ROSAT Observations of Distant Clusters of Galaxies
Session 5 -- ROSAT Observations of Clusters and Groups
Display presentation, Wednesday, January 12, 9:30-6:45, Salons I/II Room (Crystal Gateway)

## [5.03] ROSAT Observations of Distant Clusters of Galaxies

M. E. Donahue (STSCI), K. A. Arnaud (GSFC), J. T. Stocke (CU-CASA), S. L. Morris (DAO), I. M. Gioia (IFA, HI)

We have observed two distant clusters, MS0735+74 and MS0451-03, with the PSPC and HRI instruments on board the ROSAT spacecraft. MS0735+74 is a cooling flow candidate at a $z=0.216$, selected for the extended H$\alpha$ emission in its brightest central galaxy. MS0451-03, at a redshift of 0.55, is one of the most luminous clusters known with a luminosity of $>10^{45}$ erg/sec. We are able to constrain the column density of material able to absorb soft X-rays, and to place interesting limits on the gas temperatures. The MS0735+74 cluster is significantly cooler than predicted by the $L_x$ - $T_x$ relationship for low-redshift clusters. No excess soft absorption, other than that accounted for by 21 cm measurements of our own Galaxy, was seen in these objects. We estimate the core radius and isophotal extents of both galaxies, and estimate the mass cooling rate for MS0735+74. We also compare the H$\alpha$ surface brightness of the central galaxy of MS0735+74 with the HRI surface brightness contours. Preliminary analyses of the spectra of both clusters seem to indicate no significant soft-X-ray absorption owing to material intrinsic to the cluster, contrary to analyses done on Einstein spectra of other high redshift clusters by Wang \& Stocke. However, MS0735+74 is surprisingly cool for its X-ray luminosity. Cooler temperatures for a given X-ray luminosity at redshifts of order 0.3 are predicted by some models of cluster evolution.