2.4. Baryons in Groups

Plasma associated with galaxies outside the great clusters makes a significant and still quite uncertain contribution to the baryon budget. The detected X-ray emission from plasma in groups is softer than for clusters, and softest for groups dominated by late-type galaxies. It is a reasonable presumption that the absence of detections often represents a lower virial temperature, rather than the absence of plasma.

We follow two approaches in estimating the mass in plasma associated with galaxies outside clusters. In the first, we start with a characteristic value for the ratio of plasma mass to gravitational mass in groups with X-ray detections. We apply this ratio to all galaxies, on the assumption that all field galaxies are likely to have inherited a similar plasma mass, to estimate the density in the warm phase (Section 2.4.1). To this we add an estimate from quasar absorption of the plasma mass in low surface density cool clouds that certainly are not counted in the X-ray measurements (Section 2.4.2). The second approach (Section 2.4.3) assumes the plasma-to-gravitational mass ratio is the same in clusters and the field.