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Recovering the systemic redshift of galaxies from their Lyman alpha line profile

Recovering the systemic redshift of galaxies from their Lyman alpha line profile MNRAS 478, L60–L65 (2018) doi:10.1093/mnrasl/sly058 Advance Access publication 2018 April 06 Recovering the systemic redshift of galaxies from their Lyman alpha line profile 1,2 1 3 3 3 4 A. Verhamme, T. Garel, E. Ventou, T. Contini, N. Bouche, ´ E.C. Herenz, 1 1 5 6 7 6,8 J. Richard, R. Bacon, K.B. Schmidt, M. Maseda, R.A. Marino, J. Brinchmann, 7 9,10 1 5,11 1 S. Cantalupo, J. Caruana, B. Clement, ´ C. Diener, A.B. Drake, 1,12,13 1 5 14 1 1 T. Hashimoto, H. Inami, J. Kerutt, W. Kollatschny, F. Leclercq, V. Patr´ ıcio, 6 5 3 J. Schaye, L. Wisotzki and J. Zabl Univ Lyon, Univ Lyon1, Ens de Lyon, CNRS, Centre de Recherche Astrophysique de Lyon UMR5574, F-69230 Saint-Genis-Laval, France Observatoire de Geneve ` , Universited ´ eGeneve ` , 51 Ch. des Maillettes, CH-1290 Versoix, Switzerland Institut de Recherche en Astrophysique et Planetolo ´ gie (IRAP), Universite ´ de Toulouse, CNRS, UPS, F-31400 Toulouse, France Department of Astronomy, Stockholm University, AlbaNova University Centre, SE-106 91 Stockholm, Sweden Leibniz-Institut fur ¨ Astrophysik Potsdam (AIP), An der Sternwarte 16, D-14482 Potsdam, Germany Leiden Observatory, Leiden University, NL-2300 RA Leiden, the Netherlands Department of Physics, ETH Zuric ¨ h, Wolfgang-Pauli-Strasse 27, CH-8093 Zuric ¨ h, Switzerland Instituto de Astrof´ ısica e Ciencias ˆ do Espac ¸, Universidade do Porto, CAUP, Rua das Estrelas, P-PT4150-762 Porto, Portugal Department of Physics, University of Malta, Msida MSD 2080, Malta Institute for Space Sciences and Astronomy, University of Malta, Msida MSD 2080, Malta Institute of Astronomy, Madingley Road, Cambridge CB3 0HA, UK National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan College of General Education, Osaka Sangyo University, 3-1-1 Nakagaito, Daito, Osaka 574-8530, Japan Institut fur ¨ Astrophysik, Universitat ¨ Gotting ¨ en, Friedrich-Hund Platz 1, D-37077 Gotting ¨ en, Germany Accepted 2018 March 29. Received 2017 October 24; in original form 2018 March 27 ABSTRACT The Lyman alpha (Ly α) line of Hydrogen is a prominent feature in the spectra of star-forming −1 galaxies, usually redshifted by a few hundreds of km s compared to the systemic redshift. This large offset hampers follow-up surveys, galaxy pair statistics, and correlations with quasar absorption lines when only Ly α is available. We propose diagnostics that can be used to recover the systemic redshift directly from the properties of the Ly α line profile. We use spectroscopic observations of Ly α emitters for which a precise measurement of the systemic redshift is available. Our sample contains 13 sources detected between z ≈ 3 and z ≈ 6 as part of various multi-unit spectroscopic explorer guaranteed time observations. We also include a compilation of spectroscopic Ly α data from the literature spanning a wide redshift range (z ≈ 0–8). First, restricting our analysis to double-peaked Ly α spectra, we find a tight correlation red between the velocity offset of the red peak with respect to the systemic redshift, V , and peak red the separation of the peaks. Secondly, we find a correlation between V and the full width peak at half-maximum of the Ly α line. Fitting formulas to estimate systemic redshifts of galaxies −1 with an accuracy of ≤100 km s , when only the Ly α emission line is available, are given for the two methods. Key words: galaxies: high-redshift – galaxies: starburst – galaxies: statistics – ultraviolet: galaxies. 1 INTRODUCTION In the last few decades, large samples of high-redshift galaxies (z > 2) have been assembled from deep photometric surveys based on broad/narrow-band selection techniques (Steidel et al. 2003; Ouchi et al. 2008; Bouwens et al. 2015; Finkelstein et al. 2015; E-mail: [email protected] 2018 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society Downloaded from https://academic.oup.com/mnrasl/article-abstract/478/1/L60/4963748 by Ed 'DeepDyve' Gillespie user on 21 June 2018 z from Ly α L61 sys Sobral et al. 2017, and references therein). In parallel, blind spec- ancillary optical spectra from the SDSS data base containing several troscopic searches commonly rely on the Lyman alpha (Ly α) line nebular emission lines from which the redshift is determined with redshifted to the optical or the near-infrared to identify or confirm great accuracy. We present these data in the following paragraphs. red sources at z ≥ 2 (e.g. Blanc et al. 2011; Bielby et al. 2011;Le For each Ly α spectrum, we measure V as the location of the peak Fevre ` et al. 2015). The number of spectroscopic detections of Ly α maximum of the Ly α flux redwards of the systemic redshift, and emitters (LAE) is now being increased dramatically with ongoing full width at half-maximum (FWHM) as the width of the part of observational campaigns with the multi-unit spectroscopic explorer the spectrum uncorrected for instrumental broadening with flux (MUSE, Bacon et al. 2010)on European Southern Obseratory’s above half of the maximum, both directly on the data, without any Very Large Telescope (ESO VLT), allowing us to study galaxy for- modelling. mation and evolution with a homogeneous sample of sources over a large redshift range (2.8  z  6.7, e.g. Bacon et al. 2015, 2017; 2.1 LAEs from MUSE GTO data Drake et al. 2016; Herenz et al. 2017;Mahleretal. 2017; Caruana et al. 2018). Stark et al. (2014) reported the detection of C III]λλ1907, 1909 Several studies have demonstrated that the Ly α emission line emission from low mass star-forming galaxies. When observed, is not exactly tracing systemic redshift (e.g. Shapley et al. 2003; this doublet is the strongest UV emission line after Ly α, and, in McLinden et al. 2011; Rakic et al. 2011; Song et al. 2014; Hashimoto contrast to Ly α, it is an optically thin nebular line, tracing the et al. 2015). Instead, the line profiles often show a complex struc- systemic redshift of the Ly α production site. The redshift window ture that arguably originates from the propagation of resonant Ly α where Ly α and C III] are both observable within the VLT/MUSE photons in neutral gas within the interstellar medium and/or in the spectral range is 2.9 < z < 3.8. MUSE is an optical Integral Field vicinity of galaxies. Among the broad diversity of Ly α profiles Unit (IFU) spectrograph with medium spectral resolution (from R in Ly α emitting galaxies, we identify the most common two cat- ∼ 2000 in the blue to R ∼ 4000 in the red). egories: (i) spectra with a redshifted single peak [∼2/3 of Ly α Within several projects in the MUSE consortium using GTO emitting Lyman Break Galaxies, called LBGs, from Kulas et al. (Bacon et al. 2017; Brinchmann et al. 2017; Herenz et al. 2017; (2012)], and (ii) double-peaked profiles, with a prominent red peak Inami et al. 2017; Mahler et al. 2017;Masedaetal. 