The stellar mass fraction and baryon content of galaxy clusters and groups

arXiv:1004.2785v1 [astro-ph.CO] 16 Apr 2010 Mon. Not. R. Astron. Soc. 000, 1–?? (2009) Printed 17 August 2021 (MN LATEX style file v2.2) The stellar mass fraction and baryon content of galaxy clusters and groups S. Andreon,⋆ INAF–Osservatorio Astronomico di Brera, Milano, Italy Accepted ... Received ... ABSTRACT The analysis of a sample of 52 clusters with precise and hypothesis-parsimonious measurements of mass, derived from caustics based on about 208 member velocities per cluster on average, shows that low mass clusters and groups are not simple scaled- down versions of their massive cousins in terms of stellar content: lighter clusters have more stars per unit cluster mass. The same analysis also shows that the stellar content of clusters and groups displays an intrinsic spread at a given cluster mass, i.e. clusters are not similar each other in the amount of stars they contain, not even at a fixed cluster mass. The stellar mass fraction depends on halo mass with (logarithmic) slope −0.55 ± 0.08 and with 0.15 ± 0.02 dex of intrinsic scatter at a fixed cluster mass. These results are confirmed by adopting masses derived from velocity dispersion. The intrinsic scatter at a fixed cluster mass we determine for gas mass fractions taken from literature is smaller, 0.06 ± 0.01 dex. The intrinsic scatter in both the stellar and gas mass fractions is a distinctive signature that, when taken individually, the regions from which clusters and groups collected matter, a few tens of Mpc, are yet not representative, in terms of gas and baryon content, of the mean matter content of the Universe. The observed stellar mass fraction values are in marked disagreement with gasdynamics simulations with cooling and star formation of clusters and groups. Instead, amplitude and cluster mass dependency of observed stellar mass fractions are those requested not to need any AGN feedback to describe gas and stellar mass fractions and X-ray scale relations in simple semi-analytic cluster models. By adding stellar and gas masses and accounting for the intrinsic variance of both quantities, we found the the baryon fraction is fairly constant for clusters and groups with masses between 1013.7 and 1015.0 solar masses and it is offset from the WMAP-derived value by about 6 sigmas. The offset is unlikely to be due to an underestimate of the stellar mass fraction, and could be related to the possible non universality of the baryon fraction, pointed out by our measurements of the intrinsic scatter. Our analysis is the first that does not assume that clusters are identically equal at a given halo mass and it is also more accurate in many aspects. The data and code used for the stochastic computation are distributed with the paper. Key words: Galaxies: clusters: general — Galaxies: stellar content — Galaxies: luminosity function, mass function — Cosmology: observations X-ray: galaxy: clusters — methods: statistical 1 INTRODUCTION Knowledge of the baryon content of clusters and groups is a key ingredient in our understanding of the physics of these objects and in their use as cosmological probes. In fact, clus- ters have accreted matter from a region of some tens of Mpc, large enough that their content should be represen- ⋆stefano.andreon@brera.inaf.it tative of the mean matter content of the Universe (White et al. 1993). If this is the case, by measuring the baryon frac- tion in clusters, fb, and coupling it with an estimate of Ωb, for example from primordial nucleosynthesis arguments or from CMB anisotropies, gives Ωm = Ωb fb (e.g. White et al. 1993; Evrard et al. 1997). Second, the study of how baryons are distributed in gas and stars and the way this splitting depends on halo (cluster or group) mass, should provide 2 S. Andreon clues to the role played by the various physical mechanisms potentially active in clusters and groups. However, the baryon fraction is far from being fully understood: WMAP-derived value of the baryon fraction is larger than all values found in X-ray analysis (i.e. Vikhlinin et al. 2006) even accounting for baryons in stars (e.g. Gon- zalez, Zaritsky, & Zabludoff2007), and gas depletion (e.g. Nagai et al. 2007). X-ray scaling relations (e.g. halo mass vs Temperature or X-ray luminosity) predicted on the as- sumption that the thermal energy of the gas comes solely from the gravitational collapse are notoriously in disagree- ment with observed scalings (e.g. Vikhlinin et al. 2006). Ob- served and predicted scalings may be bring in agreement by allowing star formation, and, eventually a further feed- back (e.g. Kravsov et al. 2005, Nagai, Kravtsov, & Vikhlinin 2007; Bode et al. 2009; Fabjan et al. 2009). In particular, whether a further feedback, i.e. in addition to the stellar one, is needed, is largely unknown because of the uncertainty of the observed stellar mass content of clusters (e.g. Bode et al. 2009). More generally, recent works on the subject achieve to reproduce X-ray derived quantities (e.g. baryon fraction or m

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