Gus Evrard -- U. Michigan

        Advances in X-ray Clusters and Numerical Cosmology 

There is now fairly convincing observational evidence supporting the 
notion that the intracluster gas content and structure in 
galaxy clusters is strikingly regular.  An example is 
the X-ray (isophotal) size-temperature relation : a distance 
indicator for clusters with scatter competitive with the fundamental 
plane method of elliptical galaxies.  The small scatter in this and 
newly derived luminosity-temperature relations put stringent limits 
on the level of intrinsic variations in intracluster gas fraction within 
the virial regions of clusters.  The regularity of gas content makes 
determination of the population mean value straightforward 
$\bar{f}_{gas}(r_{500}) = 0.060 \pm 0.003 h^{_3/2}$, and this value 
points toward a cosmology with mass density significantly less than 
the critical value $\Omega_m h^{2/3} = 0.030 \pm 0.007 (\pm .003)$.  

Numerical simulations of cluster formation continue to play a crucial 
role in data interpretation.  The family of observed clusters is not 
self--similar (little clusters are not scaled down versions of big 
clusters), and the direction away and degree of departure from 
self-similarity are matched by models which incorporate the effects 
of galactic winds on the intracluster plasma.  But can such
simulations be trusted?  I will provide some insight into this 
question by sharing results from a blind comparison test of twelve 
independent, cosmological gas dynamics codes applied to the formation 
of a single galaxy cluster.