2017; Caruana and a smaller blue bump [∼2/3 of the remaining 1/3 of Ly α emitting et al. 2018) and Science Verification (SV) or commissioning data LBGs that are multiple peaked, from Kulas et al. (2012); 40 per cent (Patr´ ıcio et al. 2016), we find 13 LAEs with reliable C III] detections, of the LAEs observed by Yamada et al. (2012)]. We will refer to the that is, non-contaminated by sky lines and with a S/N > 3. We list latter as blue bump LAEs in the remainder of this paper. Understand- these objects in Table 1 (see Maseda et al. 2017, for a systematic ing the nature of blue bump LAEs and studying their occurrence study of CIII emitters in the MUSE GTO data from the Hubble and evolution with redshift will be the goal of a forthcoming study. Ultra Deep Field). For each of these LAEs we measure the shift of red The vast majority of objects display a red peak shifted by a variable the Ly α emission compared to C III], V , the observed FWHM, peak −1 amount peaking around ∼400 km s for LBGs (e.g. Shapley et al. and the separation of the Ly α peaks for the eight blue bump LAEs −1 2003; Kulas et al. 2012), ∼200 km s for LAEs (e.g. Hashimoto among them. et al. 2013;Erb et al. 2014; Song et al. 2014; Hashimoto et al. 2015; −1 Henry et al. 2015; Trainor et al. 2015), and less than ∼150 km s 2.2 High-z data from the literature for a small sample of five local Lyman Continuum Emitters (Ver- hamme et al. 2017). Stark et al. (2017) reported the detection of C III] from one of the red If not accounted for, this offset with respect to the systemic red- highest redshift Ly α emitters ever observed (z ∼ 7.730) with V peak shift can be problematic when addressing astrophysical issues which −1 −1 ∼ 340 km s ;the Ly α FWHM = 360 km s is measured by −70 require accurate systemic redshift measurements (e.g. galaxy inter- Oesch et al. (2015). Vanzella et al. (2016) report a narrow Ly α actions, gas kinematics, baryonic acoustic oscillations, intergalactic line observed at medium spectral resolution using VLT-Xshooter medium-galaxy emission/absorption correlations). The scope of this red of a magnified star-forming galaxy at z = 3.1169, with V (Ly α) peak paper is to investigate whether the Ly α profile shape can be used −1 −1 ∼ 100 km s and FWHM(Ly α) ∼104 km s . From Hashimoto to determine the systemic redshift of galaxies. The outline of this et al. (2015, 2017), we select the six LAEs observed with MagE (R Letter is as follows: in Section2, we gather recent spectroscopic ∼ 4100). Their systemic redshifts have been obtained with either data from MUSE Guaranteed Time Observations (GTO) surveys H α or [O III] lines. Three of these objects are blue-bump LAEs, and from the literature that have sufficient spectral resolution to for which we also measure the separation of the peaks. Kulas et al. investigate the Ly α line properties, as well as reliable systemic red- (2012) reported that a significant fraction (∼30 per cent) of their shift measurements. In Section 3, we present two diagnostics that Ly α emitting LBGs show a complex Ly α profile, with at least one can be used to recover the systemic redshift from Ly α, which we secondary peak. Blue bump objects (their Group I) represent the compare to models in Section 4. Section 5 summarizes our findings. majority of their profiles (11 out of 18 objects). We add these 11 objects to our sample of blue bumps LAEs. 2 A SAMPLE OF LAES WITH KNOWN SYSTEMIC REDSHIFT 2.3 Low-z data from the literature In order to investigate the link between the shape of the Ly α line Green Pea galaxies (hereafter GPs) are LAEs in the local Universe and the systemic redshift, we collect a diverse sample of LAEs (z ∼ 0.1 to 0.3; Jaskot & Oey 2014; Henry et al. 2015; Verhamme with a precise measure of the systemic redshift. Our sample con- sists of high-redshift (z > 2) LAEs with detected C III]λλ1907, 1909, [O III]λλ4959, 5007 or H αλ6563 emission, and low red- CIII] can be used as a redshift indicator when the two components of the shift (z < 0.4) LAEs with Ly α observations in the UV rest-frame doublet are well resolved, i.e. when R ∼ 2000, because their relative strength obtained with the Cosmic Origins Spectrograph onboard HST,and depends on the density. MNRASL 478, L60–L65 (2018) Downloaded from https://academic.oup.com/mnrasl/article-abstract/478/1/L60/4963748 by Ed 'DeepDyve' Gillespie user on 21 June 2018 L62 A. Verhamme et al. −1 Table 1. MUSE Ly α+C III] emitters. The sixth column indicates the separation of the peaks (i.e. 2 × V ,inkm s ) for blue bump LAEs, and is left empty 1/2 for single-peaked profiles. FWHM red −1 −1 −1 ID RA DEC EW (Å) V (km s ) (km s ) V (km s ) z Observations sys, CIII] peak sys 1 307.97040 −40.625694 32 176 ± 11 248 ± 9 – 3.5062 Commissioning mul 11 342.175042 −44.541031 222 215 ± 35 150 ± 35 375 ± 35 3.1163 AS1063 mul 14 342.178833 −44.535869 29 385 ± 35 300 ± 35 – 3.1150 AS1063 sys 44 64.0415559 −24.0599916 57 303 ± 35 360 ± 35 570 ± 35 3.2886 MACS0416 sys 132 64.0400838 −24.0667408 62 331 ± 35 288 ± 35 510 ± 35 3.2882 MACS0416 106 53.163726 −27.7790755 72 379 ± 13 414 ± 13 828 ± 35 3.2767 udf-10 118 53.157088 −27.7802688 65 301 ± 28 284 ± 28 568 ± 35 3.0173 udf-10 1180 53.195735 −27.7827171 80 220 ± 23 348 ± 32 – 3.3228 udf mosaic 6298 53.169249 −27.7812550 83 582 ± 38 512 ± 56 – 3.1287 udf-10 6666 53.159576 −27.7767193 52 284 ± 13 377 ± 11 754 ± 35 3.4349 udf-10 50 150.149656 2.061272 50 431 ± 42 268 ± 39 – 3.8237 GR30 48 149.852989 2.488099 68 294 ± 35 214 ± 35 705 ± 35. 3.3280 GR34 102 150.050268 2.600025 76 299 ± 15 229 ± 15 385 ± 35. 3.0400 GR84 a b c d Patricio et al. 2016; Richard et al., in preparation; Bacon et al. 2017, Inami et al. 2017, Maseda et al. 2017; Contini et al., in preparation. Figure 1. Three examples of rest-frame spectra of LAEs with detected C III]λλ1907, 1909 doublet probing the systemic redshift in the MUSE-GTO observations. In each panel, the velocity shifts of the Ly α line are shown relative to C III]1907Å. For all blue bump LAEs in our sample, the systemic redshift falls in between the blue and red peaks of the Ly α emission. et al. 2017;Yangetal. 2017). The systemic redshift of these ob- peaks, V , for blue bump LAEs with known systemic redshift. 1/2 jects was compiled from the several nebular lines contained in their We fit the data using the LTS LINEFIT program described in SDSS optical spectrum (e.g Izotov, Guseva & Thuan 2011). Note Cappellari et al. (2013), which combines the Least Trimmed Squares that the C III] emission line is out of the UV spectral range probed by robust technique of Rousseeuw & van Driessen (2006) into a least- the available HST-COS observations. For a sample of 17 GPs from squares fitting algorithm which allows for errors in both variables Jaskot & Oey (2014), Henry et al. (2015); Verhamme et al. (2017), and intrinsic scatter. The best fit, shown by the red line in Fig. 2, red we measure V and FWHM on the data. For 21 new GP observa- is given by peak red tions, we use the V and FWHM values computed by Yang et al. red −1 peak V = 1.05(±0.11) × V − 12(±37) km s (1) 1/2 peak (2017) given in their table 2. GPs nearly always exhibit blue bump Ly α profiles (Jaskot & Oey 2014; Henry et al. 2015; Verhamme This relation is so close to the one-to-one relation that we assume red et al. 2017). For the blue bump GPs, we also measure the separation from now that the underlying ‘true’ relation between V and peak of the peaks. V is one-to-one, as expected from radiation transfer modelling 1/2 (see Section 4 below). The intrinsic scatter estimated from the linear −1 regression is 53 (± 9) km s . 