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 theEinstein Extended Medium Sensitivity Survey, with the ASCA and ROSATsatellites. We find an X-ray temperature of 12.3 +3.1/-2.2 keV for thiscluster, and the ROSAT map reveals significant substructure. The hightemperature of MS1054-0321 is consistent with both its approximate velocitydispersion, based on the redshifts of 12 cluster members we have obtained atthe Keck and the Canada-France-Hawaii telescopes, and with its weak lensingsignature. The X-ray temperature of this cluster implies a virial mass ~ 7.4 x10^14 h^-1 solar masses, if the mean matter density in the universe equals thecritical value, or larger if Omega_0 < 1. Finding such a hot, massive clusterin the EMSS is extremely improbable if clusters grew from Gaussianperturbations in an Omega_0 = 1 universe. Combining the assumptions thatOmega_0 = 1 and that the intial perturbations were Gaussian with the observedX-ray temperature function at low redshift, we show that the probability ofthis cluster occurring in the volume sampled by the EMSS is less than a fewtimes 10^{-5}. Nor is MS1054-0321 the only hot cluster at high redshift; theonly two other $z > 0.5$ EMSS clusters already observed with ASCA also havetemperatures exceeding 8 keV. Assuming again that the initial perturbationswere Gaussian and Omega_0 = 1, we find that each one is improbable at the <10^{-2} level. These observations, along with the fact that these luminositiesand temperatures of the high-$z$ clusters all agree with the low-z L_X-T_Xrelation, argue strongly that Omega_0 < 1. Otherwise, the initial perturbationsmust be non-Gaussian, if these clusters' temperatures do indeed reflect theirgravitational potentials.Comment: 20 pages, 4 figures, To appear in 1 Aug 1998 ApJ (heavily revised version of original preprint