STScI Preprint #1247

A Very Hot, High Redshift Cluster of Galaxies: More Trouble for ₀= 1

We have observed the most distant (= 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 MS1054-0321 is consistent with both its approximate velocity dispersion, based on the redshifts of 12 cluster members 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 (₀ = 1), or larger if ₀ < 1. Finding such a hot, massive cluster in the EMSS is extremely improbable if clusters grew from Gaussian perturbations in an ₀ = 1 universe. Combining the assumptions that 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 MS1054-0321 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 ₀ =1, we find that each one is improbable at the < 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 ₀ < 1. Otherwise, the initial perturbations must be non-Gaussian, if these clusters' temperatures do indeed reflect their gravitational potentials.

Status:

Appeared in: The Astrophysical Journal, 502:550-557, 1998

Affiliations:

¹)Space Telescope Science Institute 3700 San Martin Drive Baltimore, MD 21218, USA
²)Institute for Astronomy 2680 Woodlawn Drive Honolulu, HI 96822
³)Department of Physics and Astronomy Rutgers University P.O. Box 849 Piscataway, NJ 08855-0849
⁴)University of Colorado Center for Astrophysics and Space Astronomy CB 389 Boulder, CO 80309
⁵) Home Institution: Istituto di Radioastronomia del CNR, Via Gobetti, 101 40129 Bologna, Italy
⁶) Visiting Astronomer at CFHT, operated by the National Research Council of Canada, le Centre National de la Recherche Scientifique de France and the University of Hawaii, and at the W. M. Keck Observatory, jointly operated by the California Institute of Technology and the University of California.