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Popesso, Paola (2006): The RASS-SDSS galaxy cluster survey.: Correlating X-ray and optical properties of galaxy clusters.. Dissertation, LMU München: Fakultät für Physik



Galaxy clusters are the largest gravitationally bound systems in the universe. Clusters consist of three components: galaxies, gas, and dark matter. The galaxies themselves contribute the least, at most a few percent, to the total mass. The remainder consists of diffuse, hot gas (the intracluster medium, or ICM) and an unseen component which is needed to explain the gravitational stability of clusters (the dark matter). The two most obvious means of studying clusters of galaxies are by observing the optical light emitted from the constituent galaxies or the X-ray emission from the ICM. Clusters of galaxies, bound ensambles of hundreds of galaxies, are an ideal environment to study galaxy evolution and to learn how this is affected by different physical processes: gravity, starbursts and star formation, interactions with the intergalactic medium and galaxy-galaxy encounters. Since the very early works of Hubble in the thirties, it has been recognized that galaxies in dense environments differ systematically from those in low-density regions in their morphological types, stellar populations and gaseous content. When during the history of the Universe and why such environmental differences were established is currently one of the subjects of most intensive investigation in the international astrophysical community. On the other hand, clusters can teach us a great deal about cosmology. The distributions of galaxies on the sky shows a net-like structure in which thin walls and filaments surround large voids. The galaxy clusters are the nodes of this network. Therefore, they trace out the Large-Scale Structure (LSS) of the universe and can be used to study the LSS formation. Moreover, if clusters provide a 'fair sample' of the universe, then the fraction of their mass in baryons should equal the universal baryon fraction, known as $\Omega_b/\Omega_m$. Moreover, the evolution of cluster number density with redshift can determine the mass density parameter, known as $\Omega_M$, and possibly determine the equation of state (and nature) of the dark energy believed to be causing the expansion of the universe to accelerate. Thus, galaxy clusters have a twofold importance: first as laboratories of galaxy formation and evolution, and second as cosmological tool. The aim of this project is to study galaxy clusters from these two perspectives. For this purpose we use the largest optical and X-ray surveys ever realized, the Sloan Digital Sky Survey (SDSS) and the Rosat All Sky Survey (RASS), respectively, to conduct a multiwavelenght study of the properties of galaxy clusters. The project is called RASS-SDSS Galaxy Cluster Survey reflecting the name of the two big surveys used for this work. All the analyses are performed on two cluster samples specially created for the survey: the X-ray selected RASS-SDSS galaxy cluster catalog and a subsample of optically selected, isolated and spectroscopically confirmed Abell clusters. The project consists of two parts. The aim of the first part is to understand which role play the gravitational processes, galaxy mergers and collisions and the interaction with ICM in the process of galaxy formation and evolution. For this purpose, we study the variations of several properties of the cluster galaxy population such as the luminosity and spatial distribution, the morphological type mix, the Star Formation Rate (SFR) and stellar mass as a function of the environmental conditions and the cluster global properties. Our detailed analysis of the cluster individual and composite luminosity functions reveals that the LF clearly shows a bimodal behavior with an upturn and a evident steepening in the faint magnitude range in any SDSS band. The LF is well fitted by the sum of two Schechter functions. The bright end of the LF is found to be universal in all the clusters. The faint end of the LF is much steeper and varies significantly from system to system, when calculated within a fixed metric aperture. The variations are not ramdom however. The more massive a cluster, the lower its fraction of dwarf galaxies. This effect disappears when the cluster LF is calculated within the physical size of the system, as the virial radius ($r_{200}$). This indicates that the previously observed variations are due to aperture effects caused by the observed increase of the fraction of dwarf galaxies with the clustercentric distance. Our conclusion is that the shape of the cluster LF is universal in all the magnitude ranges when the LF is calculated within the virial region. Moreover, the analysis of the composite cluster LF per morphological type, shows that the upturn and the steepening at the faint end of the LF is caused by dwarf early type galaxies. These systems are quite rare in low density regions and appear to be a typical cluster population. We provide evidence that the process responsible for creating the excess population of dwarf early type galaxies in clusters is a threshold process that occurs when the density exceeds $\sim 500$ times the critical density of the Universe. We interpret our results in the context of the 'harassment' scenario, where faint early-type cluster galaxies are predicted to be the descendants of tidally-stripped late-type galaxies. In the same context, we investigate whether the cluster total star formation rate ($\Sigma SFR$) depends on the cluster global properties for a sample of 90 very