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 similar to0.4), based on the UK deep X-ray survey of M-c Hardy et al. 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 (L (X) greater than or similar to10(42.5) erg s(-1) ), 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 intragroup medium (IGM) than to cosmological parameters. Energy injection into the IGM (from, for example, supernovae or active galactic nuclei winds) is required to explain the X-ray properties of nearby groups. The observed lack of evolution suggests that the energy injection occurred at redshifts z >0.5.