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 ^ { +0.5 } _ { -0.4 } \mbox { $ \mathrm { ~ { } keV } $ } and a mass ( within the virial radius ) of 4.3 ^ { +0.8 } _ { -0.7 } \times 10 ^ { 14 } M _ { \odot } .
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 ^ { +0.9 } _ { -0.8 } \mbox { $ \mathrm { ~ { } keV } $ } and 5.2 ^ { +1.1 } _ { -0.9 } \mbox { $ \mathrm { ~ { } keV } $ } respectively , and we estimate their respective masses to be 6.1 ^ { +1.7 } _ { -1.5 } \times 10 ^ { 14 } M _ { \odot } and 5.2 ^ { +1.8 } _ { -1.4 } \times 10 ^ { 14 } M _ { \odot } 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 \approx 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 \beta -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 \approx 0.8 , and thus that they were assembled at redshifts significantly higher than z = 0.8 , as predicted in low \Omega _ { M } 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 .