nearby clusters. The total cluster SFR is given by the sum of the SFR of all the cluster members within the virial region. It is found to be proportional to the number of cluster galaxies involved ($N_{gal}$). The best relation between the total SFR and the cluster mass reflects the $N_{gal}-M$ relation, which is a power law with exponent smaller than 1. As a consequence, the more massive a cluster, the lower its number of cluster galaxies and total SFR per unit mass. The mean SFR per cluster galaxy ($\Sigma SFR/N_{gal}$) is constant troughout our cluster sample and does not depend on the global properties of the system. Moreover, in order to account for projection effects, we study the galaxy surface number density profile in our cluster sample. We find that clusters of different mass exibit different profiles. In the low and intermediate mass systems the best fit is provided by a core King profile, with the core radius decreasing with cluster mass, until, at the highest cluster masses, the profile is better represented by a cuspy Navarro, Frenk \& White profile. All these different analysis converge to the conclusion that the global properties of the cluster galaxy population, such as the luminosity distribution, the galaxy type mix, the mean and total cluster SFR are only weakly dependent on the cluster mass and X-ray luminosity. This suggests that the gravitational processes and the interaction galaxy-ICM are not likely to affect those properties of the cluster galaxy population. Only the spatial distribution of the cluster galaxies depends on the cluster mass, probably reflecting the different relaxation status of systems of different masses. Instead, the variations of the LF and the galaxy type mix with the clustercentric distance reflect a link between the galaxy formation process and the galaxy-galaxy encounters, as suggested by the 'harassment' scenario. In the second part of the thesis, galaxy clusters are used as cosmological tool. The aim of this work is to elucidate which component, galaxies or ICM, traces better the cluster mass in order to understand whether different selection methods select the same cluster population. This will clarify which bias is introduced by the different selection methods in the results of the cosmological tests. This will clarify which bias is introduced by the different selection methods in the results of the cosmological tests. For this porpuse, we analyse as a first step the relation between the optical ($L_{op}$) and the X-ray ($L_X$) luminosity, respectively, to the cluster mass in the X-ray selected RASS-SDSS cluster sample. The main motivation in deriving these dependences is to evaluate $L_{op}$ and $L_X$, as predictors of the cluster mass and to compare the quality of the two quantities as predictors. Our analysis reveals that $L_{op}$ is a key measure of the cluster mass. In this respect, the optical luminosity performs even better than the X-ray luminosity, which suggests that the mass distribution of a cluster is better traced by cluster galaxies rather than by intracluster gas. On the other hand, our conclusion is at odds with the generally accepted view that a cluster main physical properties are more easily revealed in the X-ray than in the optical. Such a view was established at an epoch when the lack of optical wide field surveys precluded a reliable determination of the optical luminosities of a large sample of clusters. With the advent of the Sloan Digital Sky survey, this problem is now overcome. The application of the same analysis to an optically selected cluster sample (the Abell subsample) confirms the result. Neverthless, the Abell sample comprises a subpopulation of systems which scatter significantly in the $L_X-M$ relation and appear to be extremely X-ray underluminous (on average one order of magnitude) with regard to their mass. On the other hand, these systems do follow the general scaling relation between optical luminosity and virial mass. Therefore, we call them 'Abell X-ray Underluminous clusters' or AXU clusters for short. To understand the particular nature of these systems, we examine the properties of their galaxy population. The velocity distribution of the AXU clusters is Gaussian within the virial region but is leptokurtic (more centrally concentrated than a Gaussian) in the outskirts, as expected for the systems in accretion. In addition, the AXU clusters have a higher fraction of blue galaxies in the external region and show a marginally significant paucity of galaxies at the center. Our results seem to support the interpretation suggested by Bower et al. (1997) that the AXU clusters are systems in formation undergoing a phase of mass accretion. Their low X-ray luminosity should be due to the still accreting Intracluster gas or to an ongoing merging process. Our results give supports to the conclusion of Donahue et al. (2002) concerning the biases inherent in the selection of galaxy clusters in different wavebands. While the optical selection is prone to substantial projection effects, also the X-ray selection is not perfect or not simple to characterize. The existence of X-ray underluminous clusters, even with large masses, makes it difficult to reach the needed completeness in mass for cosmological studies. Clearly, a multi-waveband approach is needed for optimizing the completeness and reliability of clusters samples. The 'RASS-SDSS Galaxy Clusters Survey' series comprises 7 scientific papers which are inserted as part of the thesis. Four of the papers are accepted for pubblication on a scientific Journal ('Astronomy & Astrophysics') and three are submitted.