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Mass-invariance of the iron enrichment in the hot haloes of massive ellipticals, groups, and clusters of galaxies

Mass-invariance of the iron enrichment in the hot haloes of massive ellipticals, groups, and... Fe enrichment in ellipticals, groups, and clusters 1 Mass-invariance of the iron enrichment in the hot haloes of massive ellipticals, groups, and clusters of galaxies 1;2;3? 3 1;2 3;4 3 5 F. Mernier, J. de Plaa, N. Werner, J. S. Kaastra, A. J. J. Raassen, L. Gu, 3;4 3;4 1 6 J. Mao, I. Urdampilleta, N. Truong, and A. Simionescu MTA-Eotvos University Lendulet Hot Universe Research Group, P azm any P eter s et any 1/A, Budapest, 1117, Hungary    Institute of Physics, Eotvos University, P azm any P eter s etan  y 1/A, Budapest, 1117, Hungary   SRON Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands Leiden Observatory, Leiden University, P.O. Box 9513, 2300 RA Leiden, The Netherlands RIKEN Nishina Center, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan Institute of Space and Astronautical Science (ISAS), JAXA, 3-1-1 Yoshinodai, Chuo-ku, Sagamihara, Kanagawa 252-5210, Japan Accepted XXX. Received YYY; in original form ZZZ ABSTRACT X-ray measurements nd systematically lower Fe abundances in the X-ray emitting haloes pervading groups (kT . 1:7 keV) than in clusters of galaxies. These results have been dicult to reconcile with theoretical predictions. However, models using incomplete atomic data or the assumption of isothermal plasmas may have biased the best t Fe abundance in groups and giant elliptical galaxies low. In this work, we take advantage of a major update of the atomic code in the spectral tting package SPEX to re-evaluate the Fe abundance in 43 clusters, groups, and elliptical galaxies (the CHEERS sample) in a self-consistent analysis and within a common radius of 0.1r . For the rst time, we report a remarkably similar average Fe enrichment in all these systems. Unlike previous results, this strongly suggests that metals are synthesised and transported in these haloes with the same average eciency across two orders of magnitude in total mass. We show that the previous metallicity measurements in low temperature systems were biased low due to incomplete atomic data in the spectral tting codes. The reasons for such a code-related Fe bias, also implying previously unconsidered biases in the emission measure and temperature structure, are discussed. Key words: galaxies: clusters: intracluster medium { X-rays: galaxies: clusters 1 INTRODUCTION oxygen to nickel) can be robustly measured. This is espe- cially true for Fe, whose both K- and L-shell transitions have The largest gravitationally bound structures in the Uni- high emissivities and fall within the typical energy windows verse, such as giant elliptical galaxies, groups, and clus- (0.5{10 keV) of our X-ray observatories. For this reason, ters of galaxies, are pervaded by hot, X-ray emitting at- Fe abundances can be precisely measured in the X-ray ha- mospheres, which typically account for an important frac- los of both hot, massive clusters (via the Fe-K transitions) tion (up to 50{90%) of the total baryonic mass of these and cooler, less massive groups and ellipticals (via the Fe-L systems (e.g. Giodini et al. 2009). These hot atmospheres, transitions). In turn, these Fe abundance measurements are hereafter de ned for convenience as intra-cluster medium usually interpreted as a reliable tracer of the overall metal- (ICM), are also rich in heavy elements that were produced by licity in clusters and groups (e.g. de Plaa et al. 2017, and Type Ia and core-collapse supernovae within cluster/group references therein), and are thus valuable to understand the members and giant central galaxies (for recent reviews, see history of metal enrichment in these systems. Werner et al. 2008; de Plaa 2013; de Plaa & Mernier 2017). In the past, several works extensively studied the Fe Whereas observations and simulations suggest that metals abundance in the hot gas of either nearby ellipticals and in cluster outskirts were released more than 10 Gyr ago (e.g. galaxy groups (e.g. Mahdavi et al. 2005; Finoguenov et al. Urban et al. 2017; Bi et al. 2017, 2018), the epoch and ori- 2006; Grange et al. 2011; Sasaki et al. 2014; Konami et al. gin of the enrichment in the vicinity of central galaxies is less 2014), or galaxy clusters (e.g. Tamura et al. 2004; clear. de Plaa et al. 2007; De Grandi & Molendi 2001, 2009; Because the ICM is in a collisional ionisation equilib- Matsushita 2011; Zhang et al. 2011). Very few studies, how- rium (CIE), abundances of various elements (typically from ever, attempted to compare directly the metal content of all these systems together (e.g. Bregman et al. 2010; Sun 2012). E-mail: mernier@caesar.elte.hu In what has been perhaps the most comprehensive Downloaded from https://academic.oup.com/mnrasl/advance-article-abstract/doi/10.1093/mnrasl/sly080/4993253 by Ed 'DeepDyve' Gillespie user on 08 June 2018 2 F. Mernier et al. study so far, Yates et al. (2017) compiled from the litera- given within a 68% con dence interval. All the abundances ture a large number of Fe abundances measured in 79 nearby mentioned in this work are given with respect to their proto- groups and clusters and homogenised these measurements by solar values obtained by Lodders et al. (2009). extrapolating them to a radius of r . While in hot clusters, the Fe abundance was found to converge to a rather uniform value of  0:3 Solar, in low temperature groups and giant ellipticals the metallicity appeared to be on average signif- 2 REANALYSIS OF THE CHEERS SAMPLE icantly lower (see also Rasmussen & Ponman 2007, 2009). The sample, data reduction, background modelling, and These results were not reproduced by predictions from semi- spectral tting strategy are all described in detail in Pa- analytical models of galaxy evolution, in which (at least) as per I (see also Mernier et al. 2015). Compared to our much Fe was expected in groups as in clusters (Yates et al. previous work, we discard the observation of M 89 (Ob- 2017). sID:0141570101) because of its high background contamina- Do theoretical models really miss some important tion. This leaves us with XMM-Newton EPIC observations chemodynamical process at play in galaxy groups, or do of 43 nearby cool-core clusters, groups, and ellipticals, all be- spectroscopic measurements instead su er from unexpected ing part of the CHEERS project (see also Pinto et al. 2015; biases in low-temperature systems? From an observational de Plaa et al. 2017). The brightness of these nearby sources, perspective, this question remains open. In fact, homogenis- combined to their relatively moderate temperature (not ex- ing Fe abundance measurements from the literature is very ceeding 8 keV), allows a robust determination of the Fe challenging, essentially because: (i) di erent authors utilised abundance with the EPIC instruments, based on the Fe-K di erent data reduction and analysis methods, (ii) instru- lines and/or the Fe-L complex. mental calibration and spectral models continuously evolved Unlike in Paper I, where the spectra were extracted with years, and (iii) the lack of accurate measurements for within 0.05r and/or 0.2r (depending on the distance 500 500 radial Fe pro les of individual systems out to r makes the of the system), the goal of this paper is to measure the extrapolation to this radius quite uncertain. Last but not Fe abundance within the same physical scale. Therefore, all least, cooler systems (kT . 