our ﬁducial model, this of the reionization history, can still be ﬁt within the 1–σ error bars. bh sf transition stellar mass decreases with decreasing redshift. Further, As seen from the right-most panel of Fig. 5, n ˙ /n ˙ is higher by esc esc star formation in galaxies with mass < 10 M is the main driver of more than one order of magnitude compared to the ﬁducial model. MNRAS 495, 3065–3078 (2020) Downloaded from https://academic.oup.com/mnras/article/495/3/3065/5843276 by DeepDyve user on 13 July 2022 Reionization with galaxies and AGN 3073 Figure 4. The redshift evolution of the electron scattering optical depth (left-hand column) and the associated escaping ionizing emissivity (right-hand column). In the left-hand column, the dot-dashed horizontal line shows the central value for τ inferred by the latest Planck results (Planck Collaboration es VI 2018) with the grey striped region showing the 1–σ errors. In the right column, open squares show the observational results (and associated error bars) calculated following the approach of Kuhlen & Faucher-Giguere (2012). In each panel, we show results for star formation + AGN (solid line), star formation (dot-dashed line), and AGN (short-long-dashed line) for the different alternative escape fraction models (Alt1-Alt6) discussed in Section 4.1 and summarized in Table 2. The model name and the f values used for star formation and AGN are noted in each panel of the right column. esc Again, a transition stellar mass exists at z< 7 and is only slightly seen from the left and middle columns of Fig. 5. This model lower (by about 0.2–0.4 dex) compared to the ﬁducial model. naturally results in a lower AGN contribution to the escap- (iii) In the third model (Alt3, panels c1 and c2) we con- ing ionizing radiation at all masses and redshifts as com- sider the same redshift-dependent escape fraction for the ion- pared to the ﬁducial model (right most panel of the same izing radiation from both stellar radiation and AGN. Here, si- ﬁgure). Similar to the results of model Alt4 that follows, multaneously ﬁtting to the optical depth and emissivity val- in this model the AGN ionizing radiation contribution is sf bh ues yields an escape fraction that evolves as f = f = minimized and only slightly exceeds that from galaxies at esc esc 3.2 sf bh 9.5−9.8 0.017[(1 + z)/7] . The evolution of f and f can be M ∼ 10 M by z ∼ 4, i.e. stellar radiation dominates esc esc MNRAS 495, 3065–3078 (2020) Downloaded from https://academic.oup.com/mnras/article/495/3/3065/5843276 by DeepDyve user on 13 July 2022 3074 P. Dayal et al. sf bh Table 2. For the alternative models studied in Section 4.1, we summarize the model name (column 1), the parameter values for f (column 2) and f esc esc bh sf (column 3), the impact on the ratio n ˙ /n ˙ compared to the ﬁducial model (column 4) and the impact on the transition mass at which AGN start dominating esc esc the escaping H I ionizing photon production rate compared to the ﬁducial model (column 5). We note that of models Alt1 – Alt6, only Alt1, Alt3 and Alt6 simultaneously ﬁt τ (Planck Collaboration VI 2018) and the redshift evolution of the H I ionizing photon emissivity. We use the ﬁducial values of the free es parameters for galaxy formation as in Table 1. sf bh bh sf Model f f n ˙ /n ˙ Transition M esc esc esc esc 3.8 Alt1 0.017[(1 + z)/7] Merloni et al. (2014) Increases at all M Almost unchanged Alt2 ﬁducial 1 Increases at all M Decreases by 0.2 (0.4 dex) at z ∼ 6(4) 3.2 3.2 Alt3 0.017[(1 + z)/7] 0.017[(1 + z)/7] Decreases at all M – bh Alt4 ﬁducial f ∝ M ﬁducial Decreases at all M – ∗ ∗ esc Alt5 0.035 Ueda et al. (2014) Decreases at all M for z 7.5 Increases by 0.1 dex at z ∼ 6–4 ej −ζ eff 9.2 Alt6 0.1(f /f ) ∝ M ﬁducial Increases for M 10 M Decreases by 0.3 dex (unchanged) at z ∼ 6(4) ∗ ∗ ∗ sf bh Figure 5. As a function of stellar mass, we show f (left-hand column), f (middle column) and the ratio between the escaping H I ionizing photon rate esc esc for AGN and stars (right-hand column)for z ∼ 4.1 (top row) and z ∼ 6 (bottom row). We show results for the six different alternative escape