redshift galaxy clusters ClJ 1113 . 1 − 2615 and ClJ 0152 . 7 − 1357

We present an analysis of Chandra observations of two high-redshift clusters of galaxies, ClJ1113.1−2615 at z = 0.725 and ClJ0152.7−1357 at z = 0.833. We find ClJ1113.1−2615 to be morphologically relaxed with a temperature of kT = 4.3 −0.4 keV and a mass (within the virial radius) of 4.3 −0.7 × 10 M⊙. ClJ0152.7−1357, 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 −0.8 keV and 5.2 +1.1 −0.9 keV respectively, and we estimate their respective masses to be 6.1 −1.5×10 M⊙ and 5.2 +1.8 −1.4×10 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 modelling 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 normalisation, 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 they were assembled at redshifts significantly higher than z = 0.8, as predicted in low ΩM models. 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. Subject headings: cosmology: observations – galaxies: clusters: general – galaxies: high-redshift galaxies: clusters: individual: (ClJ1113.1−2615, ClJ0152.7−1357) – intergalactic medium – X-rays: galaxies