ROSAT PSPC OBSERVATIONS OF COOL RICH CLUSTERS

We present the results of ROSAT PSPC observations of three cool (kT le ss than or equal to 3 keV) Abell, clusters of galaxies (A262, A539, an d A2589). Spatially resolved X-ray spectroscopy shows that the gas in these clusters is essentially isothermal beyond the central cooling ho w region. The PSPC spectra of A262 also show excess soft X-ray absorpt ion 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 held of view with a slight increa se in absorption at large radii. We show that this excess absorption c oincides with a region of galactic cirrus observed by IRAS. Using the 100 mu m flux density toward A262, we derive a column density of molec ular gas that is comparable to the column density of atomic gas and ca n account for the excess absorption. The spatial distribution of the m olecular gas is also consistent with the observed variation in soft X- ray absorption across the PSPC held of view, Using the observed densit y and temperature profiles of the hot gas in these clusters, we determ ine their gravitating masses from the equation of hydrostatic equilibr ium. The anisotropy parameter of the member galaxies is then calculate d from the observed line-of-sight velocity dispersion and the X-ray-de termined mass. For A262, we find that the early-type galaxies are viri alized with a nearly isotropic velocity dispersion, while most of the spiral galaxies cannot be virialized for any value of the anisotropy p arameter. There is also a strong alignment between the optical isophot es of the central dominant galaxy, the cluster X-ray isophotes in both A262 and A2589, and the large-scale orientation of the host superdust er. These results are consistent with a formation scenario in which cl usters form through the accretion of gas and galaxies along large-scal e filaments. We also calculate the entropy profile of the hot gas for a sample of groups, cool clusters, and hot clusters. We find that the gas entropy in cool clusters increases very gradually with radius whil e rich clusters show a much steeper gradient. The entropy of the core gas (beyond the cooling flow region) is essentially a constant in grou ps and rich clusters. These results suggest that the diffuse gas in gr oups and clusters was preheated before, or during, gravitational colla pse. The marginal radial increase in the gas entropy in groups indicat es that gravitational heating was almost negligible and the gas passed through weak shocks only as it was accreted. The steeper entropy grad ient in rich clusters is consistent with the stronger shock heating ex pected in more massive systems.