3 DERIVING SYSTEMIC REDSHIFT FROM LYMAN-ALPHA 3.2 Method 2: an empirical correlation between FWHM and 3.1 Method 1: systemic redshift of blue bump LAEs systemic redshift In this section, only blue bump spectra, i.e. double peaks with a red In this section, both single- and double-peaked profiles are con- peak higher than the blue peak, are considered. We note that, for all sidered. The measurements are always done on the red peak, and red blue bump LAEs studied here, the systemic redshift always falls in- the red peak only. In the right-hand panel of Fig. 2 we plot V peak between the Ly α peaks, as illustrated in Fig. 1 for three blue-bump versus FWHM for the full sample of LAEs presented in Section 2 MUSE Ly α+C III] emitters (see also Kulas et al. 2012;Erb et al. 2014;Yangetal. 2016). Fig. 2, left-hand panel, shows a positive red 2 empirical correlation between V and half of the separation of the http://www-astro.physics.ox.ac.u/∼mxc/software/#lts peak MNRASL 478, L60–L65 (2018) Downloaded from https://academic.oup.com/mnrasl/article-abstract/478/1/L60/4963748 by Ed 'DeepDyve' Gillespie user on 21 June 2018 z from Ly α L63 sys Figure 2. Empirical relations to determine systemic redshift from the shape of the Ly α emission. Left: correlation between the shift of the Ly α red peak, red (V ) and half of the separation of the peaks (V ) for a sample of LAEs with a known systemic redshift: 7 Ly α+C III] emitters with blue bump Ly α spectra 1/2 peak from the MUSE GTO data (red stars), blue bump LAEs among the Yang et al. (2017) GP sample (black dots), blue bump LAEs among the Hashimoto et al. red (2015) MagE sample and Group I LBGs from Kulas et al. (2012) (black triangles). Right: correlation between V and FWHM among Ly α+C III], H α or peak [O III] emitters. The black dashed line is the one-to-one relation. We checked that the correlation remains even discarding the two most upper left points. On both sides, the red curve is our best fit to the data, described by equations (1) and (2). The Pearson coefficient and the probability of the null hypothesisare shown on each panel. 3.3 Comparison of the methods We check that the corrected redshifts from both methods give results that are closer to the systemic redshift of the objects than the ‘Ly α red redshifts’, i.e. taking V as the systemic redshift, as usually done peak (Fig. 3). The standard deviation of the red histograms (corrected redshifts), reflecting both the intrinsic scatter and measurement er- rors, are comparable for the two methods, though slightly better for the blue bump method. We therefore propose to use half of the separation of the peaks as a proxy for the red peak shift of blue bump LAEs, and the Ly α FWHM for single-peaked spectra. They allow to recover the systemic redshift from the Ly α line, with an −1 uncertainty lower than ±100 km s from z ≈ 0 to 7. This suggests that the same scattering processes, linking the line shift and the line width, are at play at every redshift, and that the effect of the IGM does not erase this correlation. 4 DISCUSSION Figure 3. Comparison of the distributions of Ly α redshift errors (=z − z , in black) with redshift distributions corrected with Method Ly α sys 4.1 Effect of the spectral resolution 1 (in red, top panel) and with Method 2 (in red, bottom panel). These two methods to retrieve the systemic redshift of a LAE from the shape of its Ly α profile rely on measurements of either the positions of the blue and red Ly α emission peaks or the (red (new MUSE LAEs measurements are reported in Table 1). There peak) FWHM. Both of these measures are affected by the spectral red is a correlation between V and FWHM, although less significant peak resolution. Although the data points presented in Section 3 were than for Method 1 (see the Pearson coefficients on each panel of collected from the literature and MUSE surveys and span a range of Fig. 2). We use the same method (Cappellari et al. 2013)tode- spectral resolutions from R ∼ 1000 (LRIS) to R ∼ 5000 (X-Shooter, termine the empirical relation, which can be used to retrieve the HST-COS), they all seem to follow the same relation. systemic redshift of a galaxy: We investigated the effect of spectral resolution on synthetic red −1 spectra constructed from Ly α radiation transfer simulations. Poorer V = 0.9(±0.14) × FWHM(Lyα) − 34(±60) km s (2) peak spectral resolution broadens the peaks, and since Ly α profiles are This relation is also compatible with the one-to-one relation, given the uncertainties in the fit parameters. The intrinsic scatter estimated −1 3 red from the linear regression is 72(±12) km s , slightly larger than We have also tested the relation between Ly α EWs and V , but did not peak with Method 1. find any significant correlation. MNRASL 478, L60–L65 (2018) Downloaded from https://academic.oup.com/mnrasl/article-abstract/478/1/L60/4963748 by Ed 'DeepDyve' Gillespie user on 21 June 2018 L64 A. Verhamme et al. Figure 4. Points show the relationship between half of the separation of the peaks and the shift of the Ly α line, and between the FWHM and the shift of the Ly α line, for synthetic spectra from expanding shells, spheres or bi-conical outflows (Schaerer et al. 2011; Zheng & Wallace 2014). The trend is driven by the −2 column density of the scattering medium, but holds for the different idealized geometries. The symbol colours scale with the column density (in cm )ofthe −1 shells and symbol sizes scale with the radial expansion velocity (from 0 to 400 km s ). The red line and dashed black line are identical as in Fig. 2. often asymmetric, it also has the effect of shifting the peak towards larger range of FWHMs in the right-hand panel of Fig. 4 (from 0 −1 longer wavelengths. The latter effect is weaker than the broadening. to 1200 km s ) compared to Fig. 2 where observed FWHMs vary −1 −1 As a consequence, the effect of spectral resolution may flatten the from 214 to 512 km s . For FWHM values less than 600 km s , red slope but seems not to break the correlation. the model predictions lie close to the FWHM–V relation derived peak in Section 3. Although the exact location of each simulated object red in the FWHM–V plane seems to depend on each parameter, we peak 4.2 Comparison with models see that models with higher HI column densities lead to broader lines and larger shifts of the peak (colour-coded circles). A similar We now compare our results with numerical simulations of Ly α trend is found by Zheng & Wallace (2014) who performed Ly α radiative transfer in expanding shells performed with the MCLya radiation transfer