2 keV) require careful attention the spectra of our sample are re-extracted and re-analysed as the Fe-L complex, which is unresolved by CCD instru- within 0.1r . The only exception is the Virgo cluster (cen- ments, may be underestimated if one assumes the plasma to tred on M 87), which could be analysed only out to 0.05r be isothermal (the "Fe-bias"; Buote & Canizares 1994; Buote within the EPIC eld-of-view. The redshift and hydrogen 2000). Since most of the baryons (and metals) are rather in column density (n ) values are adopted from Paper I. groups than in clusters, determining their accurate, unbiased metallicity is nevertheless of a crucial importance to estimate the global metal budget of the universe. Clearly, measure- ments of such metallicities in hot haloes at all masses need 2.1 From SPEXACT v2 to SPEXACT v3 to be further investigated and better understood. A key improvement with respect to Paper I is the updated In a recent work (Mernier et al. 2016, hereafter Paper version of the SPEX Atomic Code and Tables (hereafter I), we used XMM-Newton EPIC observations to measure Fe SPEXACT). While in Paper I our analysis relied on SPEX- { among other elemental abundances { in the hot haloes of ACT v2.05 (hereafter v2), in this Letter we take advan- 44 nearby cool-core ellipticals, groups, and clusters of galax- tage of the up-to-date release of SPEXACT v3.04 (here- ies (the CHEERS catalog). Interestingly, we found an ap- after v3). This most recent version is the result of a ma- parent de cit of Fe in the coolest systems, supporting the jor update started in 2016 (SPEXACT v3.00) with fur- previous ndings of Rasmussen & Ponman (2007, 2009) and ther minor improvements implemented until the end of 2017 Yates et al. (2017), which are in tension with theoretical ex- (Hitomi Collaboration et al. 2017). Compared to SPEX- pectations. In that study, however, groups and ellipticals ACT v2, the total number of energy transitions has in- were investigated only within 0.05r , making it dicult creased by a factor of 400, to reach more than 1.8 million to compare with most simulations given their limited reso- in SPEXACT v3. The new transitions include for instance lution. In addition, a major update of the plasma models higher principal quantum numbers for both H-like and He- from the SPEX tting package (Kaastra et al. 1996) has like ions. In addition, signi cant updates were performed been publicly released. As brie y noted in Mernier et al. in collisional excitation and de-excitation rates, radiative (2017), such an improvement could a ect the Fe abundance transition probabilities, auto-ionisation and dielectronic re- measured by CCD instruments in cooler plasmas and poten- combination rates (either from the literature or consistently tially revise our current picture of the ICM enrichment from calculated using the FAC code Gu 2008). Finally, signi - massive ellipticals to the largest structures of the universe. cant improvements were obtained in radiative recombination In this Letter, we revisit the observed Fe abundances in (Badnell 2006; Mao & Kaastra 2016) and collisional ionisa- the CHEERS sample by: (i) analysing EPIC spectra within a tion coecients (Urdampilleta et al. 2017). In order to com- common astrophysical radius of 0.1r { easier to compare pare the e ects of the improvements in a consistent way, in with simulations { and (ii) exploring how recent spectral the following we use successively SPEXACT v2 and SPEX- model improvements alter the measured Fe abundances and ACT v3 to t all our EPIC spectra (MOS 1, MOS 2, and pn their interpretation. Throughout this Letter, we assume H are tted simultaneously, see Paper I). 1 1 = 70 km s Mpc , = 0.3, and = 0.7. Error bars are 1 2 CHEmical Enrichment Rgs Sample https://www-amdis.iaea.org/FAC Downloaded from https://academic.oup.com/mnrasl/advance-article-abstract/doi/10.1093/mnrasl/sly080/4993253 by Ed 'DeepDyve' Gillespie user on 08 June 2018 Fe enrichment in ellipticals, groups, and clusters 3 2.2 Multi-temperature modelling Groups/Ellipticals Clusters As already demonstrated by e.g. Buote & Canizares 1.5 (1994); Buote (2000, Fe-bias) and Rasia et al. (2008); Simionescu et al. (2009, inverse Fe-bias), modelling the ICM with a multi-temperature structure is essential to derive cor- rect abundances. The most intuitive assumption would be to 0.7 consider that the temperature follows a Gaussian di eren- tial emission measure distribution (the gdem model; see e.g. de Plaa et al. 2006; Simionescu et al. 2009). Such a model, 0.5 however, requires appreciable computing resources, espe- cially when using SPEXACT v3. A cheaper, yet still rea- SPEXACT v2 sonable alternative would be to approximate a gdem distri- 0.3 SPEXACT v3 bution by modelling three temperature components (3T): (i) the main component, for which the temperature kT and mean 0.5 1 2 3 4 5 6 7 the emission measure Y are left free in the ts; (ii) a higher- kT mean and (iii) a lower-temperature components, whose tempera- tures kT and kT are left free but their Y is tied to half of up low Figure 1. Iron abundance measured as a function of the mean that of the main component. The ratio kT =kT can thus up low temperature within 0.1r of the ellipticals, galaxy groups and be seen as the typical width of the distribution. Although clusters from the CHEERS sample (only M 89 is discarded, see such a temperature distribution may somewhat deviate from text). For a given system, the corresponding SPEXACT v2 (or- Gaussianity in some cases, we verify that tting (i) a sub- ange stars) and SPEXACT v3 (blue dots) measurements, both obtained using a 3T model (see text), are tied by a green-brown sample of systems with a gdem model and (ii) gdem-simulated dashed line. Clusters and groups/ellipticals are delimited arbi- EPIC spectra with a 3T model across various mean temper- trarily beyond and below kT = 1:7 keV, respectively. mean atures have negligible impact (always less than 6%) on our measured Fe abundances. Such multi-temperature modelling is particularly rele- Mean val. vant here, as we obtain signi cantly better ts than when SPEXACT v2 Clusters we model our spectra with a single-temperature component Groups/Ell. only. Moreover, in addition to the fact that all our systems are classi ed as cool-core, they are also known to exhibit clear temperature gradients within 0.1r (see e.g. results from the ACCEPT catalog Cavagnolo et al. 2009). 