|Annu. Rev. Astron. Astrophys. 2000. 38: 289-335
Copyright © 2000 by Annual Reviews. All rights reserved
X-ray telescopes launched in the 1990s have firmly established the presence of a hot X-ray-emitting intragroup medium in nearby groups of galaxies. X-ray observations suggest that many groups are real, physical systems. The masses of X-ray groups are substantial and make a significant contribution to the mass density of the universe. Although most of the mass in groups appears to be in dark matter, the intragroup medium may be the dominant baryonic component in the nearby universe.
While we have made significant progress towards understanding groups in the last decade, there are still many outstanding issues. Ambiguities about the proper spectral model for the gas and our inability to detect gas to a large fraction of the virial radius are particularly troubling because the resulting uncertainties propagate into cosmological applications. Furthermore, the contribution of individual galaxies to the observed X-ray emission remains a point of contention. Our ability to understand the intragroup medium has largely been limited by the poor spatial and spectral resolution of the X-ray instruments. This situation is about to change drastically, however, with the availability of new powerful X-ray telescopes. Recently, NASA successfully launched CHANDRA (formerly known as AXAF). This telescope will produce high-resolution X-ray images of groups (~ 1") that will allow the relative contribution of galaxies and diffuse gas to be quantified. In late 1999, the European Space Agency (ESA) launched XMM-Newton. Although the spatial resolution of XMM-Newton is poorer than that of CHANDRA, the collecting area of this telescope is much greater. Therefore, XMM-Newton will obtain the deepest X-ray exposures ever of nearby groups and will extend the studies of the group environment to higher redshifts. The combination of CHANDRA and XMM-Newton will probably answer many of the questions raised by the recent generation of X-ray telescopes.
I would like to thank my collaborators and colleagues, particularly Arif Babul, Dave Burstein, David Buote, David Davis, Steve Helsdon, Pat Henry, Lawrence Jones, Lori Lubin, Gary Mamon, Bill Mathews, Kyoko Matsushita, Chris Mullis, Richard Mushotzky, Gus Oemler, Trevor Ponman, Matthias Steinmetz, Jack Sulentic, Ben Weiner, and Ann Zabludoff for useful discussions on X-ray groups. I would also like to thank David Davis, Richard Mushotzky and Allan Sandage for comments on the manuscript. This work was supported in part by NASA under grant NAG 5-3529.