simulations in anisotropic configurations (bipo- code (Schaerer et al. 2011; Verhamme, Schaerer & Maselli 2006). lar outflows) and inhomogeneous media (i.e. HI distributions with These models describe in a simple, idealized, way the propagation velocity or density gradients). Overall, this may suggest that the of Ly α photons emitted in HII regions through gas outflows which red FWHM–V correlation holds regardless of the assumed geome- peak seem ubiquitous in star-forming galaxies, especially at high redshift try and kinematics of the outflows, and that the HI opacity of the (Shapley et al. 2003; Steidel et al. 2010; Hashimoto et al. 2015). ISM and/or the medium surrounding galaxies (i.e. the CGM) is the Assuming a central point-source surrounded by an expanding shell main driver that shapes the observed Ly α line profiles. of gas with varying HI column density (N ), speed (V ), dust HI exp opacity (τ ) and temperature (described by the Doppler parameter b ∝ T ), shell models have proven very successful in reproducing a large diversity of Ly α line profiles. Here, we use simulations 5 CONCLUSIONS with different intrinsic Gaussian line widths (σ ) and various shell The recent increase in the number of LAEs with detected nebular parameter values (N , V , τ , b), degraded to mimic the MUSE HI exp d red lines allows to calibrate empirical methods to retrieve the systemic spectral resolution. We measure FWHM, V ,and V the same 1/2 peak redshift from the shape of the Ly α line. In addition to measure- way as for the data. red ments from the literature, we report 13 new detections from several We compare the observed correlation between V and the sep- peak MUSE GTO programs. We searched for Ly α+C III] emitters in the aration between the peaks of blue-bump LAEs (V ) with results 1/2 MUSE-Deep survey (Bacon et al. 2017), behind z ∼ 0.7 galaxy from models that produce double-peak profiles (Fig. 4, left-hand groups (Contini et al., in preparation), and lensed by three clusters panel). Predictions from expanding shell models lie very close to the (SMACSJ2031.8-4036 in Patr´ ıcio et al. 2016, AS1063, MACS0416 one-to-one relation and reproduce nicely the observed properties of red in Richard et al., in preparation). the Ly α lines. Objects with increasing V and V correspond 1/2 peak We find a robust correlation between the shift of the Ly α peak to expanding shells with larger HI column densities. This echoes red with respect to systemic redshift (V ) and half of the separation the analytical solutions for Ly α RT in static homogeneous media peak of the peaks (V ) for LAEs with blue bump spectra. The intrinsic (Neufeld 1990; Dijkstra, Haiman & Spaans 2006) that yield profiles 1/2 −1 scatter around the relation is ±53 km s . We also find a correlation with symmetric peaks around the line centre, whose positions are 1/3 red between the shift of the Ly α peak with respect to systemic redshift primarily set by the HI opacity and correspond to V ∝ τ . peak HI red (V ) and its FWHM, for LAEs with known systemic redshift. As shown in Fig. 4 (right-hand panel), the correlation between peak −1 the shift of the red peak and the FWHM of the Ly α line naturally The intrinsic scatter is of the same order (±73 km s ). These two arises from scattering processes. The slope predicted by the models relations have been approximated by linear fitting formulas as given is close to one whereas the relation derived from observations in in equations (1) and (2). These formulae have been derived for data Section 3 is shallower (≈0.9; red curve in the right-hand panel of with spectral resolution 1000 < R < 5000, they should be used on Figs 2, 4). However, it is worth pointing out that we explore a much data with similar spectral resolution. MNRASL 478, L60–L65 (2018) Downloaded from https://academic.oup.com/mnrasl/article-abstract/478/1/L60/4963748 by Ed 'DeepDyve' Gillespie user on 21 June 2018 z from Ly α L65 sys The relative redshift error if estimated from Ly α with Drake A. B. et al., 2017, A&A, 608, 6, red −1 red −3 Erb D. K. et al., 2014, ApJ, 795, 33 V = 300 km s is (z /z)(Ly α) = ((1+z) × (V / c))/z ∼10 peak peak Finkelstein S. L. et al., 2015, ApJ, 810, 71 at z = 3. The two methods presented in this letter can therefore Hashimoto T., Ouchi M., Shimasaku K., Ono Y., Nakajima K., Rauch M., help reduce systematic errors on distance measures. This is of great Lee J., Okamura S., 2013, ApJ, 765, 70 importance for redshift surveys at z  3, where spectroscopic red- Hashimoto T. et al., 2015, ApJ, 812, 157 shifts often rely on the Ly α emission line. Futures observations Hashimoto T. et al., 2017, MNRAS, 465, 1543 with better spectral resolution should allow to refine the proposed Henry A., Scarlata C., Martin C. L., Erb D., 2015, ApJ, 809, 19 relations. Herenz E. C. et al., 2017, A&A, 606, 12, Inami H. et al., 2017, A&A, 608, 2, Izotov Y. I., Guseva N. G., Thuan T. X., 2011, ApJ, 728, 161 ACKNOWLEDGEMENTS Jaskot A. E., Oey M. S., 2014, ApJ, 791, L19 We thank the anonymous referee for her/his helpful report. AV is Kulas K. R., Shapley A. E., Kollmeier J. A., Zheng Z., Steidel C. C., Hainline K. N., 2012, ApJ, 745, 33 supported by a Marie Heim Vogtlin ¨ fellowship of the Swiss National Le Fevre ` O. et al., 2015, A&A, 576, A79 Foundation. TG is grateful to the LABEX Lyon Institute of Origins Mahler G. et al., 2017, MNRAS, 473, 663, (ANR-10-LABX-0066) of the Universite ´ de Lyon for its financial Maseda M. V. et al., 2017, A&A, 608, 4, support within the program ’Investissements d’Avenir’ (ANR-11- McLinden E. M. et al., 2011, ApJ, 730, 136 IDEX-0007) of the French government operated by the National Neufeld D. A., 1990, ApJ, 350, 216 Research Agency (ANR). TC, EV, JZ acknowledge support of Oesch P. A. et al., 2015, ApJ, 804, L30 the ANR FOGHAR (ANR-13-BS05-0010-02), the OCEVU Labex Ouchi M. et al., 2008, ApJS, 176, 301 (ANR-11- LABX-0060) and the A∗MIDEX project (ANR-11- Patrıcio V. et al., 2016, MNRAS, 456, 4191 IDEX-0001-02) funded by the ’Investissements d’Avenir’ French Rakic O., Schaye J., Steidel C. C., Rudie G. C., 2011, MNRAS, 414, 3265 government program managed by the ANR. RB and FL acknowl- Schaerer D., Hayes M., Verhamme A., Teyssier R., 2011, A&A, 531, A12 Shapley A. E., Steidel C. C., Pettini M., Adelberger K. L., 2003, ApJ, 588, edges support from the ERC advanced grant 339659-MUSICOS. JR and VP acknowledge support from the ERC starting grant Sobral D. et al., 2017, MNRAS, 466, 1242 336736-CALENDS. RAM acknowledges support by the Swiss Song M. et al., 2014, ApJ, 791, 3 National Science Foundation. JS acknowledges support from the Stark D. P. et al., 2014, MNRAS, 445, 3200 ERC grant 278594-GasAroundGalaxies. JB acknowledges support Stark D. P. et al., 2017, MNRAS, 464, 469 by Fundac ¸a ˜o paraaCiencia ˆ e a Tecnologia (FCT) through na- Steidel C. C., Adelberger K. L., Shapley A. E., Pettini M., Dickinson M., tional funds (UID/FIS/04434/2013) and by FEDER through COM- Giavalisco M., 2003, ApJ, 592, 728 PETE2020 (POCI-01-0145-FEDER-007672) and Investigador FCT Steidel C. C., Erb D. 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A. et al., 2011, ApJ, 736, 31 Zheng Z., Wallace J., 2014, ApJ, 794, 116 Bouwens R. J. et al., 2015, ApJ, 803, 34 Brinchmann J. et al., 2017, A&A, 608, 3, Cappellari M. et al., 2013, MNRAS, 432, 1709 Caruana J. et al., 2018, MNRAS, 473, 30 This paper has been typeset from a T X/LT X file prepared by the author. Dijkstra M., Haiman Z., Spaans M., 2006, ApJ, 649, 14 E E MNRASL 478, L60–L65 (2018) Downloaded from https://academic.oup.com/mnrasl/article-abstract/478/1/L60/4963748 by Ed 'DeepDyve' Gillespie user on 21 June 2018 http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Monthly Notices of the Royal Astronomical Society Letters Oxford University Press