2 Mean val. SPEXACT v3 Clusters 3 RESULTS Groups/Ell. The measured Fe abundances of the 43 CHEERS systems 8 reanalysed within 0.1r are shown as a function of their kT in Figure 1. Because the overall temperature of the mean 2=3 ICM scales with the total mass M of the system as M (e.g. Giodini et al. 2013), kT can be seen as a reasonable mean 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 proxy for the total mass of our sources. Therefore, we split Fe (0.1 r ) our sample into two subsamples, namely: (i) "clusters", for Figure 2. Histograms showing the Fe abundance distribution of which kT > 1:7 keV, and (ii) "groups/ellipticals", for mean the CHEERS sample, when using successively SPEXACT v2 (top which kT < 1:7 keV. The choice of the threshold value mean panel ) and SPEXACT v3 (bottom panel ). In each case, the dis- kT = 1:7 keV is of course arbitrary, but well justi ed mean tribution for clusters (kT > 1:7 keV) and groups/ellipticals mean by the usual classi cation attributed to each system in the (kT < 1:7 keV) is shown separately. The mean value of each mean literature. distribution (and corresponding errors) is shown by the vertical While compared to SPEXACT v2, the Fe abundances dashed lines (and lled areas around them). measured in clusters remain essentially unchanged, the Fe abundances in groups and ellipticals are systematically re- vised upwards when using SPEXACT v3. This result is bet- (0:75  0:04) and groups/ellipticals (0:58  0:03) are signi - ter quanti ed in Figure 2, where the distribution of Fe abun- cantly di erent. In other words, spectral ts obtained using dances is compared between clusters and groups/ellipticals, updated atomic data indicate that the average concentra- using the two versions of the code. Based on the entire sam- tion of Fe in the hot haloes of groups and giant ellipticals is ple, the SPEXACT v3 results provide a mean Fe abundance the same as that in clusters of galaxies. of 0:74 0:03 with an intrinsic scatter of 25% (computed fol- Systems for which kT lies within 2{3 keV exhibit mean lowing the method described in Paper I). When splitting both Fe-L and Fe-K lines, hence their Fe abundance can the sample, we nd consistent average Fe abundances of be constrained by each of these two features separately. 0:75 0:04 and 0:70 0:03 for clusters and groups/ellipticals, We test this approach on M 87 and EXO 0422, both hav- respectively. This is in contrasts with the SPEXACT v2 re- ing good data quality. Compared to the Fe estimated from sults, where the average Fe abundance values for clusters the "full band" ts, we nd that the <2 keV local ts (Fe-L Downloaded from https://academic.oup.com/mnrasl/advance-article-abstract/doi/10.1093/mnrasl/sly080/4993253 by Ed 'DeepDyve' Gillespie user on 08 June 2018 Fe (0.1 r ) Number of systems 4 F. Mernier et al. 1.6 lines only) provide negligible biases (-2% and +6% for M 87 Groups/ Clusters and EXO 0422, respectively). These biases become some- Ellipticals what larger (respectively +15% and -12%) in the >2 keV 1.4 local ts (Fe-K lines only); however they are not system- atic and remain well below the typical 25% scatter reported 1.2 above. Although this issue is well known (Rasia et al. 2008; Simionescu et al. 2009) and concerns less than 14% of our 1.0 systems, this mismatch will deserve attention with future high-resolution spectroscopy missions. 0.8 3.1 The code-related Fe-bias kT mean 0.6 Fe In clusters, the Fe abundance determination is predomi- kT /kT up low nantly based on the prominent Fe-K lines. Since only the 0.4 low temperature groups/ellipticals are signi cantly a ected 1 2 3 4 5 6 kT [SPEXACT v3] by the update of SPEXACT, the reason for such a change mean is to be found in the Fe-L emission, which is dominant at Figure 3. Relative deviations on the parameters Y , kT , Fe, mean kT . 2 keV. In order to better understand the code- mean and the ratio kT =kT when EPIC mock spectra of 3T plasma up low related Fe bias that we report above, we adapt an instructive (simulated using SPEXACT v3 for various initial mean tempera- exercise previously introduced in Mernier et al. (2017) and tures) are tted using SPEXACT v2. The two horizontal dashed de Plaa et al. (2017). In short, we start by using SPEXACT lines indicate the 10% relative deviations. v3 to simulate mock EPIC spectra with 100 ks exposure on a grid of various kT values. In all these simulations mean 72 3 Y and the abundances are assumed to be 10 m and 1 proto-solar, respectively. Moreover, kT and kT are as- up low sumed such that kT =kT = 2:8. As a second step, we t up low these mock spectra using SPEXACT v2 with Y , Fe, kT , mean kT , and kT as free parameters. The relative deviation up low 1 of these SPEXACT v2 best- t parameters with respect to their input SPEXACT v3 values is shown in Figure 3 as a function of the input mean temperature. As expected from our results above, the Fe consistency between the two ver- sions of SPEXACT is excellent in the clusters regime, while 0.1 SPEXACT v3 (sim. spectrum): all parameters = [init. sim.] SPEXACT v2: all parameters = [init. sim.] it dramatically deteriorates when the plasma becomes cooler SPEXACT v2: Fe = [best-fit] than 2 keV. In addition, other interesting e ects occur SPEXACT v2: Y,Fe = [best-fit] SPEXACT v2: all parameters = [best-fit] in the groups/ellipticals regime. Below kT . 1:5 keV mean and kT . 1 keV, the ratio kT =kT and Y are re- mean up low 0.5 spectively under- and overestimated by SPEXACT v2. The 0.0 mean temperature, however, remains reasonably reproduced by SPEXACT v2, except for very hot plasmas where kT mean 0.5 1 2 is at most 15% underestimated (though without a ecting Energy (keV) the Fe abundance). Figure 4. EPIC MOS 2 simulated spectrum of a 3T plasma To better understand all the biases we observe in cool with kT = 0:7 keV, using SPEXACT v3. For comparison, mean plasmas with the EPIC instruments, we investigate further we show the same model calculated using SPEXACT v2 (red). the case of a 3T plasma simulated for 100 ks with SPEX- Then, we progressively x the Fe (orange), Y (green), and eventu- ACT v3, assuming kT = 0:7 keV (Figure 4, black data mean ally kT , kT , and kT (blue) to their a posteriori best- t mean up low points). A direct comparison of this simulated spectrum with SPEXACT v2 values. The residuals of such models with respect its equivalent model using SPEXACT v2 (Figure 4, red line) to the input simulated spectrum are shown in the bottom panel. shows signi cant discrepancies throughout the entire Fe-L complex (0.6{1.2 keV). In fact, the emissivity of many im- portant lines (e.g. Fe XVII at 0.73 keV; Fe XVIII at 0.77 keV) were revised lower with the update of SPEXACT, while new transitions were incorporated and/or updated with a ing kT to provide a formally acceptable { but incorrect { low higher emissivity (e.g. Fe XVIII at 1.18 keV). When xing best- t to the input spectrum (Figure 4, blue line). the Fe abundance to its best- t value estimated a posteriori In summary, in cool plasmas the emission measure, the by SPEXACT v2 (Figure 4, orange line), the emitting bump Fe abundance, and the width of the temperature distribution at 0.7 get smoother, in better agreement with the overall (kT =kT ) in uence each other to reproduce the observed up low shape of the Fe-L complex. However, over the entire soft shape of the unresolved Fe-L complex. As a consequence, band the ux signi cantly decreases, which the t attempts even outdated spectral codes can reasonably t the Fe-L to "correct" by increasing Y (Figure 4, green line). Finally, complex, yet providing strongly biased measurements. This the t smooths the residual bumps (in particular around conspiracy between all these parameters explain the code- 0.9{1 keV) by simultaneously decreasing kT and increas- related Fe-bias that we report in this Letter. up Downloaded from https://academic.oup.com/mnrasl/advance-article-abstract/doi/10.1093/mnrasl/sly080/4993253 by Ed 'DeepDyve' Gillespie user on 08 June 2018 [SPEXACT v2]/[SPEXACT v3] deviations Rel. err. Counts/s/keV Fe enrichment in