fraction models (Alt1- Alt6) discussed in Section 4.1 and summarized in Table 2 and also plot the ﬁducial model for comparison. In the right-most column, the horizontal line shows a ratio of unity. the ionizing budget at effectively all masses and redshifts As seen from the bottom panels of Fig. 4, this model is unable to although the AGN contribution still increases with increasing stellar simultaneously reproduce both the values of τ and the emissivity. es sf mass. In this model, the value of f is decreased (increased) at z esc (iv) In the fourth model (Alt4, panels d1 and d2) we assume 7.5( 7.5) compared to the ﬁducial case as shown in the left-hand sf bh bh f = f using the ﬁducial f value from Ueda et al. (2014) panel of Fig. 5. Compared to the ﬁducial model, this results in esc esc esc sf bh sf for galaxies that have a black hole; we use f = 0 for galaxies a lower value of n ˙ /n ˙ by about 0.3 (0.8 dex) at z ∼ 6(z ∼ esc esc esc sf bh that do not host a black hole. This results in both f and f 4.1) and the transition mass increases negligibly (by ∼0.1 dex) at esc esc scaling positively with the stellar mass as shown in the left-most z = 4−6. and middle panels of Fig. 5. As in the previous model, this identical (vi) In the sixth model (Alt6, panels f1 and f2), while we use the bh sf escape fraction for both stellar radiation and AGN results in stellar ﬁducial value for f , we assume that f scales with the ejected esc esc ej sf eff radiation dominating the ionizing budget at almost all masses and gas fraction such that f = f (f /f ). This naturally results in 0 ∗ esc ∗ sf redshifts; the AGN ionizing radiation contribution only slightly f decreasing with an increasing halo (and stellar) mass. A value esc exceeds that from galaxies at M ∼ 10 M by z ∼ 4. However, ∗ of f = 0.1 is required to simultaneously ﬁt both the optical depth and we note that this model overpredicts the emissivity from stellar emissivity constraints as shown in the same ﬁgure. In this model, sources at all redshifts and is unable to simultaneously reproduce the increasing suppression of the star formation rate in low-mass both the values of τ the the emissivity. es haloes due to both supernova and reionization feedback naturally (v) In the ﬁfth model (Alt5, panels e1 and e2) we assume a leads to a downturn in the stellar emissivity with decreasing redshift. sf bh sf constant f = 3.5 per cent and use the ﬁducial value for f . As showninFig. 5, in this model the f values lie below the esc esc esc MNRAS 495, 3065–3078 (2020) Downloaded from https://academic.oup.com/mnras/article/495/3/3065/5843276 by DeepDyve user on 13 July 2022 Reionization with galaxies and AGN 3075 8.4 Table 3. The parameter values for the z-evolution of the escape fraction, ﬁducial one for all M 10 M at z ∼ 6. However, by z ∼ 4, sf β sf 8.6 f = f [(1 + z)/7] for different models constrained to simultaneously esc the f values for the lowest mass haloes (∼ 10 M ) approach esc ﬁt τ (Planck Collaboration VI 2018) that combines polarization, lensing, es the values for the ﬁducial model. Compared to the ﬁducial model, and temperature data, and the redshift evolution of the H I ionizing photon bh sf this results in an increasing n ˙ /n ˙ with increasing stellar mass, esc esc bh emissivity (see the text). We use the ﬁducial value for f and the same 9.2 esc specially for M 10 M . This naturally leads a transition mass values of the free parameters for galaxy formation as in Table 1. that is lower than that in the ﬁducial model by about 0.3 dex at z ∼ 6, whilst being almost identical at z ∼ 4. SPS Model Sources f × 100 β sf bh To summarize, the possible range of f and f combinations SB99 SF 2.0 2.8 esc esc (ranging from redshift-dependent to constant to scaling both pos- SB99 SF + AGN 1.85 2.8 BPB SF 0.46 2.8 itively and negatively with stellar mass) have conﬁrmed our key BPB SF + AGN 0.43 2.8 results: the AGN contribution of ionizing photons is subdominant SB99 + sb SF 1.7 2.8 at all galaxy masses at z> 7 and increases with stellar mass at z< SB99 + sb SF + AGN 1.6 2.8 7. Additionally, we have conﬁrmed the existence of