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MNRAS 478, L60–L65 (2018) doi:10.1093/mnrasl/sly058 Advance Access publication 2018 April 06 Recovering the systemic redshift of galaxies from their Lyman alpha line profile 1,2 1 3 3 3 4 A. Verhamme, T. Garel, E. Ventou, T. Contini, N. Bouche, ´ E.C. Herenz, 1 1 5 6 7 6,8 J. Richard, R. Bacon, K.B. Schmidt, M. Maseda, R.A. Marino, J. Brinchmann, 7 9,10 1 5,11 1 S. Cantalupo, J. Caruana, B. Clement, ´ C. Diener, A.B. Drake, 1,12,13 1 5 14 1 1 T. Hashimoto, H. Inami, J. Kerutt, W. Kollatschny, F. Leclercq, V. Patr´ ıcio, 6 5 3 J. Schaye, L. Wisotzki and J. Zabl Univ Lyon, Univ Lyon1, Ens de Lyon, CNRS, Centre de Recherche Astrophysique de Lyon UMR5574, F-69230 Saint-Genis-Laval, France Observatoire de Geneve ` , Universited ´ eGeneve ` , 51 Ch. des Maillettes, CH-1290 Versoix, Switzerland Institut de Recherche en Astrophysique et Planetolo ´ gie (IRAP), Universite ´ de Toulouse, CNRS, UPS, F-31400 Toulouse, France Department of Astronomy, Stockholm University, AlbaNova University Centre, SE-106 91 Stockholm, Sweden Leibniz-Institut fur ¨ Astrophysik Potsdam (AIP), An der Sternwarte 16, D-14482 Potsdam, Germany Leiden Observatory, Leiden University, NL-2300 RA Leiden, the Netherlands Department of Physics, ETH Zuric ¨ h, Wolfgang-Pauli-Strasse 27, CH-8093 Zuric ¨ h, Switzerland Instituto de Astrof´ ısica e Ciencias ˆ do Espac ¸, Universidade do Porto, CAUP, Rua das Estrelas, P-PT4150-762 Porto, Portugal Department of Physics, University of Malta, Msida MSD 2080, Malta Institute for Space Sciences and Astronomy, University of Malta, Msida MSD 2080, Malta Institute of Astronomy, Madingley Road, Cambridge CB3 0HA, UK National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan College of General Education, Osaka Sangyo University, 3-1-1 Nakagaito, Daito, Osaka 574-8530, Japan Institut fur ¨ Astrophysik, Universitat ¨ Gotting ¨ en, Friedrich-Hund Platz 1, D-37077 Gotting ¨ en, Germany Accepted 2018 March 29. Received 2017 October 24; in original form 2018 March 27 ABSTRACT The Lyman alpha (Ly α) line of Hydrogen is a prominent feature in the spectra of star-forming −1 galaxies, usually redshifted by a few hundreds of km s compared to the systemic redshift. This large offset hampers follow-up surveys, galaxy pair statistics, and correlations with quasar absorption lines when only Ly α is available. We propose diagnostics that can be used to recover the systemic redshift directly from the properties of the Ly α line profile. We use spectroscopic observations of Ly α emitters for which a precise measurement of the systemic redshift is available. Our sample contains 13 sources detected between z ≈ 3 and z ≈ 6 as part of various multi-unit spectroscopic explorer guaranteed time observations. We also include a compilation of spectroscopic Ly α data from the literature spanning a wide redshift range (z ≈ 0–8). First, restricting our analysis to double-peaked Ly α spectra, we find a tight correlation red between the velocity offset of the red peak with respect to the systemic redshift, V , and peak red the separation of the peaks. Secondly, we find a correlation between V and the full width peak at half-maximum of the Ly α line. Fitting formulas to estimate systemic redshifts of galaxies −1 with an accuracy of ≤100 km s , when only the Ly α emission line is available, are given for the two methods. Key words: galaxies: high-redshift – galaxies: starburst – galaxies: statistics – ultraviolet: galaxies. 1 INTRODUCTION In the last few decades, large samples of high-redshift galaxies (z > 2) have been assembled from deep photometric surveys based on broad/narrow-band selection techniques (Steidel et al. 2003; Ouchi et al. 2008; Bouwens et al. 2015; Finkelstein et al. 2015; E-mail: [email protected] 2018 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society Downloaded from https://academic.oup.com/mnrasl/article-abstract/478/1/L60/4963748 by Ed 'DeepDyve' Gillespie user on 21 June 2018 z from Ly α L61 sys Sobral et al. 2017, and references therein). In parallel, blind spec- ancillary optical spectra from the SDSS data base containing several troscopic searches commonly rely on the Lyman alpha (Ly α) line nebular emission lines from which the redshift is determined with redshifted to the optical or the near-infrared to identify or confirm great accuracy. We present these data in the following paragraphs. red sources at z ≥ 2 (e.g. Blanc et al. 2011; Bielby et al. 2011;Le For each Ly α spectrum, we measure V as the location of the peak Fevre ` et al. 2015). The number of spectroscopic detections of Ly α maximum of the Ly α flux redwards of the systemic redshift, and emitters (LAE) is now being increased dramatically with ongoing full width at half-maximum (FWHM) as the width of the part of observational campaigns with the multi-unit spectroscopic explorer the spectrum uncorrected for instrumental broadening with flux (MUSE, Bacon et al. 2010)on European Southern Obseratory’s above half of the maximum, both directly on the data, without any Very Large Telescope (ESO VLT), allowing us to study galaxy for- modelling. mation and evolution with a homogeneous sample of sources over a large redshift range (2.8  z  6.7, e.g. Bacon et al. 2015, 2017; 2.1 LAEs from MUSE GTO data Drake et al. 2016; Herenz et al. 2017;Mahleretal. 2017; Caruana et al. 2018). Stark et al. (2014) reported the detection of C III]λλ1907, 1909 Several studies have demonstrated that the Ly α emission line emission from low mass star-forming galaxies. When observed, is not exactly tracing systemic redshift (e.g. Shapley et al. 2003; this doublet is the strongest UV emission line after Ly α, and, in McLinden et al. 2011; Rakic et al. 2011; Song et al. 2014; Hashimoto contrast to Ly α, it is an optically thin nebular line, tracing the et al. 2015). Instead, the line profiles often show a complex struc- systemic redshift of the Ly α production site. The redshift window ture that arguably originates from the propagation of resonant Ly α where Ly α and C III] are both observable within the VLT/MUSE photons in neutral gas within the interstellar medium and/or in the spectral range is 2.9 < z < 3.8. MUSE is an optical Integral Field vicinity of galaxies. Among the broad diversity of Ly α profiles Unit (IFU) spectrograph with medium spectral resolution (from R in Ly α emitting galaxies, we identify the most common two cat- ∼ 2000 in the blue to R ∼ 4000 in the red). egories: (i) spectra with a redshifted single peak [∼2/3 of Ly α Within several projects in the MUSE consortium using GTO emitting Lyman Break Galaxies, called LBGs, from Kulas et al. (Bacon et al. 2017; Brinchmann et al. 2017; Herenz et al. 2017; (2012)], and (ii) double-peaked profiles, with a prominent red peak Inami et al. 2017; Mahler et al. 2017;Masedaetal. 