ellipticals, groups, and clusters 5 4 IMPLICATIONS FOR THE IRON CONTENT recent chemo- and hydrodynamical simulations in a com- IN ELLIPTICALS, GROUPS AND panion paper (Truong et al. submitted), to which we refer CLUSTERS the reader for a more detailed discussion. We remind that these integrated measurements cover By measuring Fe abundances within 0.1r in a self- 0.1r , without further information on their inner or outer consistent way and using the latest SPEXACT version avail- spatial distributions. The question of whether clusters and able to date, we report for the rst time similar Fe abun- groups/ellipticals are really self-similar in terms of metal dances in ellipticals, galaxy groups, and galaxy clusters. In enrichment would require at least to derive the individual other words, gas-phase metallicities remain constant across abundance pro les for the entire sample using SPEXACT v3 two orders of magnitude in halo mass. (for a similar work using SPEXACT v2, see Mernier et al. These new results contradict previous papers (e.g. 2017). Because of the non-negligible time required by SPEX- Rasmussen & Ponman 2009; Bregman et al. 2010; Sun 2012; ACT v3 to t each spectrum, we leave such a study for future Yates et al. 2017), which reported systematically lower Fe work. abundances in groups and/or ellipticals with respect to the In addition to the code-related Fe bias discussed in this hotter clusters of galaxies (although Konami et al. (2014) work, we also note from Figure 3 that tting the spectra reported similar average Fe abundances as reported here, of cool systems with an outdated plasma code may also albeit for ellipticals only). Rather than SPEX, most of bias the emission measure, the mean temperature and the those previous studies used many (very di erent) versions kT =kT ratio by +35%, +7%, and -24%, respectively. In up low of APEC to t their data, making a direct comparison turn, these biases may have consequences on the estimates of with this work dicult. All these (mostly outdated) atomic further interesting quantities. For instance, we estimate that codes, however, likely encountered similar problems of a too the ICM pressure, usually de ned as P = n kT , can be biased simplistic modelling of the Fe-L transitions. From a theo- high by 19% in the case of a 0.7 keV plasma. Unlike the 2=3 retical perspective, that trend was not trivial to explain. pressure, the ICM entropy, usually de ned as K = kT=n , For example, when comparing the observational trend with remains very close to its true value, with a underestimate a semi-analytic model, Yates et al. (2017) did not succeed of less than 1%. Similarly, the total hydrostatic mass is to reproduce the previously reported positive temperature- not expected to be a ected by more than a few percent, as metallicity correlation in galaxy groups. Instead, the metal temperature and density gradients do not change dramati- content in low-mass systems is systematically overestimated cally. A more precise quanti cation, however, is left to future by their model. work. Our results also reveal the complication of measuring Our present results have interesting consequences, in accurately the temperature structure of lower-mass systems, particular given that the investigated systems exhibit very as long as the Fe-L complex remains unresolved by the ob- di erent stellar- to ICM-mass fractions. Because this frac- serving instruments. tion is lower in rich clusters than in less massive systems, Finally, it should be reminded that no spectral code is invariant Fe abundances could be explained only if the ef- perfect. It is certain that further improvements on SPEX- fective ICM enrichment considerably increases with the mass ACT will be pursued in the future, with potential implica- of the system. Such requirements have been dicult to rec- tions on the interpretation of moderate resolution spectra of oncile with the observed stellar populations in clusters so X-ray sources. In that respect, micro-calorimeters onboard far (e.g. Loewenstein 2013; Renzini & Andreon 2014). The future missions such as XARM and Athena will enable us story, however, is di erent if the Fe present in the ICM to observe the Fe-L complex with unprecedented resolution. is unrelated to the current stellar population of these sys- These observations will be invaluable to better understand tems. In addition to the increasing evidence towards an all the radiation processes in the ICM and push our knowl- early ICM enrichment in cluster outskirts (e.g. Werner et al. edge of astrophysical plasma emission to the next level. 2013; Simionescu et al. 2017; Urban et al. 2017), central Fe peaks were also found to be in place already at z  1 (De Grandi et al. 2014; Mantz et al. 2017) and exhibit the same radial distribution as SNcc products (Mernier et al. ACKNOWLEDGEMENTS 2017). These recent ndings suggest that recent SNIa ex- plosions and stellar mass loss from central galaxies do not The authors thank the referee for their constructive com- signi cantly contribute to the ICM enrichment. ments that helped to improve the manuscript as well as Ki- In this context, the similar Fe abundances found in hot ran Lakhchaura for fruitful discussions. F.M. is supported haloes spanning di erent mass ranges constitute an addi- by the Lendulet LP2016-11 grant awarded by the Hungar- tional support toward this early enrichment scenario, even ian Academy of Sciences. This work is partly based on the in their central parts. Since they grow hierarchically, iso- XMM-Newton AO-12 proposal \The XMM-Newton view of lated massive ellipticals and assembling groups can be seen chemical enrichment in bright galaxy clusters and groups " as the rst steps of the formation of more massive clus- (PI: de Plaa), and is a follow-up of the CHEERS (CHEmi- ters. Although, admittedly, nearby groups may have di er- cal Evolution Rgs cluster Sample) collaboration; the authors ent speci c properties (star formation, AGN feedback, etc.) thank all its members. This work is based on observations than high-redshift proto-clusters, the mass-invariance of Fe obtained with XMM-Newton, an ESA science mission with abundances at low redshift suggests that the bulk of met- instruments and contributions directly funded by ESA mem- als in hot haloes was already in place well before clusters ber states and the USA (NASA). The SRON Netherlands In- e ectively assembled. Our new measurements are directly stitute for Space Research is supported nancially by NWO, confronted to (and are found to be in good agreement with) the Netherlands Organisation for Scienti c Research. Downloaded from https://academic.oup.com/mnrasl/advance-article-abstract/doi/10.1093/mnrasl/sly080/4993253 by Ed 'DeepDyve' Gillespie user on 08 June 2018 6 F. Mernier et al. 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W., Urban O., 2017, MNRAS, 469, 1476 Sun M., 2012, New Journal of Physics, 14, 045004 Tamura T., Kaastra J. S., den Herder J. W. A., Bleeker J. A. M., Peterson J. R., 2004, A&A, 420, 135 Urban O., Werner N., Allen S. W., Simionescu A., Mantz A., 2017, MNRAS, 470, 4583 Urdampilleta I., Kaastra J. S., Mehdipour M., 2017, A&A, 601, A85 Werner N., Durret F., Ohashi T., Schindler S., Wiersma R. P. C., 2008, Space Sci. Rev., 134, 337 Werner N., Urban O., Simionescu A., Allen S. W., 2013, Nature, 502, 656 Yates R. M., Thomas P. A., Henriques B. M. B., 2017, MNRAS, 464, 3169 Downloaded from https://academic.oup.com/mnrasl/advance-article-abstract/doi/10.1093/mnrasl/sly080/4993253 by Ed 'DeepDyve' Gillespie user on 08 June 2018 http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Monthly Notices of the Royal Astronomical Society Letters Oxford University Press

