The detection and X-ray evolution of galaxy groups at high redshift
  We describe some of the first X-ray detections of groups of galaxies at high redshifts (z~0.4), based on the UK deep X-ray survey of McHardy et al (1998). Combined with other deep ROSAT X-ray surveys with nearly complete optical identifications, we investigate the X-ray evolution of these systems. We find no evidence for evolution of the X-ray luminosity function up to z=0.5 at the low luminosities of groups of galaxies and poor clusters (Lx>3e42 erg/s), although the small sample size precludes very accurate measurements. This result confirms and extends to lower luminosities current results based on surveys at brighter X-ray fluxes. The evolution of the X-ray luminosity function of these low luminosity systems is more sensitive to the thermal history of the intra-group medium (IGM) than to cosmological parameters. Energy injection into the IGM (from for example supernovae or AGN winds) is required to explain the X-ray properties of nearby groups. The observed lack of evolution suggests that the energy injection occured at redshifts z>0.5. 
L. R. Jones, I. McHardy, A. Newsam, K. Mason