a ‘transition’ stellar mass (at which AGN overtake the stellar contribution to the escaping ionizing radiation) which decreases with decreasing redshift. Stars dominate all the way through the mass function only sf bh 5 REIONIZATION HISTORY AND THE when the stellar mass dependence of f is steeper than f or esc esc CUMULATIVE AGN CONTRIBUTION if we assume the same f values for both star formation and AGN esc (i.e. the Alt3 and Alt4 models); in this case, naturally, the transition We start with a recap of the total (star formation + AGN) ionizing mass no longer exists. emissivity for all the different models considered in this work in the left-hand panel of Fig. 6. In all models, the ionizing emissivity from star formation dominates at z> 6 and is virtually indistinguishable for all the models (ﬁducial, Alt1, Alt2, and Alt3) that use a redshift 4.2 Alternative stellar population synthesis models sf dependent f value. The redshift evolution of the emissivity esc In addition to the ﬁducial SB99 model, we have considered two sf is the steepest for the Alt4 model where f ∝ M . With its esc other population synthesis models: BPASS binaries (BPB; Eldridge sf constant value of f = 0.035, model Alt5 shows the shallowest esc et al. 2017) and Starburst99 including stripped binaries (SB99 + sb; sf slope. Given its lower f values for all stellar masses at high esc Gotberg et al. 2019). The time evolution of the intrinsic ionizing redshifts, the Alt6 model naturally shows a lower ionizing emissivity and UV photons from star formation in the BPB model can be compared to ﬁducial; the stellar emissivity from the Alt6 model expressed as: converges to the ﬁducial one by z ∼ 9 as a result of the decreasing sf f values for the latter. As expected, the AGN contribution is t esc sf 47.25 −1 sf bh n ˙ (t ) = 10 − 2.28 log + 0.6[s ], (14) int the lowest for the model Alt3 where f = f = a decreasing esc esc 2Myr function of redshift (as shown in the same panel). It then increases by a factor of 3 from the ﬁducial case to the Alt1 case and reaches bh its maximum for the Alt2 case where f = 1. esc 33.0 −1 −1 We then discuss reionization history, expressed through the L (t ) = 10 − 1.2log + 0.5[ergs Å ]. (15) UV 2Myr redshift evolution of the volume ﬁlling fraction of ionized hydrogen (Q ), as shown in the right-panel of Fig. 6. Interestingly, despite the II In the SB99 + sb model, these quantities evolve as: sf bh range and trends used for f and f , reionization is 50 per cent esc esc complete in all cases in the very narrow redshift range of z ∼ 6.6– sf 46.7 −1 n ˙ (t ) = 10 − 2.3log [s ], (16) int 7.6. Further, we ﬁnd an end redshift of reionization value of z ∼ 2Myr re 5–6.5 in all the models studied here except Alt 3. In this model, the 33.01 −1 −1 L (t ) = 10 − 1.3log + 0.49 [erg s Å ]. (17) UV decrease in the star formation emissivity (driven by the decrease of 2Myr sf f ) with decreasing redshift is not compensated by an increasing esc The rest-frame UV luminosity has almost the same normalization AGN contribution as in the other models; as a result, reionization and time-evolution in all three models (SB99, BPB, SB99 + sb) does not ﬁnish even by z ∼ 4. Given that star formation in low-mass resulting in the same UV LFs. However, as seen from equations haloes is the key driver of reionization, it is not surprising to see that (5), (15). and (17), the slope of the time evolution of n ˙ is much reionization ﬁnishes ﬁrst (z ∼ 6.5) in the Alt4 model that has the int re sf shallower in the BPB and SB99 + sb models compared to the largest value of f . Models Alt2 and Alt5 show a similar z ∼ 5.8 re esc sf ﬁducial (SB99) model. We re-tune f for each of these models driven by an increasing contribution from star formation and AGN, esc to match to the reionization data (τ and the emissivity) using the respectively. Finally, given their lower values of the total ionizing es bh ﬁducial f values, the results of which are summarized in Table 3. emissivity at z 7, reionization ends at z ∼ 5 in the ﬁducial, Alt1 re esc sf As seen, while the slope of the redshift dependence of f remains and