Mass-invariance of the iron enrichment in the hot haloes of massive ellipticals, groups, and clusters of galaxies

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Abstract

Fe enrichment in ellipticals, groups, and clusters 1 Mass-invariance of the iron enrichment in the hot haloes of massive ellipticals, groups, and clusters of galaxies 1;2;3? 3 1;2 3;4 3 5 F. Mernier, J. de Plaa, N. Werner, J. S. Kaastra, A. J. J. Raassen, L. Gu, 3;4 3;4 1 6 J. Mao, I. Urdampilleta, N. Truong, and A. Simionescu MTA-Eotvos University Lendulet Hot Universe Research Group, P azm any P eter s et any 1/A, Budapest, 1117, Hungary    Institute of Physics, Eotvos University, P azm any P eter s etan  y 1/A, Budapest, 1117, Hungary   SRON Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands Leiden Observatory, Leiden University, P.O. Box 9513, 2300 RA Leiden, The Netherlands RIKEN Nishina Center, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan Institute of Space and Astronautical Science (ISAS), JAXA, 3-1-1 Yoshinodai, Chuo-ku, Sagamihara, Kanagawa 252-5210, Japan Accepted XXX. Received YYY; in original form ZZZ ABSTRACT X-ray measurements nd systematically lower Fe abundances in the X-ray emitting haloes pervading groups (kT . 1:7 keV) than in clusters of galaxies. These results have been dicult to reconcile with theoretical predictions. However, models using incomplete atomic data or the assumption of isothermal plasmas may have biased the best t Fe abundance in groups and giant elliptical galaxies low. In this work, we take advantage of a major update of the atomic code in the spectral tting package SPEX to re-evaluate the Fe abundance in 43 clusters, groups, and elliptical galaxies (the CHEERS sample) in a self-consistent analysis and within a common radius of 0.1r . For the rst time, we report a remarkably similar average Fe enrichment in all these systems. Unlike previous results, this strongly suggests that metals are synthesised and transported in these haloes with the same average eciency across two orders of magnitude in total mass. We show that the previous metallicity measurements in low temperature systems were biased low due to incomplete atomic data in the spectral tting codes. The reasons for such a code-related Fe bias, also implying previously unconsidered biases in the emission measure and temperature structure, are discussed. Key words: galaxies: clusters: intracluster medium { X-rays: galaxies: clusters 1 INTRODUCTION oxygen to nickel) can be robustly measured. This is espe- cially true for Fe, whose both K- and L-shell transitions have The largest gravitationally bound structures in the Uni- high emissivities and fall within the typical energy windows verse, such as giant elliptical galaxies, groups, and clus- (0.5{10 keV) of our X-ray observatories. For this reason, ters of galaxies, are pervaded by hot, X-ray emitting at- Fe abundances can be precisely measured in the X-ray ha- mospheres, which typically account for an important frac- los of both hot, massive clusters (via the Fe-K transitions) tion (up to 50{90%) of the total baryonic mass of these and cooler, less massive groups and ellipticals (via the Fe-L systems (e.g. Giodini et al. 2009). These hot atmospheres, transitions). In turn, these Fe abundance measurements are hereafter de ned for convenience as intra-cluster medium usually interpreted as a reliable tracer of the overall metal- (ICM), are also rich in heavy elements that were produced by licity in clusters and groups (e.g. de Plaa et al. 2017, and Type Ia and core-collapse supernovae within cluster/group references therein), and are thus valuable to understand the members and giant central galaxies (for recent reviews, see history of metal enrichment in these systems. Werner et al. 2008; de Plaa 2013; de Plaa & Mernier 2017). In the past, several works extensively studied the Fe Whereas observations and simulations suggest that metals abundance in the hot gas of either nearby ellipticals and in cluster outskirts were released more than 10 Gyr ago (e.g. galaxy groups (e.g. Mahdavi et al. 2005; Finoguenov et al. Urban et al. 2017; Bi et al. 2017, 2018), the epoch and ori- 2006; Grange et al. 2011; Sasaki et al. 2014; Konami et al. gin of the enrichment in the vicinity of central galaxies is less 2014), or galaxy clusters (e.g. Tamura et al. 2004; clear. de Plaa et al. 2007; De Grandi & Molendi 2001, 2009; Because the ICM is in a collisional ionisation equilib- Matsushita 2011; Zhang et al. 2011). Very few studies, how- rium (CIE), abundances of various elements (typically from ever, attempted to compare directly the metal content of all these systems together (e.g. Bregman et al. 2010; Sun 2012). E-mail: mernier@caesar.elte.hu In what has been perhaps the most comprehensive Downloaded from https://academic.oup.com/mnrasl/advance-article-abstract/doi/10.1093/mnrasl/sly080/4993253 by Ed 'DeepDyve' Gillespie user on 08 June 2018 2 F. Mernier et al. study so far, Yates et al. (2017) compiled from the litera- given within a 68% con dence interval. All the abundances ture a large number of Fe abundances measured in 79 nearby mentioned in this work are given with respect to their proto- groups and clusters and homogenised these measurements by solar values obtained by Lodders et al. (2009). extrapolating them to a radius of r . While in hot clusters, the Fe abundance was found to converge to a rather uniform value of  0:3 Solar, in low temperature groups and giant ellipticals the metallicity appeared to be on average signif- 2 REANALYSIS OF THE CHEERS SAMPLE icantly lower (see also Rasmussen & Ponman 2007, 2009). The sample, data reduction, background modelling, and These results were not reproduced by predictions from semi- spectral tting strategy are all described in detail in Pa- analytical models of galaxy evolution, in which (at least) as per I (see also Mernier et al. 2015). Compared to our much Fe was expected in groups as in clusters (Yates et al. previous work, we discard the observation of M 89 (Ob- 2017). sID:0141570101) because of its high background contamina- Do theoretical models really miss some important tion. This leaves us with XMM-Newton EPIC observations chemodynamical process at play in galaxy groups, or do of 43 nearby cool-core clusters, groups, and ellipticals, all be- spectroscopic measurements instead su er from unexpected ing part of the CHEERS project (see also Pinto et al. 2015; biases in low-temperature systems? From an observational de Plaa et al. 2017). The brightness of these nearby sources, perspective, this question remains open. In fact, homogenis- combined to their relatively moderate temperature (not ex- ing Fe abundance measurements from the literature is very ceeding 8 keV), allows a robust determination of the Fe challenging, essentially because: (i) di erent authors utilised abundance with the EPIC instruments, based on the Fe-K di erent data reduction and analysis methods, (ii) instru- lines and/or the Fe-L complex. mental calibration and spectral models continuously evolved Unlike in Paper I, where the spectra were extracted with years, and (iii) the lack of accurate measurements for within 0.05r and/or 0.2r (depending on the distance 500 500 radial Fe pro les of individual systems out to r makes the of the system), the goal of this paper is to measure the extrapolation to this radius quite uncertain. Last but not Fe abundance within the same physical scale. Therefore, all least, cooler systems (kT . 