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Register a new userThe similarity of the stellar mass fractions of galaxy groups and clusters
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We employ a large sample of 20171 optically-selected groups and clusters at 0.15 < z < 0.4 in the SDSS to investigate how the stacked stellar mass fraction varies across a wide range of total mass, $M_{500}$. Our study improves upon previous observational studies in a number of important ways, including having a much larger sample size, an explicit inclusion of the intracluster light (ICL) component, and a thorough examination of the accuracy of our total mass estimates via comparisons to simulations and weak lensing observations. We find that the stellar mass fraction depends only weakly on total mass and that the contribution of ICL to the total stellar mass fraction is significant (typically 20-40 per cent). Both of these findings are in excellent accordance with the predictions of cosmological simulations. Under the assumption of a Chabrier (Salpeter) IMF, the derived star formation efficiency ($f_{star}$/$f_{b}$, where $f_b=Omega_b/Omega_m$) is relatively low at 8 per cent (14 per cent) and is consistent with the global star formation efficiency of semi-analytic models that reproduce the galaxy stellar mass function. When our measured stellar mass fractions are combined with the observed relation between hot gas mass fraction and total mass from X-ray observations, our results imply that galaxy groups have significantly lower baryon fractions than massive clusters. Ejection of gas due to energetic AGN feedback (most likely at high redshift) provides a plausible mechanism for explaining the trends we observe.
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We investigate if the discrepancy between estimates of the total baryon mass fraction obtained from observations of the cosmic microwave background (CMB) and of galaxy groups/clusters persists when a large sample of groups is considered. To this purpose, 91 candidate X-ray groups/poor clusters at redshift 0.1 < z < 1 are selected from the COSMOS 2 deg^2 survey, based only on their X-ray luminosity and extent. This sample is complemented by 27 nearby clusters with a robust, analogous determination of the total and stellar mass inside R_500. The total sample of 118 groups and clusters with z < 1 spans a range in M_500 of ~10^13--10^15 M_sun. We find that the stellar mass fraction associated with galaxies at R_500 decreases with increasing total mass as (M_500)^-0.37 pm 0.04, independent of redshift. Estimating the total gas mass fraction from a recently derived, high quality scaling relation, the total baryon mass fraction (f_500^stars+gas=f_500^stars+f_500^gas) is found to increase by ~ 25% when M_500 increases from <M>=5 X 10^13 M_sun to <M> = 7 X 10^14 M_sun. After consideration of a plausible contribution due to intra--cluster light (11--22% of the total stellar mass), and gas depletion through the hierarchical assembly process (10% of the gas mass), the estimated values of the total baryon mass fraction are still lower than the latest CMB measure of the same quantity (WMAP5), at a significance level of 3.3sigma for groups of <M>=5 X 10^13~M_sun. The discrepancy decreases towards higher total masses, such that it is 1sigma at <M>= 7 X 10^14~M_sun. We discuss this result in terms of non--gravitational processes such as feedback and filamentary heating.
We analyse the stellar and hot gas content of 18 nearby, low-mass galaxy clusters, detected in redshift space and selected to have a dynamical mass 3E14<M/Msun<6E14, as measured from the 2dF Galaxy Redshift Survey. We combine X-ray measurements from both Chandra and XMM with ground-based near-infrared observations from CTIO, AAT and CFHT to compare the mass in hot gas and stars to the dynamical mass and state of the clusters. Only 13 of the clusters are detected in X-ray emission, and for these systems we find that a range of 7-20 per cent of their baryonic mass, and <3 per cent of their dynamical mass, is detected in starlight, similar to what is observed in more massive clusters. In contrast, the five undetected clusters are underluminous in X-ray emission, by up to a factor 10, given their stellar mass. Although the velocity distribution of cluster members in these systems is indistinguishable from a Gaussian, all show subtle signs of being unrelaxed: either they lack a central, dominant galaxy, or the bright galaxy distribution is less concentrated and/or more elongated than the rest of the sample. Thus we conclude that low-mass clusters and groups selected from the velocity distribution of their galaxies exhibit a dichotomy in their hot gas properties. Either they are detected in X-ray, in which case they generally lie on the usual scaling relations, or they are completely undetected in X-ray emission. The non-detections may be partly related to the apparently young dynamical state of the clusters, but it remains a distinct possibility that some of these systems are exceptionally devoid of hot emitting gas as the result of its expulsion or rarefaction.