Alt6 models. esc unchanged (β = 2.8), the normalization (f ) is the lowest for the Finally, we show the AGN contribution to the cumulative ionizing BPB model as compared to SB99 by a factor 4.6; the SB99 and emissivity as a function of redshift in Fig. 7.Asseen, AGN SB99 + sb models on the other hand only differ by a factor 1.17. contribute at most 1 per cent of the total escaping ionizing photon sf Finally, the lower f values compensate for a higher intrinsic rate by z ∼ 4inthe Alt3 model. This increases to ∼ 10 per cent esc sf production rate to result in the same n ˙ value as a function of M . of the total ionizing emissivity for the ﬁducial and Alt4-Alt6 cases. esc bh These different stellar populations, therefore, have no bearing on Compared to the ﬁducial case, the higher f in the Alt1 case esc our result regarding the relative AGN/starlight contribution to the results in an AGN contribution as high as 25 per cent by z ∼ 4. bh ionizing radiation for different galaxy stellar masses. Finally, the Alt2 case (f = 1) provides the upper limit to the esc MNRAS 495, 3065–3078 (2020) Downloaded from https://academic.oup.com/mnras/article/495/3/3065/5843276 by DeepDyve user on 13 July 2022 3076 P. Dayal et al. Figure 6. Left-hand panel: As a function of redshift, we show the escaping H I ionizing photon emissivity. The different lines show the emissivity from star formation + AGN while the shaded regions (of the same lighter colour) show the contribution from AGN only. Right-hand panel: The reionization history, expressed through the redshift evolution of the volume ﬁlling fraction of H II. The horizontal dashed line shows Log(Q ) =−0.301, i.e. when reionization II is 50 per cent complete. The different colours in both panels show results for the ﬁducial and alternative escape fraction models (discussed in Section4.1)as marked in the right-hand panel. Figure 7. The cumulative fraction of ionizing photons contributed by AGN as a function of redshift; the horizontal short-dashed line shows the 50 per cent contribution to the cumulative ionizing emissivity for the various models discussed in this work (see Section 4.1 for details), as marked. AGN contribution. Here, AGN contribute as much as galaxies to in galaxies via suppression of black hole accretion in low-mass the cumulative emissivity by z ∼ 4.4. galaxies, caused by supernova feedback. Furthermore, in our model In addition to the ﬁducial model, only Alt1, Alt3,and Alt6 are each accreting black hole has a spectral energy distribution that able to simultaneously reproduce the emissivity and optical depth depends on the black hole mass and accretion rate. Given that the constraints. However, as seen above, the Alt3 model does not have escape fractions for both star formation and AGN remain poorly enough ionizing photons to ﬁnish the process of reionization. This understood, we have explored a wide range of combinations for leaves us with three physically plausible models – the ﬁducial one, these (ranging from redshift-dependent to constant to scaling both Alt1,and Alt6. In these, the AGN contribution to the total emissivity positively and negatively with stellar mass). Using these models, is sub-dominant at all z; AGN contribute about 0.5 − 1 per cent we ﬁnd the following key results: to the cumulative ionizing emissivity by z ∼ 6 that increases to 10 − 25 per cent by z = 4. (i) The intrinsic production rate of ionizing photons for both star formation and AGN scales positively with stellar mass with star formation dominating at all masses and redshifts. 