Annu. Rev. Astron. Astrophys. 2000. 38: 289-335 Copyright © 2000 by Annual Reviews. All rights reserved

5.5. Moderate Redshift Groups

Despite the cosmological significance of groups, remarkably little is known about these systems at high redshift. Optical studies of high redshift groups have been limited because low galaxy densities make groups difficult to recognize even at moderate redshifts. X-ray emission from the intragroup medium provides a potentially useful method for finding groups at high redshift. A number of searches for faint, extended X-ray sources have been performed in recent years using deep ROSAT PSPC observations (e.g. Rosati et al 1995, Griffiths et al 1995, Scharf et al 1997, Burke et al 1997, Jones et al 1998, Schmidt et al 1998, Vikhlinin et al 1998, Zamorani et al 1999). Although the goal of these surveys is often to find rich clusters of galaxies at high redshift, many X-ray groups at redshifts z = 0.1-0.6 have also been found. Unfortunately, the ROSAT observations of these groups generally contain very few counts, so it is not possible to determine the temperature or the metallicity of the gas with the existing data. However, studies of the spectral properties of the intragroup medium out to z ~ 0.3 will be possible with both XMM and CHANDRA. Furthermore, deep images with these telescopes will likely uncover X-ray groups at even higher redshifts. Therefore, the first studies of the evolution of the intragroup medium should be possible within the next decade.