ROSATPSPC Observations of Cool Rich Clusters

Abstract

We present the results of ROSAT PSPC observations of three cool (kT ≤ 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 spectra of A262 also show 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 with a slight increase in absorption at large radii. We show that this excess absorption coincides with a region of galactic cirrus observed by IRAS. Using the 100 μm flux density toward A262, we derive a column density of molecular gas that is comparable to the column density of atomic gas and can account for the excess absorption. The spatial distribution of the molecular gas is also consistent with the observed variation in soft X-ray absorption across the PSPC field of view. Using the observed density and temperature profiles of the hot gas in these clusters, we determine their gravitating masses from the equation of hydrostatic equilibrium. The anisotropy parameter of the member galaxies is then calculated from the observed line-of-sight velocity dispersion and the X-ray—determined mass. For A262, we find that the early-type galaxies are virialized with a nearly isotropic velocity dispersion, while most of the spiral galaxies cannot be virialized for any value of the anisotropy parameter. There is also a strong alignment between the optical isophotes of the central dominant galaxy, the cluster X-ray isophotes in both A262 and A2589, and the large-scale orientation of the host supercluster. These results are consistent with a formation scenario in which clusters form through the accretion of gas and galaxies along large-scale 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 while rich clusters show a much steeper gradient. The entropy of the core gas (beyond the cooling flow region) is essentially a constant in groups and rich clusters. These results suggest that the diffuse gas in groups and clusters was preheated before, or during, gravitational collapse. The marginal radial increase in the gas entropy in groups indicates that gravitational heating was almost negligible and the gas passed through weak shocks only as it was accreted. The steeper entropy gradient in rich clusters is consistent with the stronger shock heating expected in more massive systems.