|Annu. Rev. Astron. Astrophys. 2000. 38: 289-335
Copyright © 2000 by Annual Reviews. All rights reserved
3.1. First ROSAT Results
The great potential of ROSAT for group studies was demonstrated in early papers by Mulchaey et al (1993), Ponman & Bertram (1993). Each of these papers presented a detailed look at the X-ray properties of an individual group. Mulchaey et al (1993) studied the NGC 2300 group, a poor group dominated by an elliptical-spiral pair. The X-ray emission in the NGC 2300 group is not centered on any particular galaxy, but is instead offset from the elliptical galaxy NGC 2300 by several arcminutes. The X-ray emission can be traced to a radius of at least ~ 150 h-1100 kpc (~ 25"). Ponman & Bertram (1993) studied Hickson Compact Group 62 (HCG 62). In this case, the X-ray emission is extended to a radius of at least 210 h-1100 kpc (~ 18"). Although the presence of intragroup gas had been suggested by earlier Einstein observations, these ROSAT PSPC results were the first to unambiguously separate a diffuse component related to the group from emission associated with individual galaxies. The intragroup medium interpretation was also supported by the ROSAT PSPC spectra, which are well-fit by a thermal model with a temperature of approximately 1.0 keV (~ 107 K). The ROSAT PSPC spectrum of HCG 62 contained enough counts that Ponman & Bertram (1993) could also derive a temperature profile for the gas. Ponman & Bertram (1993) found evidence for cooler gas near the center of the group, which they interpreted as evidence for a cooling flow. Many of the X-ray properties of the NGC 2300 group and HCG 62 are consistent with the idea of these systems being scaled-down versions of more massive clusters.
The early ROSAT observations of groups also provided some surprises. For both groups, the gas metallicity derived from the X-ray spectra was much lower than the value found for rich clusters (~ 6% solar for NGC 2300 and ~ 15% solar for HCG 62, compared with ~ 20-30% solar found for clusters; Fukazawa et al 1998). The X-ray data were also used to estimate the total masses of the groups. In each case, the mass of the group is approximately 1013 h-1100 M. Comparing the total mass as measured by the X-ray data with the total mass in observed baryons, Mulchaey et al (1993), Ponman & Bertram (1993) concluded that the majority of mass in these groups is dark. In the case of the NGC 2300 group, Mulchaey et al (1993) estimated a baryon fraction that was low enough to be consistent with = 1 and the baryon fraction predicted by standard big bang nucleosynthesis. However, subsequent analysis of the ROSAT PSPC data suggests the true baryon fraction is higher in this group (David et al 1995, Pildis et al 1995, Davis et al 1996).