The RASS-SDSS galaxy cluster survey.
Galaxy clusters are the largest gravitationally bound systems in\nthe universe. Clusters consist of three components: galaxies, gas, and\ndark matter. The galaxies themselves contribute the least, at most a\nfew percent, to the total mass. The remainder consists of diffuse, hot gas (the intracluster medium, or ICM) and an unseen component which is\nneeded to explain the gravitational stability of clusters (the dark\nmatter).\n\nThe two most obvious means of studying clusters of galaxies are by\nobserving the optical light emitted from the constituent galaxies or\nthe X-ray emission from the ICM. Clusters of galaxies, bound ensambles\nof hundreds of galaxies, are an ideal environment to study galaxy\nevolution and to learn how this is affected by different physical\nprocesses: gravity, starbursts and star formation, interactions with\nthe intergalactic medium and galaxy-galaxy encounters. Since the very\nearly works of Hubble in the thirties, it has been recognized that\ngalaxies in dense environments differ systematically from those in\nlow-density regions in their morphological types, stellar populations\nand gaseous content. When during the history of the Universe and why\nsuch environmental differences were established is currently one of\nthe subjects of most intensive investigation in the international\nastrophysical community.\n\n\nOn the other hand, clusters can teach us a great deal about\ncosmology. The distributions of galaxies on the sky shows a net-like\nstructure in which thin walls and filaments surround large voids. The\ngalaxy clusters are the nodes of this network. Therefore, they trace\nout the Large-Scale Structure (LSS) of the universe and can be used to\nstudy the LSS formation. Moreover, if clusters provide a 'fair sample'\nof the universe, then the fraction of their mass in baryons should\nequal the universal baryon fraction, known as\n$\\Omega_b/\\Omega_m$. Moreover, the evolution of cluster number density\nwith redshift can determine the mass density parameter, known as\n$\\Omega_M$, and possibly determine the equation of state (and nature)\nof the dark energy believed to be causing the expansion of the\nuniverse to accelerate.\n\nThus, galaxy clusters have a twofold importance: first as laboratories\nof galaxy formation and evolution, and second as cosmological\ntool. The aim of this project is to study galaxy clusters from these\ntwo perspectives. For this purpose we use the largest optical and\nX-ray surveys ever realized, the Sloan Digital Sky Survey (SDSS) and\nthe Rosat All Sky Survey (RASS), respectively, to conduct a\nmultiwavelenght study of the properties of galaxy clusters. The\nproject is called RASS-SDSS Galaxy Cluster Survey reflecting the name\nof the two big surveys used for this work. All the analyses are\nperformed on two cluster samples specially created for the survey: the\nX-ray selected RASS-SDSS galaxy cluster catalog and a subsample of\noptically selected, isolated and spectroscopically confirmed Abell\nclusters.\n\nThe project consists of two parts. The aim of the first part is to\nunderstand which role play the gravitational processes, galaxy mergers\nand collisions and the interaction with ICM in the process of galaxy\nformation and evolution. For this purpose, we study the variations of\nseveral properties of the cluster galaxy population such as the\nluminosity and spatial distribution, the morphological type mix, the\nStar Formation Rate (SFR) and stellar mass as a function of the\nenvironmental conditions and the cluster global properties.\n\nOur detailed analysis of the cluster individual and composite\nluminosity functions reveals that the LF clearly shows a bimodal\nbehavior with an upturn and a evident steepening in the faint\nmagnitude range in any SDSS band. The LF is well fitted by the sum of\ntwo Schechter functions. The bright end of the LF is found to be\nuniversal in all the clusters. The faint end of the LF is much steeper\nand varies significantly from system to system, when calculated within\na fixed metric aperture. The variations are not ramdom however. The\nmore massive a cluster, the lower its fraction of dwarf galaxies. This\neffect disappears when the cluster LF is calculated within the\nphysical size of the system, as the virial radius ($r_{200}$). This\nindicates that the previously observed variations are due to aperture\neffects caused by the observed increase of the fraction of dwarf\ngalaxies with the clustercentric distance. Our conclusion is that the\nshape of the cluster LF is universal in all the magnitude ranges when\nthe LF is calculated within the virial region. Moreover, the analysis\nof the composite cluster LF per morphological type, shows that the\nupturn and the steepening at the faint end of the LF is caused by\ndwarf early type galaxies. These systems are quite rare in low density\nregions and appear to be a typical cluster population. We provide\nevidence that the process responsible for creating the excess\npopulation of dwarf early type galaxies in clusters is a threshold\nprocess that occurs when the density exceeds $\\sim 500$ times the\ncritical density of the Universe. We interpret our results in the\ncontext of the 'harassment' scenario, where faint early-type cluster\ngalaxies are predicted to be the descendants of tidally-stripped\nlate-type galaxies.