A Very Hot, High Redshift Cluster of Galaxies: More Trouble for Omega_0 = 1

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Abstract
We present X-ray spectra and images, from ASCA and ROSAT, and ground-based optical spectroscopy from CFHT and Keck, of MS1054-0321, the most distant (z =0.828) cluster of galaxies in the Extended Medium Sensitivity Survey. This cluster has a temperature of 14.7 +/- 4 keV, possibly as hot as any cluster yet measured. The high temperature is consistent with its velocity dispersion, based on 12 cluster redshifts, and with the high mass implied by the weak lensing signature seen by Luppino & Kaiser (1997). This temperature is nearly commensurate with its high X-ray luminosity (2-10 keV rest frame) of 2.3x10^45 h50^-2 erg/s if the relationship between L_x and T_x is the same at redshift 0.8 as nearby. This observation implies that, at least for the most luminous clusters, the L_x-T_x relationship has indeed not changed since redshift 0.8. Furthermore, the high temperature of this and other high-redshift clusters observed by ASCA implies that the number density of hot clusters has not changed dramatically since z=0.5-0.8. This lack of evolution directly contradicts all CDM-like models where the mass density Omega = 1, and therefore implies that Omega cannot be 1. A high resolution X-ray map shows that the cluster is not very regular, with extensions and sublumps. We do not detect any iron in the cluster gas, with a 90% upper limit on the iron abundance of 0.22 solar.

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