2 keV) require careful attention the spectra of our sample are re-extracted and re-analysed as the Fe-L complex, which is unresolved by CCD instru- within 0.1r . The only exception is the Virgo cluster (cen- ments, may be underestimated if one assumes the plasma to tred on M 87), which could be analysed only out to 0.05r be isothermal (the "Fe-bias"; Buote & Canizares 1994; Buote within the EPIC eld-of-view. The redshift and hydrogen 2000). Since most of the baryons (and metals) are rather in column density (n ) values are adopted from Paper I. groups than in clusters, determining their accurate, unbiased metallicity is nevertheless of a crucial importance to estimate the global metal budget of the universe. Clearly, measure- ments of such metallicities in hot haloes at all masses need 2.1 From SPEXACT v2 to SPEXACT v3 to be further investigated and better understood. A key improvement with respect to Paper I is the updated In a recent work (Mernier et al. 2016, hereafter Paper version of the SPEX Atomic Code and Tables (hereafter I), we used XMM-Newton EPIC observations to measure Fe SPEXACT). While in Paper I our analysis relied on SPEX- { among other elemental abundances { in the hot haloes of ACT v2.05 (hereafter v2), in this Letter we take advan- 44 nearby cool-core ellipticals, groups, and clusters of galax- tage of the up-to-date release of SPEXACT v3.04 (here- ies (the CHEERS catalog). Interestingly, we found an ap- after v3). This most recent version is the result of a ma- parent de cit of Fe in the coolest systems, supporting the jor update started in 2016 (SPEXACT v3.00) with fur- previous ndings of Rasmussen & Ponman (2007, 2009) and ther minor improvements implemented until the end of 2017 Yates et al. (2017), which are in tension with theoretical ex- (Hitomi Collaboration et al. 2017). Compared to SPEX- pectations. In that study, however, groups and ellipticals ACT v2, the total number of energy transitions has in- were investigated only within 0.05r , making it dicult creased by a factor of 400, to reach more than 1.8 million to compare with most simulations given their limited reso- in SPEXACT v3. The new transitions include for instance lution. In addition, a major update of the plasma models higher principal quantum numbers for both H-like and He- from the SPEX tting package (Kaastra et al. 1996) has like ions. In addition, signi cant updates were performed been publicly released. As brie y noted in Mernier et al. in collisional excitation and de-excitation rates, radiative (2017), such an improvement could a ect the Fe abundance transition probabilities, auto-ionisation and dielectronic re- measured by CCD instruments in cooler plasmas and poten- combination rates (either from the literature or consistently tially revise our current picture of the ICM enrichment from calculated using the FAC code Gu 2008). Finally, signi - massive ellipticals to the largest structures of the universe. cant improvements were obtained in radiative recombination In this Letter, we revisit the observed Fe abundances in (Badnell 2006; Mao & Kaastra 2016) and collisional ionisa- the CHEERS sample by: (i) analysing EPIC spectra within a tion coecients (Urdampilleta et al. 2017). In order to com- common astrophysical radius of 0.1r { easier to compare pare the e ects of the improvements in a consistent way, in with simulations { and (ii) exploring how recent spectral the following we use successively SPEXACT v2 and SPEX- model improvements alter the measured Fe abundances and ACT v3 to t all our EPIC spectra (MOS 1, MOS 2, and pn their interpretation. Throughout this Letter, we assume H are tted simultaneously, see Paper I). 1 1 = 70 km s Mpc , = 0.3, and = 0.7. Error bars are 1 2 CHEmical Enrichment Rgs Sample https://www-amdis.iaea.org/FAC Downloaded from https://academic.oup.com/mnrasl/advance-article-abstract/doi/10.1093/mnrasl/sly080/4993253 by Ed 'DeepDyve' Gillespie user on 08 June 2018 Fe enrichment in ellipticals, groups, and clusters 3 2.2 Multi-temperature modelling Groups/Ellipticals Clusters As already demonstrated by e.g. Buote & Canizares 1.5 (1994); Buote (2000, Fe-bias) and Rasia et al. (2008); Simionescu et al. (2009, inverse Fe-bias), modelling the ICM with a multi-temperature structure is essential to derive cor- rect abundances. The most intuitive assumption would be to 0.7 consider that the temperature follows a Gaussian di eren- tial emission measure distribution (the gdem model; see e.g. de Plaa et al. 2006; Simionescu et al. 2009). Such a model, 0.5 however, requires appreciable computing resources, espe- cially when using SPEXACT v3. A cheaper, yet still rea- SPEXACT v2 sonable alternative would be to approximate a gdem distri- 0.3 SPEXACT v3 bution by modelling three temperature components (3T): (i) the main component, for which the temperature kT and mean 0.5 1 2 3 4 5 6 7 the emission measure Y are left free in the ts; (ii) a higher- kT mean and (iii) a lower-temperature components, whose tempera- tures kT and kT are left free but their Y is tied to half of up low Figure 1. Iron abundance measured as a function of the mean that of the main component. The ratio kT =kT can thus up low temperature within 0.1r of the ellipticals, galaxy groups and be seen as the typical width of the distribution. Although clusters from the CHEERS sample (only M 89 is discarded, see such a temperature distribution may somewhat deviate from text). For a given system, the corresponding SPEXACT v2 (or- Gaussianity in some cases, we verify that tting (i) a sub- ange stars) and SPEXACT v3 (blue dots) measurements, both obtained using a 3T model (see text), are tied by a green-brown sample of systems with a gdem model and (ii) gdem-simulated dashed line. Clusters and groups/ellipticals are delimited arbi- EPIC spectra with a 3T model across various mean temper- trarily beyond and below kT = 1:7 keV, respectively. mean atures have negligible impact (always less than 6%) on our measured Fe abundances. Such multi-temperature modelling is particularly rele- Mean val. vant here, as we obtain signi cantly better ts than when SPEXACT v2 Clusters we model our spectra with a single-temperature component Groups/Ell. only. Moreover, in addition to the fact that all our systems are classi ed as cool-core, they are also known to exhibit clear temperature gradients within 0.1r (see e.g. results from the ACCEPT catalog Cavagnolo et al. 2009). 