X-ray bright groups and their galaxies

Combining X-ray data from the ROSAT PSPC and optical data drawn from the literature, we examine in detail the relationship between the X-ray and optical properties of X-ray bright galaxy groups. We find a relationship between optical luminosity and X-ray temperature consistent with that expected from self-similar scaling of galaxy systems, L-B proportional to T1.6+/-0.2. The self-similar form and continuity of the L-B:T relation from clusters to groups and the limited scatter seen in this relation, implies that the star formation efficiency is rather similar in all of these systems. We find that the bright extended X-ray components associated with many central galaxies in groups appear to be more closely related to the group than the galaxy itself, and we suggest that these are group cooling flows rather than galaxy haloes. In addition we find that the optical light in these groups appears to be more centrally concentrated than the light in clusters. We also use the optical and X-ray data to investigate whether early- or late-type galaxies are primarily responsible for pre-heating in groups. Using three different methods, we conclude that spiral galaxies appear to play a comparable role to early types in the pre-heating of the intragroup medium. This tends to favour models in which the pre-heating arises primarily from galaxy winds rather than active galactic nuclei and implies that spirals have played a significant role in the metal enrichment of the intragroup medium.