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^+3.1_-2.2 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 ∼7.4 × 10¹⁴ h^-1 M⊙, if the mean matter density in the universe equals the critical value (Ω₀ = 1), or larger if Ω₀ < 1. Finding such a hot, massive cluster in the EMSS is extremely improbable if clusters grew from Gauss an perturbations in an Ω₀ = 1 universe. Combining the assumptions that Ω₀ = 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 Ω₀ = 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 Ω₀ < 1. Otherwise, the initial perturbations must be non-Gaussian, if these clusters' temperatures do indeed reflect their gravitational potentials.