2 Mean val. SPEXACT v3 Clusters 3 RESULTS Groups/Ell. The measured Fe abundances of the 43 CHEERS systems 8 reanalysed within 0.1r are shown as a function of their kT in Figure 1. Because the overall temperature of the mean 2=3 ICM scales with the total mass M of the system as M (e.g. Giodini et al. 2013), kT can be seen as a reasonable mean 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 proxy for the total mass of our sources. Therefore, we split Fe (0.1 r ) our sample into two subsamples, namely: (i) "clusters", for Figure 2. Histograms showing the Fe abundance distribution of which kT > 1:7 keV, and (ii) "groups/ellipticals", for mean the CHEERS sample, when using successively SPEXACT v2 (top which kT < 1:7 keV. The choice of the threshold value mean panel ) and SPEXACT v3 (bottom panel ). In each case, the dis- kT = 1:7 keV is of course arbitrary, but well justi ed mean tribution for clusters (kT > 1:7 keV) and groups/ellipticals mean by the usual classi cation attributed to each system in the (kT < 1:7 keV) is shown separately. The mean value of each mean literature. distribution (and corresponding errors) is shown by the vertical While compared to SPEXACT v2, the Fe abundances dashed lines (and lled areas around them). measured in clusters remain essentially unchanged, the Fe abundances in groups and ellipticals are systematically re- vised upwards when using SPEXACT v3. This result is bet- (0:75  0:04) and groups/ellipticals (0:58  0:03) are signi - ter quanti ed in Figure 2, where the distribution of Fe abun- cantly di erent. In other words, spectral ts obtained using dances is compared between clusters and groups/ellipticals, updated atomic data indicate that the average concentra- using the two versions of the code. Based on the entire sam- tion of Fe in the hot haloes of groups and giant ellipticals is ple, the SPEXACT v3 results provide a mean Fe abundance the same as that in clusters of galaxies. of 0:74 0:03 with an intrinsic scatter of 25% (computed fol- Systems for which kT lies within 2{3 keV exhibit mean lowing the method described in Paper I). When splitting both Fe-L and Fe-K lines, hence their Fe abundance can the sample, we nd consistent average Fe abundances of be constrained by each of these two features separately. 0:75 0:04 and 0:70 0:03 for clusters and groups/ellipticals, We test this approach on M 87 and EXO 0422, both hav- respectively. This is in contrasts with the SPEXACT v2 re- ing good data quality. Compared to the Fe estimated from sults, where the average Fe abundance values for clusters the "full band" ts, we nd that the <2 keV local ts (Fe-L Downloaded from https://academic.oup.com/mnrasl/advance-article-abstract/doi/10.1093/mnrasl/sly080/4993253 by Ed 'DeepDyve' Gillespie user on 08 June 2018 Fe (0.1 r ) Number of systems 4 F. Mernier et al. 1.6 lines only) provide negligible biases (-2% and +6% for M 87 Groups/ Clusters and EXO 0422, respectively). These biases become some- Ellipticals what larger (respectively +15% and -12%) in the >2 keV 1.4 local ts (Fe-K lines only); however they are not system- atic and remain well below the typical 25% scatter reported 1.2 above. Although this issue is well known (Rasia et al. 2008; Simionescu et al. 2009) and concerns less than 14% of our 1.0 systems, this mismatch will deserve attention with future high-resolution spectroscopy missions. 0.8 3.1 The code-related Fe-bias kT mean 0.6 Fe In clusters, the Fe abundance determination is predomi- kT /kT up low nantly based on the prominent Fe-K lines. Since only the 0.4 low temperature groups/ellipticals are signi cantly a ected 1 2 3 4 5 6 kT [SPEXACT v3] by the update of SPEXACT, the reason for such a change mean is to be found in the Fe-L emission, which is dominant at Figure 3. Relative deviations on the parameters Y , kT , Fe, mean kT . 2 keV. In order to better understand the code- mean and the ratio kT =kT when EPIC mock spectra of 3T plasma up low related Fe bias that we report above, we adapt an instructive (simulated using SPEXACT v3 for various initial mean tempera- exercise previously introduced in Mernier et al. (2017) and tures) are tted using SPEXACT v2. The two horizontal dashed de Plaa et al. (2017). In short, we start by using SPEXACT lines indicate the 10% relative deviations. v3 to simulate mock EPIC spectra with 100 ks exposure on a grid of various kT values. In all these simulations mean 72 3 Y and the abundances are assumed to be 10 m and 1 proto-solar, respectively. Moreover, kT and kT are as- up low sumed such that kT =kT = 2:8. As a second step, we t up low these mock spectra using SPEXACT v2 with Y , Fe, kT , mean kT , and kT as free parameters. The relative deviation up low 1 of these SPEXACT v2 best- t parameters with respect to their input SPEXACT v3 values is shown in Figure 3 as a function of the input mean temperature. As expected from our results above, the Fe consistency between the two ver- sions of SPEXACT is excellent in the clusters regime, while 0.1 SPEXACT v3 (sim. spectrum): all parameters = [init. sim.] SPEXACT v2: all parameters = [init. sim.] it dramatically deteriorates when the plasma becomes cooler SPEXACT v2: Fe = [best-fit] than 2 keV. In addition, other interesting e ects occur SPEXACT v2: Y,Fe = [best-fit] SPEXACT v2: all parameters = [best-fit] in the groups/ellipticals regime. Below kT . 1:5 keV mean and kT . 1 keV, the ratio kT =kT and Y are re- mean up low 0.5 spectively under- and overestimated by SPEXACT v2. The 0.0 mean temperature, however, remains reasonably reproduced by SPEXACT v2, except for very hot plasmas where kT mean 0.5 1 2 is at most 15% underestimated (though without a ecting Energy (keV) the Fe abundance). Figure 4. EPIC MOS 2 simulated spectrum of a 3T plasma To better understand all the biases we observe in cool with kT = 0:7 keV, using SPEXACT v3. For comparison, mean plasmas with the EPIC instruments, we investigate further we show the same model calculated using SPEXACT v2 (red). the case of a 3T plasma simulated for 100 ks with SPEX- Then, we progressively x the Fe (orange), Y (green), and eventu- ACT v3, assuming kT = 0:7 keV (Figure 4, black data mean ally kT , kT , and kT (blue) to their a posteriori best- t mean up low points). A direct comparison of this simulated spectrum with SPEXACT v2 values. The residuals of such models with respect its equivalent model using SPEXACT v2 (Figure 4, red line) to the input simulated spectrum are shown in the bottom panel. shows signi cant discrepancies throughout the entire Fe-L complex (0.6{1.2 keV). In fact, the emissivity of many im- portant lines (e.g. Fe XVII at 0.73 keV; Fe XVIII at 0.77 keV) were revised lower with the update of SPEXACT, while new transitions were incorporated and/or updated with a ing kT to provide a formally acceptable { but incorrect { low higher emissivity (e.g. Fe XVIII at 1.18 keV). When xing best- t to the input spectrum (Figure 4, blue line). the Fe abundance to its best- t value estimated a posteriori In summary, in cool plasmas the emission measure, the by SPEXACT v2 (Figure 4, orange line), the emitting bump Fe abundance, and the width of the temperature distribution at 0.7 get smoother, in better agreement with the overall (kT =kT ) in uence each other to reproduce the observed up low shape of the Fe-L complex. However, over the entire soft shape of the unresolved Fe-L complex. As a consequence, band the ux signi cantly decreases, which the t attempts even outdated spectral codes can reasonably t the Fe-L to "correct" by increasing Y (Figure 4, green line). Finally, complex, yet providing strongly biased measurements. This the t smooths the residual bumps (in particular around conspiracy between all these parameters explain the code- 0.9{1 keV) by simultaneously decreasing kT and increas- related Fe-bias that we