The enrichment history of the intracluster medium: a Bayesian approach

This work measures the evolution of the iron content in galaxy clusters by a rigorous analysis of the data of 130 clusters at 0.1<z<1.3. This task is made difficult by a) the low signal-to-noise ratio of abundance measurements and the upper limits, b) possible selection effects, c) boundaries in the parameter space, d) non-Gaussian errors, e) the intrinsic variety of the objects studied, and f) abundance systematics. We introduce a Bayesian model to address all these issues at the same time, thus allowing cross-talk (covariance). On simulated data, the Bayesian fit recovers the input enrichment history, unlike in standard analysis. After accounting for a possible dependence on X-ray temperature, for metal abundance systematics, and for the intrinsic variety of studied objects, we found that the present-day metal content is not reached either at high or at low redshifts, but gradually over time: iron abundance increases by a factor 1.5 in the 7 Gyr sampled by the data. Therefore, feedback in metal abundance does not end at high redshift. Evolution is established with a moderate amount of evidence, 19 to 1 odds against faster or slower metal enrichment histories. We quantify, for the first time, the intrinsic spread in metal abundance, 18+/-3 %, after correcting for the effect of evolution, X-ray temperature, and metal abundance systematics. Finally, we also present an analytic approximation of the X-ray temperature and metal abundance likelihood functions, which are useful for other regression fitting involving these parameters. The data for the 130 clusters and code used for the stochastic computation are provided with the paper.Comment: A&A, in pres

Making the observational parsimonious richness a working mass proxy

Richness, i.e., the number of bright cluster galaxies, is known to correlate with the cluster mass, however, to exploit it as mass proxy we need a way to estimate the aperture in which galaxies should be counted that minimizes the scatter between mass and richness. In this work, using a sample of 39 clusters with accurate caustic masses at 0.1<z<0.22, we first show that the scatter between mass and richness derived from survey data is negligibly small, as small as best mass proxies. The scatter turns out to be smaller than in some previous works and has a 90% upper limit of 0.05 dex in mass. The current sample, adjoining 76 additional clusters analyzed in previous works, establishes an almost scatterless, minimally evolving (if at all), mass-richness scaling in the redshift range 0.03<z<0.55. We then exploit this negligible scatter to derive the reference aperture to be used to compute richness and to predict the mass of cluster samples. These predicted masses have a total 0.16 dex scatter with caustic mass, about half of which is not intrinsic to the proxy, but related to the noisiness of the caustic masses used for test proxy performances. These results make richness-based masses of best quality and available for large samples at a low observational cost.Comment: A&A, in pres

Dim galaxies and outer halos of galaxies missed by 2MASS ? The near-infrared luminosity function and density

By using high-resolution and deep Ks band observations of early-type galaxies of the nearby Universe and of a cluster at z=0.3 we show that the two luminosity functions (LFs) of the local universe derived from 2MASS data miss a fair fraction of the flux of the galaxies (more than 20 to 30%) and a whole population of galaxies of central brightness fainter than the isophote used for detection, but bright enough to be included in the published LFs. In particular, the fraction of lost flux increases as the galaxy surface brightness become fainter. Therefore, the so far derived LF slopes and characteristic luminosity as well as luminosity density are underestimated. Other published near-infrared LFs miss flux in general, including the LF of the distant field computed in a 3 arcsec aperture.Comment: A&A in pres

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

[Abridged] The analysis of a sample of 52 clusters with precise and hypothesis-parsimonious measurements of mass 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. The intrinsic scatter at a fixed cluster mass we determine for gas mass fractions is smaller, 0.06+/-0.01 dex. The intrinsic scatter in both the stellar and gas mass fractions is a distinctive signature that 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. We found the the baryon (gas+stellar) fraction is fairly constant for clusters and groups with 13.7<lg(mass)<15.0 solar masses and it is offset from the WMAP-derived value by about 6 sigmas. The offset 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.Comment: MNRAS, in pres

A low-scatter survey-based mass proxy for clusters of galaxies

Estimates of cosmological parameters using galaxy clusters have the scatter in the observable at a given mass as a fundamental parameter. This work computes the amplitude of the scatter for a newly introduced mass proxy, the product of the cluster total luminosity times the mass-to-light ratio, usually referred as stellar mass. The analysis of 12 galaxy clusters with excellent total masses shows a tight correlation between the stellar mass, or stellar fraction, and total mass within r500 with negligible intrinsic scatter: the 90% upper limit is 0.06 dex, the posterior mean is 0.027 dex. This scatter is similar to the one of best-determined mass proxies, such as Yx, i.e. the product of X-ray temperature and gas mass. The size of the cluster sample used to determine the intrinsic scatter is small, as in previous works proposing low-scatter proxies because very accurate masses are needed to infer very small values of intrinsic scatter. Three-quarters of the studied clusters have lgM <~14 Msol, which is advantageous from a cosmological perspective because these clusters are far more abundant than more massive clusters. At the difference of other mass proxies such as Yx, stellar mass can be determined with survey data up to at least z=0.9 using upcoming optical near-infrared surveys, such as DES and Euclid, or even with currently available surveys, covering however smaller solid angles. On the other end, the uncertainty about the predicted mass of a single cluster is large, 0.21 to 0.32 dex, depending on cluster richness. This is largely because the proxy itself has ~0.10 dex errors for clusters of lgM<~ 14 Msol mass.Comment: A&A in pres