The second most distant cluster of galaxies in the extended medium sensitivity survey

We report on our ASCA, Keck, and ROSAT observations of MS 1137.5 + 6625, the second most distant cluster of galaxies in the Einstein Extended Medium Sensitivity Survey (EMSS), at redshift 0.78. We now have a full set of X-ray temperatures, optical velocity dispersions, and X-ray images for a complete, high-redshift sample of clusters of galaxies drawn from the EMSS. Our ASCA observations of MS 1137.5 + 6625 yield a temperature of 5.7^+2.1_-1.1 keV and a metallicity of 0.43^+0.40_-0.37 solar, with 90% confidence limits. Keck II spectroscopy of 22 clster members reveals a velocity dispersion of 884⁺¹⁸⁵_-124 km s^-1. This cluster is the most distant in the sample with a detected iron line. We also derive a mean abundance at z = 0.8 by simultaneously fitting X-ray data for the two z = 0.8 clusters, and obtain an abundance of Z_Fe = 0.33 ±^0.26_0.23. Our ROSAT observations show that MS 1137.5 + 6625 is regular and highly centrally concentrated. Fitting of a β model to the X-ray surface brightness yields a core radius of only 71 h^-1 kpc (q₀ = 0.1) with β= 0.70 ± ^0.45_0.15. The gas mass interior to 0.5 h^-1 Mpc is thus 1.2 ± ^0.2_0.3 x 10¹³ h^-5/2 M_⊙ (q₀ = 0.1). If the cluster's gas is nearly isothermal and in hydrostatic equilibrium with the cluster potential, the total mass of the cluster within this same region is 2.1 ± ^1.5_0.8 × 10¹⁴ h^-1 M_⊙, giving a gas fraction of 0.06 ± 0.04 h^-3/2. This cluster is the highest redshift EMSS cluster showing evidence for a possible cooling flow (∼ 20-400 M⊙ yr^-1). The velocity dispersion, temperature, gas fraction, and iron abundance of MS 1137.5 + 6625 are all statistically the same as those properties in lower redshift clusters of similar luminosity. With this cluster's temperature now in hand, we derive a high-redshift temperature function for EMSS clusters at 0.5 < z < 0.9 and compare it with temperature functions at lower redshifts, showing that the evolution of the temperature function is relatively modest. Supplementing our high-redshift sample with other data from the literature, we demonstrate that neither the cluster luminosity-temperature relation, nor cluster metallicities, nor the cluster gas fraction has detectably evolved with redshift. The very modest degree of evolution in the luminosity-temperature relation inferred from these data is inconsistent with the absence of evolution in the X-ray luminosity functions derived from ROSAT cluster surveys if a critical density structure formation model is assumed.