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

Reprinted with kind permission from Annual Reviews, 4139 El Camino Way, Palo Alto, California, USA

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Abstract. ROSAT observations indicate that approximately half of all nearby groups of galaxies contain spatially extended X-ray emission. The radial extent of the X-ray emission is typically 50-500 h^-1₁₀₀ kpc or approximately 10-50% of the virial radius of the group. Diffuse X-ray emission is generally restricted to groups that contain at least one early-type galaxy. X-ray spectroscopy suggests the emission mechanism is most likely a combination of thermal bremsstrahlung and line emission. This interpretation requires that the entire volume of groups be filled with a hot, low-density gas known as the intragroup medium. ROSAT and ASCA observations indicate that the temperature of the diffuse gas in groups ranges from approximately 0.3 keV to 2 keV. Higher temperature groups tend to follow the correlations found for rich clusters between X-ray luminosity, temperature, and velocity dispersion. However, groups with temperatures below approximately 1 keV appear to fall off the cluster L_X-T relationship (and possibly the L_X- and -T cluster relationships, although evidence for these latter departures is at the present time not very strong). Deviations from the cluster L_X-T relationship are consistent with preheating of the intragroup medium by an early generation of stars and supernovae.

There is now considerable evidence that most X-ray groups are real, physical systems and not chance superpositions or large-scale filaments viewed edge-on. Assuming the intragroup gas is in hydrostatic equilibrium, X-ray observations can be used to estimate the masses of individual systems. ROSAT observations indicate that the typical mass of an X-ray group is ~ 10¹³ h^-1₁₀₀ M out to the radius to which X-ray emission is currently detected. The observed baryonic masses of groups are a small fraction of the X-ray determined masses, which implies that groups are dominated by dark matter. On scales of the virial radius, the dominant baryonic component in groups is likely the intragroup medium.

Key Words: intragroup medium, temperature, metallicity, masses, dark matter

Table of Contents

• INTRODUCTION
• X-RAY TELESCOPES
• ROSAT
• ASCA
• PROPERTIES OF THE INTRAGROUP MEDIUM
• First ROSAT Results
• ROSAT Surveys of Groups
• Spatial Properties of the Intragroup Medium
• Spectral Properties
• CORRELATIONS
• T- Relation
• L_X- and L_X-T Relations
• Galaxy Richness and Optical Luminosity
• Morphological Content
• COSMOLOGICAL IMPLICATIONS OF X-RAY GROUPS
• The Physical Nature of Groups
• Mass Estimates
• Baryon Fraction
• Large-Scale Structure
• Moderate Redshift Groups
• Gravitational Lensing
• Cooling Flows
• Fossil Groups
• The Origin and Evolution of the Intragroup Medium
• The Local Group
• FUTURE WORK
• REFERENCES