There exist discrepancies in measurements of the number and evolution of galaxy pairs. The pair fraction appears to be sensitive to both the criteria used to select pair fraction and the methods used to analyze survey data. This paper explores the connection between stellar mass estimation and the pair fraction of galaxies making use of STEEL, the Statistical sEmi-Emprical modeL. Previous results have found the pair fraction is sensitive to choices made when selecting what qualifies as a pair, for example luminosity or stellar mass selections. We find that different estimations of stellar mass such as photometric choice mass-to-light ratio or IMF that effect the stellar mass function also significantly affect the derived galaxy pair fraction. By making use of the galaxy halo connection we investigate these systematic affects on the pair fraction. We constrain the galaxy halo connection using the stellar-mass-halo-mass relationship for two observed stellar mass functions, and the Illustris TNG stellar mass function. Furthermore, we also create a suite of toy models where the stellar-mass-halo-mass relationship is manually changed. For each stellar-mass-halo-mass relation the pair fraction, and its evolution, are generated. We find that enhancements to the number density of high mass galaxies cause steepening of the stellar-mass-halo mass relation, resulting in a reduction of the pair fraction. We argue this is a considerable cause of bias that must be accounted for when comparing pair fractions.
By means of the abundance matching technique, we infer the local stellar and baryonic mass-halo mass (Ms-Mh and Mb-Mh) relation for central blue and red galaxies separately in the mass range Ms~10^8.5-10^12.0 Msun. The observational inputs are the SDSS central blue and red Galaxy Stellar Mass Functions reported in Yang et al. 2009, and the measured local gas mass-Ms relations for blue and red galaxies. For the Halo Mass Function associated to central blue galaxies, the distinct LCDM one is used and set up to exclude: (i) the observed group/cluster mass function (blue galaxies are rare as centers of groups/clusters), and (ii) halos with a central major merger at resdshifts z<0.8 (dry late major mergers destroy the disks of blue galaxies). For red galaxies, we take the complement of this function to the total. The obtained mean Ms-Mh and Mb-Mh relations of central blue and red galaxies do not differ significantly from the respective relations for all central galaxies. For Mh>10^11.5 Msun, the Mss of red galaxies tend to be higher than those of blue ones for a given Mh, the difference not being larger than 1.7. For Mh<10^11.5 Msun, this trend is inverted. For blue (red) galaxies: (a) the maximum value of fs=Ms/Mh is 0.021^{+0.016}_{-0.009} (0.034{+0.026}_{-0.015}) and it is attained atlog(Mh/Msun)~12.0 (log(Mh/Msun)~11.9); (b) fspropto Mh (fspropto Mh^3) at the low-mass end while at the high-mass end, fspropto Mh^-0.4 (fspropto Mh^-0.6). The baryon mass fractions, fb=Mb/Mh, of blue and red galaxies reach maximum values of fb=0.028^{+0.018}_{-0.011} and fb=0.034^{+0.025}_{-0.014}, respectively. For Mh<10^11.3 Msun, a much steeper dependence of fb on Mh is obtained for the red galaxies than for the blue ones. We discuss on the differences found in the fs-Mh and fb-Mh relations between blue and red galaxies in the light of of semi-empirical galaxy models.
We present a catalogue of 348 galaxy clusters and groups with $0.2<z<1.2$ selected in the 2.78 $deg^2$ ALHAMBRA Survey. The high precision of our photometric redshifts, close to $1%$, and the wide spread of the seven ALHAMBRA pointings ensure that this catalogue has better mass sensitivity and is less affected by cosmic variance than comparable samples. The detection has been carried out with the Bayesian Cluster Finder (BCF), whose performance has been checked in ALHAMBRA-like light-cone mock catalogues. Great care has been taken to ensure that the observable properties of the mocks photometry accurately correspond to those of real catalogues. From our simulations, we expect to detect galaxy clusters and groups with both $70%$ completeness and purity down to dark matter halo masses of $M_hsim3times10^{13}rm M_{odot}$ for $z<0.85$. Cluster redshifts are expected to be recovered with $sim0.6%$ precision for $z<1$. We also expect to measure cluster masses with $sigma_{M_h|M^*_{CL}}sim0.25-0.35, dex$ precision down to $sim3times10^{13}rm M_{odot}$, masses which are $50%$ smaller than those reached by similar work. We have compared these detections with previous optical, spectroscopic and X-rays work, finding an excellent agreement with the rates reported from the simulations. We have also explored the overall properties of these detections such as the presence of a colour-magnitude relation, the evolution of the photometric blue fraction and the clustering of these sources in the different ALHAMBRA fields. Despite the small numbers, we observe tentative evidence that, for a fixed stellar mass, the environment is playing a crucial role at lower redshifts (z$<$0.5).
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