The X-ray properties of a sample of high redshift ( z > 0.6 ) , massive clusters observed with XMM - Newton and Chandra are described , including two exceptional systems .
One , at z=0.89 , has an X-ray temperature of T=11.5 ^ { +1.1 } _ { -0.9 } keV ( the highest temperature of any cluster known at z > 0.6 ) , an estimated mass of \approx ( 1.4 \pm 0.2 ) x10 ^ { 15 } \thinspace M _ { \odot } , and appears relaxed .
The other , at z=0.83 , has at least three sub-clumps , probably in the process of merging , and may also show signs of faint filamentary structure at large radii , observed in X-rays .
In general there is a mix of X-ray morphologies , from those clusters which appear relaxed and containing little substructure to some highly non-virialized and probably merging systems .
The X-ray gas metallicities and gas mass fractions of the relaxed systems are similar to those of low redshift clusters of the same temperature , suggesting that the gas was in place , and containing its metals , by z \approx 0.8 .
The evolution of the mass-temperature relation may be consistent with no evolution or with the ‘ ‘ late formation ’ ’ assumption .
The effect of point source contamination in the ROSAT survey from which these clusters were selected is estimated , and the implications for the ROSAT X-ray luminosity function discussed .