2017; Caruana and a smaller blue bump [∼2/3 of the remaining 1/3 of Ly α emitting et al. 2018) and Science Verification (SV) or commissioning data LBGs that are multiple peaked, from Kulas et al. (2012); 40 per cent (Patr´ ıcio et al. 2016), we find 13 LAEs with reliable C III] detections, of the LAEs observed by Yamada et al. (2012)]. We will refer to the that is, non-contaminated by sky lines and with a S/N > 3. We list latter as blue bump LAEs in the remainder of this paper. Understand- these objects in Table 1 (see Maseda et al. 2017, for a systematic ing the nature of blue bump LAEs and studying their occurrence study of CIII emitters in the MUSE GTO data from the Hubble and evolution with redshift will be the goal of a forthcoming study. Ultra Deep Field). For each of these LAEs we measure the shift of red The vast majority of objects display a red peak shifted by a variable the Ly α emission compared to C III], V , the observed FWHM, peak −1 amount peaking around ∼400 km s for LBGs (e.g. Shapley et al. and the separation of the Ly α peaks for the eight blue bump LAEs −1 2003; Kulas et al. 2012), ∼200 km s for LAEs (e.g. Hashimoto among them. et al. 2013;Erb et al. 2014; Song et al. 2014; Hashimoto et al. 2015; −1 Henry et al. 2015; Trainor et al. 2015), and less than ∼150 km s 2.2 High-z data from the literature for a small sample of five local Lyman Continuum Emitters (Ver- hamme et al. 2017). Stark et al. (2017) reported the detection of C III] from one of the red If not accounted for, this offset with respect to the systemic red- highest redshift Ly α emitters ever observed (z ∼ 7.730) with V peak shift can be problematic when addressing astrophysical issues which −1 −1 ∼ 340 km s ;the Ly α FWHM = 360 km s is measured by −70 require accurate systemic redshift measurements (e.g. galaxy inter- Oesch et al. (2015). Vanzella et al. (2016) report a narrow Ly α actions, gas kinematics, baryonic acoustic oscillations, intergalactic line observed at medium spectral resolution using VLT-Xshooter medium-galaxy emission/absorption correlations). The scope of this red of a magnified star-forming galaxy at z = 3.1169, with V (Ly α) peak paper is to investigate whether the Ly α profile shape can be used −1 −1 ∼ 100 km s and FWHM(Ly α) ∼104 km s . From Hashimoto to determine the systemic redshift of galaxies. The outline of this et al. (2015, 2017), we select the six LAEs observed with MagE (R Letter is as follows: in Section2, we gather recent spectroscopic ∼ 4100). Their systemic redshifts have been obtained with either data from MUSE Guaranteed Time Observations (GTO) surveys H α or [O III] lines. Three of these objects are blue-bump LAEs, and from the literature that have sufficient spectral resolution to for which we also measure the separation of the peaks. Kulas et al. investigate the Ly α line properties, as well as reliable systemic red- (2012) reported that a significant fraction (∼30 per cent) of their shift measurements. In Section 3, we present two diagnostics that Ly α emitting LBGs show a complex Ly α profile, with at least one can be used to recover the systemic redshift from Ly α, which we secondary peak. Blue bump objects (their Group I) represent the compare to models in Section 4. Section 5 summarizes our findings. majority of their profiles (11 out of 18 objects). We add these 11 objects to our sample of blue bumps LAEs. 2 A SAMPLE OF LAES WITH KNOWN SYSTEMIC REDSHIFT 2.3 Low-z data from the literature In order to investigate the link between the shape of the Ly α line Green Pea galaxies (hereafter GPs) are LAEs in the local Universe and the systemic redshift, we collect a diverse sample of LAEs (z ∼ 0.1 to 0.3; Jaskot & Oey 2014; Henry et al. 2015; Verhamme with a precise measure of the systemic redshift. Our sample con- sists of high-redshift (z > 2) LAEs with detected C III]λλ1907, 1909, [O III]λλ4959, 5007 or H αλ6563 emission, and low red- CIII] can be used as a redshift indicator when the two components of the shift (z < 0.4) LAEs with Ly α observations in the UV rest-frame doublet are well resolved, i.e. when R ∼ 2000, because their relative strength obtained with the Cosmic Origins Spectrograph onboard HST,and depends on the density. MNRASL 478, L60–L65 (2018) Downloaded from https://academic.oup.com/mnrasl/article-abstract/478/1/L60/4963748 by Ed 'DeepDyve' Gillespie user on 21 June 2018 L62 A. Verhamme et al. −1 Table 1. MUSE Ly α+C III] emitters. The sixth column indicates the separation of the peaks (i.e. 2 × V ,inkm s ) for blue bump LAEs, and is left empty 1/2 for single-peaked profiles. FWHM red −1 −1 −1 ID RA DEC EW (Å) V (km s ) (km s ) V (km s ) z Observations sys, CIII] peak sys 1 307.97040 −40.625694 32 176 ± 11 248 ± 9 – 3.5062 Commissioning mul 11 342.175042 −44.541031 222 215 ± 35 150 ± 35 375 ± 35 3.1163 AS1063 mul 14 342.178833 −44.535869 29 385 ± 35 300 ± 35 – 3.1150 AS1063 sys 44 64.0415559 −24.0599916 57 303 ± 35 360 ± 35 570 ± 35 3.2886 MACS0416 sys 132 64.0400838 −24.0667408 62 331 ± 35 288 ± 35 510 ± 35 3.2882 MACS0416 106 53.163726 −27.7790755 72 379 ± 13 414 ± 13 828 ± 35 3.2767 udf-10 118 53.157088 −27.7802688 65 301 ± 28 284 ± 28 568 ± 35 3.0173 udf-10 1180 53.195735 −27.7827171 80 220 ± 23 348 ± 32 – 3.3228 udf mosaic 6298 53.169249 −27.7812550 83 582 ± 38 512 ± 56 – 3.1287 udf-10 6666 53.159576 −27.7767193 52 284 ± 13 377 ± 11 754 ± 35 3.4349 udf-10 50 150.149656 2.061272 50 431 ± 42 268 ± 39 – 3.8237 GR30 48 149.852989 2.488099 68 294 ± 35 214 ± 35 705 ± 35. 3.3280 GR34 102 150.050268 2.600025 76 299 ± 15 229 ± 15 385 ± 35. 3.0400 GR84 a b c d Patricio et al. 2016; Richard et al., in preparation; Bacon et al. 2017, Inami et al. 2017, Maseda et al. 2017; Contini et al., in preparation. Figure 1. Three examples of rest-frame spectra of LAEs with detected C III]λλ1907, 1909 doublet probing the systemic redshift in the MUSE-GTO observations. In each panel, the velocity shifts of the Ly α line are shown relative to C III]1907Å. For all blue bump LAEs in our sample, the systemic redshift falls in between the blue and red peaks of the Ly α emission. et al. 2017;Yangetal. 2017). The systemic redshift of these ob- peaks, V , for blue bump LAEs with known systemic redshift. 