Session 111 - X-Ray Observations of Clusters of Galaxies.
Display session, Thursday, January 18
North Banquet Hall, Convention Center

## [111.03] ROSAT Observations of Cool Clusters

L. P. David, C. Jones, W. Forman (SAO)

We present the results of ROSAT PSPC observations of 3 cool (KT \leq 3 keV) Abell clusters of galaxies (A262, A539, and A2589). Spatially resolved x-ray spectroscopy shows that the gas in these clusters is essentially isothermal beyond the central cooling flow region. The PSPC data for A262 show evidence for excess soft x-ray absorption above that expected from the galactic hydrogen column density. The excess absorption is not confined to the central cooling flow region, but is distributed across the entire field of view. We show that this excess absorption coincides with galactic cirrus emission in the direction of A262 and is probably due to molecular gas within our galaxy. Using the x-ray determined gravitating masses of these clusters, we are able to calculate the anisotropy parameter of the member galaxies from their line-of-sight velocity dispersion. For example, in A262 we find that the early-type galaxies must have an isotropic velocity dispersion, while most of the spiral galaxies are not yet virialized. We also show that the entropy of the gas in the central regions of groups and rich clusters is essentially a constant, which suggests that the gas was preheated before, or during, cluster collapse.