\n\nIn the same context, we investigate whether the cluster total star\nformation rate ($\\Sigma SFR$) depends on the cluster global properties\nfor a sample of 90 very nearby clusters. The total cluster SFR is\ngiven by the sum of the SFR of all the cluster members within the\nvirial region. It is found to be proportional to the number of cluster\ngalaxies involved ($N_{gal}$). The best relation between the total SFR\nand the cluster mass reflects the $N_{gal}-M$ relation, which is a\npower law with exponent smaller than 1. As a consequence, the more\nmassive a cluster, the lower its number of cluster galaxies and total\nSFR per unit mass. The mean SFR per cluster galaxy ($\\Sigma\nSFR/N_{gal}$) is constant troughout our cluster sample and does not\ndepend on the global properties of the system.\n\nMoreover, in order to account for projection effects, we study the\ngalaxy surface number density profile in our cluster sample. We find\nthat clusters of different mass exibit different profiles. In the low\nand intermediate mass systems the best fit is provided by a core King\nprofile, with the core radius decreasing with cluster mass, until, at\nthe highest cluster masses, the profile is better represented by a\ncuspy Navarro, Frenk \\& White profile.\n\nAll these different analysis converge to the conclusion that the\nglobal properties of the cluster galaxy population, such as the\nluminosity distribution, the galaxy type mix, the mean and total\ncluster SFR are only weakly dependent on the cluster mass and X-ray\nluminosity. This suggests that the gravitational processes and the\ninteraction galaxy-ICM are not likely to affect those properties of\nthe cluster galaxy population. Only the spatial distribution of the\ncluster galaxies depends on the cluster mass, probably reflecting the\ndifferent relaxation status of systems of different masses. Instead,\nthe variations of the LF and the galaxy type mix with the\nclustercentric distance reflect a link between the galaxy formation\nprocess and the galaxy-galaxy encounters, as suggested by the\n'harassment' scenario.\n\n\nIn the second part of the thesis, galaxy clusters are used as\ncosmological tool. The aim of this work is to elucidate which\ncomponent, galaxies or ICM, traces better the cluster mass in order to\nunderstand whether different selection methods select the same cluster\npopulation. This will clarify which bias is introduced by the\ndifferent selection methods in the results of the cosmological\ntests. This will clarify which bias is introduced by the different\nselection methods in the results of the cosmological tests. For this\nporpuse, we analyse as a first step the relation between the optical\n($L_{op}$) and the X-ray ($L_X$) luminosity, respectively, to the\ncluster mass in the X-ray selected RASS-SDSS cluster sample. The main\nmotivation in deriving these dependences is to evaluate $L_{op}$ and\n$L_X$, as predictors of the cluster mass and to compare the quality of\nthe two quantities as predictors. Our analysis reveals that $L_{op}$\nis a key measure of the cluster mass. In this respect, the optical\nluminosity performs even better than the X-ray luminosity, which\nsuggests that the mass distribution of a cluster is better traced by\ncluster galaxies rather than by intracluster gas. On the other hand,\nour conclusion is at odds with the generally accepted view that a\ncluster main physical properties are more easily revealed in the X-ray\nthan in the optical. Such a view was established at an epoch when the\nlack of optical wide field surveys precluded a reliable determination\nof the optical luminosities of a large sample of clusters. With the\nadvent of the Sloan Digital Sky survey, this problem is now overcome.\n\nThe application of the same analysis to an optically selected cluster\nsample (the Abell subsample) confirms the result. Neverthless, the\nAbell sample comprises a subpopulation of systems which scatter\nsignificantly in the $L_X-M$ relation and appear to be extremely X-ray\nunderluminous (on average one order of magnitude) with regard to their\nmass. On the other hand, these systems do follow the general scaling\nrelation between optical luminosity and virial mass. Therefore, we\ncall them 'Abell X-ray Underluminous clusters' or AXU clusters for\nshort. To understand the particular nature of these systems, we\nexamine the properties of their galaxy population. The velocity\ndistribution of the AXU clusters is Gaussian within the virial region\nbut is leptokurtic (more centrally concentrated than a Gaussian) in\nthe outskirts, as expected for the systems in accretion. In addition,\nthe AXU clusters have a higher fraction of blue galaxies in the\nexternal region and show a marginally significant paucity of galaxies\nat the center. Our results seem to support the interpretation\nsuggested by Bower et al. (1997) that the AXU clusters are systems in\nformation undergoing a phase of mass accretion. Their low X-ray\nluminosity should be due to the still accreting Intracluster gas or to\nan ongoing merging process.\n\n\nOur results give supports to the conclusion of Donahue et al. (2002)\nconcerning the biases inherent in the selection of galaxy clusters in\ndifferent wavebands. While the optical selection is prone to\nsubstantial projection effects, also the X-ray selection is not\nperfect or not simple to characterize. The existence of X-ray\nunderluminous clusters, even with large masses, makes it difficult to\nreach the needed completeness in mass for cosmological\nstudies. Clearly, a multi-waveband approach is needed for optimizing\nthe completeness and reliability of clusters samples.\n\nThe 'RASS-SDSS Galaxy Clusters Survey' series comprises 7 scientific\npapers which are inserted as part of the thesis. Four of the papers\nare accepted for pubblication on a scientific Journal ('Astronomy & Astrophysics') and three are submitted.