1/2 jects was compiled from the several nebular lines contained in their We fit the data using the LTS LINEFIT program described in SDSS optical spectrum (e.g Izotov, Guseva & Thuan 2011). Note Cappellari et al. (2013), which combines the Least Trimmed Squares that the C III] emission line is out of the UV spectral range probed by robust technique of Rousseeuw & van Driessen (2006) into a least- the available HST-COS observations. For a sample of 17 GPs from squares fitting algorithm which allows for errors in both variables Jaskot & Oey (2014), Henry et al. (2015); Verhamme et al. (2017), and intrinsic scatter. The best fit, shown by the red line in Fig. 2, red we measure V and FWHM on the data. For 21 new GP observa- is given by peak red tions, we use the V and FWHM values computed by Yang et al. red −1 peak V = 1.05(±0.11) × V − 12(±37) km s (1) 1/2 peak (2017) given in their table 2. GPs nearly always exhibit blue bump Ly α profiles (Jaskot & Oey 2014; Henry et al. 2015; Verhamme This relation is so close to the one-to-one relation that we assume red et al. 2017). For the blue bump GPs, we also measure the separation from now that the underlying ‘true’ relation between V and peak of the peaks. V is one-to-one, as expected from radiation transfer modelling 1/2 (see Section 4 below). The intrinsic scatter estimated from the linear −1 regression is 53 (± 9) km s . 3 DERIVING SYSTEMIC REDSHIFT FROM LYMAN-ALPHA 3.2 Method 2: an empirical correlation between FWHM and 3.1 Method 1: systemic redshift of blue bump LAEs systemic redshift In this section, only blue bump spectra, i.e. double peaks with a red In this section, both single- and double-peaked profiles are con- peak higher than the blue peak, are considered. We note that, for all sidered. The measurements are always done on the red peak, and red blue bump LAEs studied here, the systemic redshift always falls in- the red peak only. In the right-hand panel of Fig. 2 we plot V peak between the Ly α peaks, as illustrated in Fig. 1 for three blue-bump versus FWHM for the full sample of LAEs presented in Section 2 MUSE Ly α+C III] emitters (see also Kulas et al. 2012;Erb et al. 2014;Yangetal. 2016). Fig. 2, left-hand panel, shows a positive red 2 empirical correlation between V and half of the separation of the http://www-astro.physics.ox.ac.u/∼mxc/software/#lts peak MNRASL 478, L60–L65 (2018) Downloaded from https://academic.oup.com/mnrasl/article-abstract/478/1/L60/4963748 by Ed 'DeepDyve' Gillespie user on 21 June 2018 z from Ly α L63 sys Figure 2. Empirical relations to determine systemic redshift from the shape of the Ly α emission. Left: correlation between the shift of the Ly α red peak, red (V ) and half of the separation of the peaks (V ) for a sample of LAEs with a known systemic redshift: 7 Ly α+C III] emitters with blue bump Ly α spectra 1/2 peak from the MUSE GTO data (red stars), blue bump LAEs among the Yang et al. (2017) GP sample (black dots), blue bump LAEs among the Hashimoto et al. red (2015) MagE sample and Group I LBGs from Kulas et al. (2012) (black triangles). Right: correlation between V and FWHM among Ly α+C III], H α or peak [O III] emitters. The black dashed line is the one-to-one relation. We checked that the correlation remains even discarding the two most upper left points. On both sides, the red curve is our best fit to the data, described by equations (1) and (2). The Pearson coefficient and the probability of the null hypothesisare shown on each panel. 3.3 Comparison of the methods We check that the corrected redshifts from both methods give results that are closer to the systemic redshift of the objects than the ‘Ly α red redshifts’, i.e. taking V as the systemic redshift, as usually done peak (Fig. 3). The standard deviation of the red histograms (corrected redshifts), reflecting both the intrinsic scatter and measurement er- rors, are comparable for the two methods, though slightly better for the blue bump method. We therefore propose to use half of the separation of the peaks as a proxy for the red peak shift of blue bump LAEs, and the Ly α FWHM for single-peaked spectra. They allow to recover the systemic redshift from the Ly α line, with an −1 uncertainty lower than ±100 km s from z ≈ 0 to 7. This suggests that the same scattering processes, linking the line shift and the line width, are at play at every redshift, and that the effect of the IGM does not erase this correlation. 4 DISCUSSION Figure 3. Comparison of the distributions of Ly α redshift errors (=z − z , in black) with redshift distributions corrected with Method Ly α sys 4.1 Effect of the spectral resolution 1 (in red, top panel) and with Method 2 (in red, bottom panel). These two methods to retrieve the systemic redshift of a LAE from the shape of its Ly α profile rely on measurements of either the positions of the blue and red Ly α emission peaks or the (red (new MUSE LAEs measurements are reported in Table 1). There peak) FWHM. Both of these measures are affected by the spectral red is a correlation between V and FWHM, although less significant peak resolution. Although the data points presented in Section 3 were than for Method 1 (see the Pearson coefficients on each panel of collected from the literature and MUSE surveys and span a range of Fig. 2). We use the same method (Cappellari et al. 2013)tode- spectral resolutions from R ∼ 1000 (LRIS) to R ∼ 5000 (X-Shooter, termine the empirical relation, which can be used to retrieve the HST-COS), they all seem to follow the same relation. systemic redshift of a galaxy: We investigated the effect of spectral resolution on synthetic red −1 spectra constructed from Ly α radiation transfer simulations. Poorer V = 0.9(±0.14) × FWHM(Lyα) − 34(±60) km s (2) peak spectral resolution broadens the peaks, and since Ly α profiles are This relation is also compatible with the one-to-one relation, given the uncertainties in the fit parameters. The intrinsic scatter estimated −1 3 red from the linear regression is 72(±12) km s , slightly larger than We have also tested the relation between Ly α EWs and V , but did not peak with Method 1. find any significant correlation. MNRASL 478, L60–L65 (2018) Downloaded from https://academic.oup.com/mnrasl/article-abstract/478/1/L60/4963748 by Ed 'DeepDyve' Gillespie user on 21 June 2018 L64 A. Verhamme et al. Figure 4. Points show the relationship between half of the separation of the peaks and the shift of the Ly α line, and between the FWHM and the shift of the Ly α line, for synthetic spectra from expanding shells, spheres or bi-conical outflows (Schaerer et al. 2011; Zheng & Wallace 2014). The trend is driven by the −2 column density of the scattering medium, but holds for the different idealized geometries. The symbol colours scale with the column density (in cm )ofthe −1 shells and symbol sizes scale with the radial expansion velocity (from 0 to 400 km s ). The red line and dashed black line are identical as in Fig. 2. often asymmetric, it also has the effect of shifting the peak towards larger range of FWHMs in the right-hand panel of Fig. 4 (from 0 −1 longer wavelengths. The latter effect is weaker than the broadening. to 1200 km s ) compared to Fig. 2 where observed FWHMs vary −1 −1 As a consequence, the effect of spectral resolution may flatten the from 214 to 512 km s . For FWHM values less than 600 km s , red slope but seems not to break the correlation. the model