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Paper: X-ray Observations of Poor Groups
Volume: 209, Small Galaxy Groups, IAU Colloquium 174
Page: 174
Authors: Mulchaey, John S.
Abstract: X-ray observations of groups have provided considerable new insight into these cosmologically important systems. Recent ROSAT surveys of loose and compact groups suggest that &sim 50% of all nearby groups contain a hot intragroup medium, including many spiral-rich groups. However, spiral-only groups tend not to be X-ray detected. Spectroscopic studies of X-ray groups indicate that these systems contain a large number of faint galaxies. The existence of a large population of faint galaxies, combined with the extended X-ray halos, is consistent with X-ray detected groups being real, bound systems. The nature of non-X-ray detected groups, which do not contain an extensive faint galaxy population, is still unclear. ROSAT observations have also revealed the existence of individual elliptical galaxies with X-ray properties and total masses similar to those of X-ray groups. These objects may be the merger remnants of X-ray groups or `failed groups' (i.e. overdensities that formed only a single galaxy). The number density of these objects suggests they make a non-negligible contribution to the mass density of the universe.
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