A Very Hot High‐Redshift Cluster of Galaxies: More Trouble for Ω₀= 1

We present X-ray spectra and images, obtained with the ASCA and ROSAT satellites, and ground-based optical spectroscopy, performed in Hawaii at the Canada-France-Hawaii Telescope and Keck Telescopes, of MS1054-03, the most distant (z = 0.828) cluster of galaxies in the Extended Medium Sensitivity Survey (EMSS). We find that the hot gas in this cluster has a temperature of 14.7 ± 4.6 3.5 keV, possibly as hot as any cluster yet measured. The high temperature is consistent with our preliminary 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 2.3 × 10 45 h −2 50 erg s −1 or a bolometric luminosity of 4.2 × 10 45 erg s −1, if the relationship between Lx and Tx is the same at redshift 0.8 as nearby. This observation implies that, at least for