report in this Letter. up Downloaded from https://academic.oup.com/mnrasl/advance-article-abstract/doi/10.1093/mnrasl/sly080/4993253 by Ed 'DeepDyve' Gillespie user on 08 June 2018 [SPEXACT v2]/[SPEXACT v3] deviations Rel. err. Counts/s/keV Fe enrichment in ellipticals, groups, and clusters 5 4 IMPLICATIONS FOR THE IRON CONTENT recent chemo- and hydrodynamical simulations in a com- IN ELLIPTICALS, GROUPS AND panion paper (Truong et al. submitted), to which we refer CLUSTERS the reader for a more detailed discussion. We remind that these integrated measurements cover By measuring Fe abundances within 0.1r in a self- 0.1r , without further information on their inner or outer consistent way and using the latest SPEXACT version avail- spatial distributions. The question of whether clusters and able to date, we report for the rst time similar Fe abun- groups/ellipticals are really self-similar in terms of metal dances in ellipticals, galaxy groups, and galaxy clusters. In enrichment would require at least to derive the individual other words, gas-phase metallicities remain constant across abundance pro les for the entire sample using SPEXACT v3 two orders of magnitude in halo mass. (for a similar work using SPEXACT v2, see Mernier et al. These new results contradict previous papers (e.g. 2017). Because of the non-negligible time required by SPEX- Rasmussen & Ponman 2009; Bregman et al. 2010; Sun 2012; ACT v3 to t each spectrum, we leave such a study for future Yates et al. 2017), which reported systematically lower Fe work. abundances in groups and/or ellipticals with respect to the In addition to the code-related Fe bias discussed in this hotter clusters of galaxies (although Konami et al. (2014) work, we also note from Figure 3 that tting the spectra reported similar average Fe abundances as reported here, of cool systems with an outdated plasma code may also albeit for ellipticals only). Rather than SPEX, most of bias the emission measure, the mean temperature and the those previous studies used many (very di erent) versions kT =kT ratio by +35%, +7%, and -24%, respectively. In up low of APEC to t their data, making a direct comparison turn, these biases may have consequences on the estimates of with this work dicult. All these (mostly outdated) atomic further interesting quantities. For instance, we estimate that codes, however, likely encountered similar problems of a too the ICM pressure, usually de ned as P = n kT , can be biased simplistic modelling of the Fe-L transitions. From a theo- high by 19% in the case of a 0.7 keV plasma. Unlike the 2=3 retical perspective, that trend was not trivial to explain. pressure, the ICM entropy, usually de ned as K = kT=n , For example, when comparing the observational trend with remains very close to its true value, with a underestimate a semi-analytic model, Yates et al. (2017) did not succeed of less than 1%. Similarly, the total hydrostatic mass is to reproduce the previously reported positive temperature- not expected to be a ected by more than a few percent, as metallicity correlation in galaxy groups. Instead, the metal temperature and density gradients do not change dramati- content in low-mass systems is systematically overestimated cally. A more precise quanti cation, however, is left to future by their model. work. Our results also reveal the complication of measuring Our present results have interesting consequences, in accurately the temperature structure of lower-mass systems, particular given that the investigated systems exhibit very as long as the Fe-L complex remains unresolved by the ob- di erent stellar- to ICM-mass fractions. Because this frac- serving instruments. tion is lower in rich clusters than in less massive systems, Finally, it should be reminded that no spectral code is invariant Fe abundances could be explained only if the ef- perfect. It is certain that further improvements on SPEX- fective ICM enrichment considerably increases with the mass ACT will be pursued in the future, with potential implica- of the system. Such requirements have been dicult to rec- tions on the interpretation of moderate resolution spectra of oncile with the observed stellar populations in clusters so X-ray sources. In that respect, micro-calorimeters onboard far (e.g. Loewenstein 2013; Renzini & Andreon 2014). The future missions such as XARM and Athena will enable us story, however, is di erent if the Fe present in the ICM to observe the Fe-L complex with unprecedented resolution. is unrelated to the current stellar population of these sys- These observations will be invaluable to better understand tems. In addition to the increasing evidence towards an all the radiation processes in the ICM and push our knowl- early ICM enrichment in cluster outskirts (e.g. Werner et al. edge of astrophysical plasma emission to the next level. 2013; Simionescu et al. 2017; Urban et al. 2017), central Fe peaks were also found to be in place already at z  1 (De Grandi et al. 2014; Mantz et al. 2017) and exhibit the same radial distribution as SNcc products (Mernier et al. ACKNOWLEDGEMENTS 2017). These recent ndings suggest that recent SNIa ex- plosions and stellar mass loss from central galaxies do not The authors thank the referee for their constructive com- signi cantly contribute to the ICM enrichment. ments that helped to improve the manuscript as well as Ki- In this context, the similar Fe abundances found in hot ran Lakhchaura for fruitful discussions. F.M. is supported haloes spanning di erent mass ranges constitute an addi- by the Lendulet LP2016-11 grant awarded by the Hungar- tional support toward this early enrichment scenario, even ian Academy of Sciences. This work is partly based on the in their central parts. Since they grow hierarchically, iso- XMM-Newton AO-12 proposal \The XMM-Newton view of lated massive ellipticals and assembling groups can be seen chemical enrichment in bright galaxy clusters and groups " as the rst steps of the formation of more massive clus- (PI: de Plaa), and is a follow-up of the CHEERS (CHEmi- ters. Although, admittedly, nearby groups may have di er- cal Evolution Rgs cluster Sample) collaboration; the authors ent speci c properties (star formation, AGN feedback, etc.) thank all its members. This work is based on observations than high-redshift proto-clusters, the mass-invariance of Fe obtained with XMM-Newton, an ESA science mission with abundances at low redshift suggests that the bulk of met- instruments and contributions directly funded by ESA mem- als in hot haloes was already in place well before clusters ber states and the USA (NASA). The SRON Netherlands In- e ectively assembled. Our new measurements are directly stitute for Space Research is supported nancially by NWO, confronted to (and are found to be in good agreement with) the Netherlands Organisation for Scienti c Research. Downloaded from https://academic.oup.com/mnrasl/advance-article-abstract/doi/10.1093/mnrasl/sly080/4993253 by Ed 'DeepDyve' Gillespie user on 08 June 2018 6 F. Mernier et al. 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Journal

Monthly Notices of the Royal Astronomical Society LettersOxford University Press

Published: May 5, 2018

Keywords: galaxies: clusters: intracluster medium; X-rays: galaxies: clusters; X-rays: galaxies

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