A very hot high-redshift cluster of galaxies:: More trouble for Ω0=1

We have observed the most distant (z = 0.829) cluster of galaxies in the Einstein Extended Medium Sensitivity Survey (EMSS) with the ASCA and ROSAT satellites. We find an X-ray temperature of 12.3(-2.2)(+3.1) keV for this cluster, and the ROSAT map reveals significant substructure. The high temperature of MS 1054-0321 is consistent with both its approximate velocity dispersion, based on the redshifts of 12 cluster members that we have obtained at the Keck and the Canada-France-Hawaii telescopes, and with its weak lensing signature. The X-ray temperature of this cluster implies a virial mass similar to 7.4 x 10(14) h(-1) M., if the mean matter density in the universe equals the critical value (Ohm(0) = 1), or larger if Ohm(0) < 1. Finding such a hot, massive cluster in the EMSS is extremely improbable if clusters grew from Gaussian perturbations in an Ohm(0) = 1 universe. Combining the assumptions that Ohm(0) = 1 and that the intial perturbations were Gaussian with the observed X-ray temperature function at low redshift, we show that this probability of this cluster occurring in the volume sampled by the EMSS is less than a few times 10(-5). Nor is MS 1054-0321 the only hot cluster at high redshift; the only two other z > 0.5 EMSS clusters already observed with ASCA also have temperatures exceeding 8 keV. Assuming again that the initial perturbations were Gaussian and Ohm(0) = 1, we find that each one is improbable at the less than 10(-2) level. These observations, along with the fact that these luminosities and temperatures of the high-z clusters all agree with the low-z L-X-T-X relation, argue strongly that Ohm(0) < 1. Otherwise, the initial perturbations must be non-Gaussian, if these clusters' temperatures do indeed reflect their gravitational potentials.