predictions lie close to the FWHM–V relation derived peak in Section 3. Although the exact location of each simulated object red in the FWHM–V plane seems to depend on each parameter, we peak 4.2 Comparison with models see that models with higher HI column densities lead to broader lines and larger shifts of the peak (colour-coded circles). A similar We now compare our results with numerical simulations of Ly α trend is found by Zheng & Wallace (2014) who performed Ly α radiative transfer in expanding shells performed with the MCLya radiation transfer simulations in anisotropic configurations (bipo- code (Schaerer et al. 2011; Verhamme, Schaerer & Maselli 2006). lar outflows) and inhomogeneous media (i.e. HI distributions with These models describe in a simple, idealized, way the propagation velocity or density gradients). Overall, this may suggest that the of Ly α photons emitted in HII regions through gas outflows which red FWHM–V correlation holds regardless of the assumed geome- peak seem ubiquitous in star-forming galaxies, especially at high redshift try and kinematics of the outflows, and that the HI opacity of the (Shapley et al. 2003; Steidel et al. 2010; Hashimoto et al. 2015). ISM and/or the medium surrounding galaxies (i.e. the CGM) is the Assuming a central point-source surrounded by an expanding shell main driver that shapes the observed Ly α line profiles. of gas with varying HI column density (N ), speed (V ), dust HI exp opacity (τ ) and temperature (described by the Doppler parameter b ∝ T ), shell models have proven very successful in reproducing a large diversity of Ly α line profiles. Here, we use simulations 5 CONCLUSIONS with different intrinsic Gaussian line widths (σ ) and various shell The recent increase in the number of LAEs with detected nebular parameter values (N , V , τ , b), degraded to mimic the MUSE HI exp d red lines allows to calibrate empirical methods to retrieve the systemic spectral resolution. We measure FWHM, V ,and V the same 1/2 peak redshift from the shape of the Ly α line. In addition to measure- way as for the data. red ments from the literature, we report 13 new detections from several We compare the observed correlation between V and the sep- peak MUSE GTO programs. We searched for Ly α+C III] emitters in the aration between the peaks of blue-bump LAEs (V ) with results 1/2 MUSE-Deep survey (Bacon et al. 2017), behind z ∼ 0.7 galaxy from models that produce double-peak profiles (Fig. 4, left-hand groups (Contini et al., in preparation), and lensed by three clusters panel). Predictions from expanding shell models lie very close to the (SMACSJ2031.8-4036 in Patr´ ıcio et al. 2016, AS1063, MACS0416 one-to-one relation and reproduce nicely the observed properties of red in Richard et al., in preparation). the Ly α lines. Objects with increasing V and V correspond 1/2 peak We find a robust correlation between the shift of the Ly α peak to expanding shells with larger HI column densities. This echoes red with respect to systemic redshift (V ) and half of the separation the analytical solutions for Ly α RT in static homogeneous media peak of the peaks (V ) for LAEs with blue bump spectra. The intrinsic (Neufeld 1990; Dijkstra, Haiman & Spaans 2006) that yield profiles 1/2 −1 scatter around the relation is ±53 km s . We also find a correlation with symmetric peaks around the line centre, whose positions are 1/3 red between the shift of the Ly α peak with respect to systemic redshift primarily set by the HI opacity and correspond to V ∝ τ . peak HI red (V ) and its FWHM, for LAEs with known systemic redshift. As shown in Fig. 4 (right-hand panel), the correlation between peak −1 the shift of the red peak and the FWHM of the Ly α line naturally The intrinsic scatter is of the same order (±73 km s ). These two arises from scattering processes. The slope predicted by the models relations have been approximated by linear fitting formulas as given is close to one whereas the relation derived from observations in in equations (1) and (2). These formulae have been derived for data Section 3 is shallower (≈0.9; red curve in the right-hand panel of with spectral resolution 1000 < R < 5000, they should be used on Figs 2, 4). However, it is worth pointing out that we explore a much data with similar spectral resolution. MNRASL 478, L60–L65 (2018) Downloaded from https://academic.oup.com/mnrasl/article-abstract/478/1/L60/4963748 by Ed 'DeepDyve' Gillespie user on 21 June 2018 z from Ly α L65 sys The relative redshift error if estimated from Ly α with Drake A. B. et al., 2017, A&A, 608, 6, red −1 red −3 Erb D. K. et al., 2014, ApJ, 795, 33 V = 300 km s is (z /z)(Ly α) = ((1+z) × (V / c))/z ∼10 peak peak Finkelstein S. L. et al., 2015, ApJ, 810, 71 at z = 3. The two methods presented in this letter can therefore Hashimoto T., Ouchi M., Shimasaku K., Ono Y., Nakajima K., Rauch M., help reduce systematic errors on distance measures. This is of great Lee J., Okamura S., 2013, ApJ, 765, 70 importance for redshift surveys at z  3, where spectroscopic red- Hashimoto T. et al., 2015, ApJ, 812, 157 shifts often rely on the Ly α emission line. Futures observations Hashimoto T. et al., 2017, MNRAS, 465, 1543 with better spectral resolution should allow to refine the proposed Henry A., Scarlata C., Martin C. L., Erb D., 2015, ApJ, 809, 19 relations. Herenz E. C. et al., 2017, A&A, 606, 12, Inami H. et al., 2017, A&A, 608, 2, Izotov Y. I., Guseva N. G., Thuan T. X., 2011, ApJ, 728, 161 ACKNOWLEDGEMENTS Jaskot A. E., Oey M. S., 2014, ApJ, 791, L19 We thank the anonymous referee for her/his helpful report. AV is Kulas K. R., Shapley A. E., Kollmeier J. A., Zheng Z., Steidel C. C., Hainline K. 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C., 2011, MNRAS, 414, 3265 government program managed by the ANR. RB and FL acknowl- Schaerer D., Hayes M., Verhamme A., Teyssier R., 2011, A&A, 531, A12 Shapley A. E., Steidel C. C., Pettini M., Adelberger K. L., 2003, ApJ, 588, edges support from the ERC advanced grant 339659-MUSICOS. JR and VP acknowledge support from the ERC starting grant Sobral D. et al., 2017, MNRAS, 466, 1242 336736-CALENDS. RAM acknowledges support by the Swiss Song M. et al., 2014, ApJ, 791, 3 National Science Foundation. JS acknowledges support from the Stark D. P. et al., 2014, MNRAS, 445, 3200 ERC grant 278594-GasAroundGalaxies. JB acknowledges support Stark D. P. et al., 2017, MNRAS, 464, 469 by Fundac ¸a ˜o paraaCiencia ˆ e a Tecnologia (FCT) through na- Steidel C. C., Adelberger K. L., Shapley A. E., Pettini M., Dickinson M., tional funds (UID/FIS/04434/2013) and by FEDER through COM- Giavalisco M., 2003, ApJ, 592, 728 PETE2020 (POCI-01-0145-FEDER-007672) and Investigador FCT Steidel C. C., Erb D. 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Journal

Monthly Notices of the Royal Astronomical Society LettersOxford University Press

Published: Apr 6, 2018

Keywords: galaxies: high-redshift; galaxies: starburst; galaxies: statistics; ultraviolet: galaxies

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