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Register a new userCosmological Implications and Physical Properties of an X-Ray Flux-Limited Sample of Galaxy Clusters
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Thomas H. Reiprich
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The original abstract significantly exceeds the space available here, so heres a brief summary. The abstract is similar to the abstract of astro-ph/0111285 (ApJ, 567, 716) which describes the X-ray galaxy cluster sample HIFLUGCS, the X-ray luminosity--gravitational mass relation, the cluster mass function, and the derived cosmological constraints. Additionally, the fraction of the total gravitating mass in the universe which is contained in intracluster gas is quantified. Furthermore, physical properties of the cluster sample have been studied and analyses of relations between different cluster parameters (including the gas mass fraction, gas temperature, X-ray luminosity, gas mass, gravitational mass, beta, and core radius) are discussed. Also, results from an analysis of XMM-Newton performance verification phase data of Abell 1835 are described.
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We present first strong observational evidence that the X-ray cool-core bias or the apparent bias in the abundance of relaxed clusters is absent in our REFLEX volume-limited sample (ReVols). We show that these previously observed biases are due to the survey selection method such as for an flux-limited survey, and are not due to the inherent nature of X-ray selection. We also find that the X-ray luminosity distributions of clusters for the relaxed and for the disturbed clusters are distinct and a displacement of approximately 60 per cent is required to match two distributions. Our results suggest that to achieve more precise scaling relation one may need to take the morphology of clusters and their fractional abundance into account.
The thermal properties of hydrodynamical simulations of galaxy clusters are usually compared to observations by relying on the emission-weighted temperature T_ew, instead of on the spectroscopic X-ray temperature T_spec, which is obtained by actual observational data. In a recent paper Mazzotta et al. show that, if the cluster is thermally complex, T_ew fails at reproducing T_spec, and propose a new formula, the spectroscopic-like temperature, T_sl, which approximates T_spec better than a few per cent. By analyzing a set of hydrodynamical simulations of galaxy clusters, we find that T_sl is lower than T_ew by 20-30 per cent. As a consequence, the normalization of the M-T_sl relation from the simulations is larger than the observed one by about 50 per cent. If masses in simulated clusters are estimated by following the same assumptions of hydrostatic equilibrium and beta--model gas density profile, as often done for observed clusters, then the M-T relation decreases by about 40 per cent, and significantly reduces its scatter. Based on this result, we conclude that using the observed M-T relation to infer the amplitude of the power spectrum from the X-ray temperature function could bias low sigma_8 by 10-20 per cent. This may alleviate the tension between the value of sigma_8 inferred from the cluster number density and those from cosmic microwave background and large scale structure.
We present results on the X-ray properties of clusters and groups of galaxies, extracted from a large hydrodynamical simulation. We used the GADGET code to simulate a LambdaCDM model within a box of 192 Mpc/h on a side, with 480^3 dark matter particles and as many gas particles. The simulation includes radiative cooling, star formation and supernova feedback. The simulated M-T relation is consistent with observations once we mimic the procedure for mass estimates applied to real clusters. Also, with the adopted choices of Omega_m=0.3 and sigma_8=0.8 the resulting XTF agrees with observational determinations. The L-T relation also agrees with observations for clusters with T>2 keV, with no change of slope at the scale of groups. The entropy in central cluster regions is higher than predicted by gravitational heating alone, the excess being almost the same for clusters and groups. The simulated clusters appear to have suffered some overcooling, with f*~0.2, thus about twice as large as the value observed. Interestingly, temperature profiles are found to steadily increase toward cluster centers. They decrease in the outer regions, much like observational data do at r>0.2r_vir, while not showing an isothermal regime followed by a smooth temperature decline in the innermost regions.
The mass function of galaxy clusters is a sensitive tracer of the gravitational evolution of the cosmic large-scale structure and serves as an important census of the fraction of matter bound in large structures. We obtain the mass function by fitting the observed cluster X-ray luminosity distribution from the REFLEX galaxy cluster survey to models of cosmological structure formation. We marginalise over uncertainties in the cosmological parameters as well as those of the relevant galaxy cluster scaling relations. The mass function is determined with an uncertainty less than 10% in the mass range 3 x 10^12 to 5 x 10^14 M$_odot$. For the cumulative mass function we find a slope at the low mass end consistent with a value of -1, while the mass rich end cut-off is milder than a Schechter function with an exponential term exp($- M^delta$) with $delta$ smaller than 1. Changing the Hubble parameter in the range $H_0 = 67 - 73 km s^-1 Mpc^{-1}$ or allowing the total neutrino mass to have a value between 0 - 0.4 eV causes variations less than the uncertainties. We estimate the fraction of mass locked up in galaxy clusters: about 4.4% of the matter in the Universe is bound in clusters (inside $r_200$) with a mass larger than 10^14 M$_odot$ and 14% to clusters and groups with a mass larger than 10^13 M$_odot$ at the present Universe. We also discuss the evolution of the galaxy cluster population with redshift. Our results imply that there is hardly any clusters with a mass > 10^15 M$_odot$ above a redshift of z = 1.
X-ray observations of galaxy clusters potentially provide powerful cosmological probes if systematics due to our incomplete knowledge of the intracluster medium (ICM) physics are understood and controlled. In this paper, we present mock Chandra analyses of cosmological cluster simulations and assess X-ray measurements of galaxy cluster properties using a model and procedure essentially identical to that used in real data analysis. We show that reconstruction of three-dimensional ICM density and temperature profiles is excellent for relaxed clusters, but still reasonably accurate for unrelaxed systems. The total ICM mass is measured quite accurately (<6%) in all clusters, while the hydrostatic estimate of the gravitationally bound mass is biased low by about 5%-20% through the virial region, primarily due to additional pressure support provided by subsonic bulk motions in the ICM, ubiquitous in our simulations even in relaxed systems. Gas fraction determinations are therefore biased high; the bias increases toward cluster outskirts and depends sensitively on its dynamical state, but we do not observe significant trends of the bias with cluster mass or redshift. We also find that different average ICM temperatures, such as the X-ray spectroscopic Tspec and gas-mass-weighted Tmg, are related to each other by a constant factor with a relatively small object-to-object scatter and no systematic trend with mass, redshift or the dynamical state of clusters. We briefly discuss direct applications of our results for different cluster-based cosmological tests.
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