4 Conclusion3 Physical Cosmology3.5 Ly Forest

3.6 Galaxy Clusters 

Clusters of galaxies are the largest gravitationally bound systems known to be in quasi-equilibrium. This allows for reliable estimates to be made of their mass as well as their dynamical and thermal attributes. The richest clusters, arising from 3 tex2html_wrap_inline1028 density fluctuations, can be as massive as tex2html_wrap_inline1030 solar masses, and the environment in these structures is composed of shock heated gas with temperatures of order tex2html_wrap_inline1032 degrees Kelvin which emits thermal bremsstrahlung and line radiation at X-ray energies. Also, because of their spatial size tex2html_wrap_inline1034 Mpc and separations of order tex2html_wrap_inline1036 Mpc, they provide a measure of nonlinearity on scales close to the perturbation normalization scale tex2html_wrap_inline1038 Mpc. Observations of the substructure, distribution, luminosity, and evolution of galaxy clusters are therefore likely to provide signatures of the underlying cosmology of our Universe, and can be used as cosmological probes in the easily observable redshift range tex2html_wrap_inline1040 .

3.6.1 Internal Structure 

Thomas et al. [59] have investigated the internal structure of galaxy clusters formed in high resolution N-body simulations of four different cosmological models, including standard, open, and flat but low density universes. They find that the structure of relaxed clusters is similar in the critical and low density universes, although the critical density models contain relatively more disordered clusters due to the freeze-out of fluctuations in open universes at late times. The profiles of relaxed clusters are very similar in the different simulations since most clusters are in a quasi-equilibrium state inside the virial radius and follow the universal density profile of Navarro et al. [52]. There does not appear to be a strong cosmological dependence in the profiles as suggested by previous studies of clusters formed from pure power law initial density fluctuations [27]. However, because more young and dynamically evolving clusters are found in critical density universes, Thomas et al. suggest that it may be possible to discriminate among the density parameters by looking for multiple cores in the substructure of the dynamic cluster population. They note that a statistical population of 20 clusters can distinguish between open and critically closed universes.

3.6.2 Number Density Evolution 

The evolution of the number density of rich clusters of galaxies can be used to compute tex2html_wrap_inline948 and tex2html_wrap_inline1044 (the power spectrum normalization on scales of tex2html_wrap_inline1038 Mpc) when numerical simulation results are combined with the constraint tex2html_wrap_inline1048, derived from observed present-day abundances of rich clusters. Bahcall et al. [10] computed the evolution of the cluster mass function in five different cosmological model simulations and find that the number of high mass (Coma-like) clusters in flat, low tex2html_wrap_inline1044 models (ie. the standard CDM model with tex2html_wrap_inline1052) decreases dramatically by a factor of approximately tex2html_wrap_inline1054 from z =0 to tex2html_wrap_inline1058 . For low tex2html_wrap_inline948, high tex2html_wrap_inline1044 models, the data results in a much slower decrease in the number density of clusters over the same redshift interval. Comparing these results to observations of rich clusters in the real Universe, which indicate only a slight evolution of cluster abundances to redshifts tex2html_wrap_inline1064, they conclude that critically closed standard CDM and Mixed Dark Matter (MDM) models are not consistent with the observed data. The models which best fit the data are the open models with low bias (tex2html_wrap_inline1066 and tex2html_wrap_inline1068), and flat low density models with a cosmological constant (tex2html_wrap_inline1070 and tex2html_wrap_inline1072).

3.6.3 X-Ray Luminosity Function 

The evolution of the X-ray luminosity function, and the size and temperature distribution of rich clusters of galaxies are all potentially important discriminants of cosmological models. Bryan et al. [17] investigated these properties in a high resolution numerical simulation of a standard CDM model normalized to COBE. Although the results are highly sensitive to grid resolution (see [7] for a discussion of the effects from resolution constraints on the properties of rich clusters), their primary conclusion, that the standard CDM model predicts too many bright X-ray emitting clusters and too much integrated X-ray intensity, is robust since an increase in resolution will only exaggerate these problems.

4 Conclusion3 Physical Cosmology3.5 Ly Forest

image Physical and Relativistic Numerical Cosmology
Peter Anninos
© Max-Planck-Gesellschaft. ISSN 1433-8351
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