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Session 12 - Cosmology, Large-Scale Structure and Distance Scales.
Display session, Monday, June 10
The observed distributions of velocity dispersions and X-ray temperatures in clusters of galaxies are related to the theoretical mass function of these systems in the standard (Ømega_0=1) CDM cosmogony. We obtain M(\sigma_los) and M(kT_X) relations as a function of the linear bias parameter b in spheres of 8\ h^-1 Mpc. The inferred relations are unique inside any finite range of the variables. We find that, for b\mathrel\hbox\rlap\hbox\lower4pt\hbox\sim\hbox< 1.5, \sigma_los and T_X vary very slowly with M, as observed in cosmological simulations. The comparison of these correlations with the ones predicted for different cluster models is used to further constrain the biasing. Two models are examined: the classical \beta-model, which assumes that the intracluster medium and the galaxies are isotropic isothermal gases in hydrostatic equilibrium with the cluster potential; and (2) a less constrained model, with the same spatial distributions of the luminous components adopted for the former model, but with non-uniform velocity dispersions and temperatures, and a slight galaxy velocity anisotropy. For the \beta-model, the two values of b obtained from the galaxies and from the intracluster gas are higher than 2.0 and different from each other, reflecting the incompatibility of the kinematics adopted for these components with their observed spatial repartitions. Consistent values of b can only be obtained by means of the non-isothermal and anisotropic cluster model. This latter gives b\sim 1.5, in reasonable agreement with other independent estimates. Such a relatively low bias yields also total cluster masses similar to those inferred from gravitational lensing techniques and \sim 3 times larger than the ones derived in classical dynamical analysis. Accordingly, this second cluster model leads to a barion fraction three times smaller than conventionally found. This is just about the right amount needed to solve the problem of the barion fraction excess in flat universes.
Program listing for Monday