2003ApJ...587..589M

We present an analysis of Chandra observations of two high-redshift clusters of galaxies, Cl J1113.1-2615 at z=0.725 and Cl J0152.7-1357 at z=0.833. We find Cl J1113.1-2615 to be morphologically relaxed with a temperature of kT=4.3^+0.5_–0.4keV and a mass (within the virial radius) of 4.3^+0.8_–0.7x10¹⁴ M_☉. Cl J0152.7-1357, by contrast, is resolved into a northern and southern subcluster, each massive and X-ray-luminous, in the process of merging. The temperatures of the subclusters are found to be 5.5^+0.9_–0.8and 5.2^+1.1_–0.9 keV, respectively, and we estimate their respective masses to be 6.1^+1.7_–1.5x10¹⁴ and 5.2^+1.8_–1.4x10¹⁴ M_☉within the virial radii. A dynamical analysis of the system shows that the subclusters are likely to be gravitationally bound. If the subclusters merge, they will form a system with a mass similar to that of the Coma Cluster. Two-dimensional modeling of the X-ray surface brightness reveals excess emission between the subclusters, suggestive, but not conclusive, evidence of a shock front. We make a first attempt at measuring the cluster M-T relation at z~0.8 and find no evolution in its normalization, supporting the previous assumption of an unevolving M-T relation when constraining cosmological parameters from cluster evolution studies. A comparison of the cluster properties with those of nearby systems also finds little or no evolution in the L-T relation, the gas fraction-T relation, the β-T relation, or the metallicity. These results suggest that, in at least some massive clusters, the hot gas was in place, and containing its metals, at z~0.8 and thus that the clusters were assembled at redshifts significantly higher than z=0.8, as predicted in low-Ω_Mmodels. We also highlight the need to correct for the degradation of the Chandra ACIS low-energy quantum efficiency in high-redshift cluster studies when the low-energy absorption is often assumed to be the Galactic value, rather than measured.