6 CONCLUSIONS (ii) Irrespective of the escape fraction values used, the AGN In this paper, we have studied the contribution of AGN to hydrogen contribution to the escaping ionizing photons is always sub- reionization. Our model includes a delayed growth of black holes dominant at all galaxy masses at z> 7. In the case that the MNRAS 495, 3065–3078 (2020) Downloaded from https://academic.oup.com/mnras/article/495/3/3065/5843276 by DeepDyve user on 13 July 2022 Reionization with galaxies and AGN 3077 sf bh Dayal P., Ferrara A., 2018, Phys. Rep., 780, 1 stellar mass dependence of f is shallower than f ,at z< esc esc Dayal P., Ferrara A., Dunlop J. S., Pacucci F., 2014, MNRAS, 445, 2545 7 a ‘transition’ stellar mass exists above which AGN dominate the Dayal P., Choudhury T. R., Bromm V., Pacucci F., 2017a, ApJ, 836, 16 escaping ionizing photon production rate. This transition stellar Dayal P., Choudhury T. R., Pacucci F., Bromm V., 2017b, MNRAS, 472, mass decreases with redshift from being equal to the knee of the stellar mass function at z ∼ 6.8 to being an order of magnitude less Dayal P., Rossi E. M., Shiralilou B., Piana O., Choudhury T. R., Volonteri than the knee by z = 4. M., 2019, MNRAS, 486, 2336 (iii) Overall, the ionizing budget is dominated by stellar radiation Done C., Davis S. W., Jin C., Blaes O., Ward M., 2012, MNRAS, 420, 1848 from low-mass (M < 10 M ) galaxies down to z 6inall ∗ Dubois Y., Volonteri M., Silk J., Devriendt J., Slyz A., Teyssier R., 2015, models. 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However, Haardt F., Madau P., 2012, ApJ, 746, 125 AGN only contribute 0.5 − 1 per cent to the cumulative ionizing Hassan S., Dave R., Mitra S., Finlator K., Ciardi B., Santos M. G., 2018, emissivity by z ∼ 6 that increases to 10 − 25 per cent by z = 4. MNRAS, 473, 227 Kakiichi K., Graziani L., Ciardi B., Meiksin A., Compostella M., Eide M. B., Zaroubi S., 2017, MNRAS, 468, 3718 ACKNOWLEDGEMENTS Klypin A., Kravtsov A. V., Valenzuela O., Prada F., 1999, ApJ, 522, 82 PD acknowledges support from the European Research Council’s Kuhlen M., Faucher-Giguere C.-A., 2012, MNRAS, 423, 862 Kulkarni G., Worseck G., Hennawi J. F., 2019, MNRAS, 488, 1035 starting grant ERC StG-717001 (‘DELPHI’), from the NWO grant Liu C., Mutch S. J., Angel P. W., Duffy A. R., Geil P. M., Poole G. B., 016.VIDI.189.162 (‘ODIN’) and the European Commission’s and Mesinger A., Wyithe J. S. B., 2016, MNRAS, 462, 235 University of Groningen’s CO-FUND Rosalind Franklin program. Madau P., Fragos T., 2017, ApJ, 840, 39 MV and MT acknowledge funding from the European Research Madau P., Haardt F., 2015, ApJ, 813, L8 Council under the European Community’s Seventh Framework Madau P., Haardt F., Rees M. J., 1999, ApJ, 514, 648 Programme (FP7/2007-2013 Grant Agreement no. 614199, project McGreer I. D., Fan X., Jiang L., Cai Z., 2018, AJ, 155, 131 ‘BLACK’). TRC acknowledges support from the Associateship Merloni A. et al., 2014, MNRAS, 437, 3550 Scheme of ICTP. MV and RS acknowledge support from the Amaldi Micheva G., Iwata I., Inoue A. K., 2017, MNRAS, 465, 302 Research Center funded by the MIUR program ‘Dipartimento di Ec- Mitra S., Choudhury T. R., Ferrara A., 2015, MNRAS, 454, L76 cellenza’ (CUP:B81I18001170001). MT is supported by Deutsche Mitra S., Choudhury T. R., Ferrara A., 2018, MNRAS, 473, 1416 Forschungsgemeinschaft (DFG, German Research Foundation) un- Moore B., Quinn T., Governato F., Stadel J., Lake G., 1999, MNRAS, 310, der Germany’s Excellence Strategy EXC-2181/1 – 390900948 (the Naidu R. P., Tacchella S., Mason C. A., Bose S., Oesch P. A., Conroy C., Heidelberg STRUCTURES Cluster of Excellence). HA is supported 2020, ApJ, 892, 109 by the Centre National d’Etudes Spatiales (CNES). MH acknowl- Oke J. B., Gunn J. E., 1983, ApJ, 266, 713 edges ﬁnancial support from the Carlsberg Foundation via a Semper Onoue M. et al., 2017, ApJ, 847, L15 Ardens grant (CF15-0384). Finally, PD and MV thank La Sapienza Paardekooper J.-P., Khochfar S., Dalla Vecchia C., 2015, MNRAS, 451, for their hospitality, where the bulk of this work was carried out and thank D. Stark and P. Oesch for their insightful comments. Pawlik A. 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Solid: for a black hole at the Eddington luminosity; dashed: for a black hole at 10 per cent of the Eddington luminosity; dot-dashed: for a black hole at 1 per cent of the Eddington luminosity. This paper has been typeset from a T X/LT X ﬁle prepared by the author. E E MNRAS 495, 3065–3078 (2020)

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Reionization with galaxies and active galactic nuclei

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