The intragroup medium in loose groups of galaxies

We have used the ROSAT PSPC to study the properties of a sample of 24 X-ray-bright galaxy groups, representing the largest sample examined in detail to date. Hot plasma models are fitted to the spectral data to derive temperatures, and modified King models are used to characterize the surface brightness profiles.
In agreement with previous work, we find evidence for the presence of two components in the surface brightness profiles. The extended component is generally found to be much flatter than that observed in galaxy clusters, and there is evidence that the profiles follow a trend with system mass. We derive relationships between X-ray luminosity, temperature and optical velocity dispersion. The relation between X-ray luminosity and temperature is found to be L _X∝ T ^4.9, which is significantly steeper than the same relation in galaxy clusters. These results are in good agreement with pre-heating models, in which galaxy winds raise the internal energy of the gas, inhibiting its collapse into the shallow potential wells of poor systems.