Baryon budget in groups and clusters of galaxies

This (http://arxiv.org/abs/0705.1726) is a very careful study of baryon budget in massive halos (M_500>5.e13 M_sun). They find that the total baryon fraction (including ICM gas, BCG+ICL+stars from other galaxies) in halos > 5.e13 M_sun is only slightly smaller than the universal baryon fraction (f_b=0.17; their Fig. 2). Thus clusters represent a fair sample of baryons in the universe, considering that the small deficit from the universal value is within systematic uncertainties, and that they only go out to R_500 and not R_200. Moreover, the fractional contribution of stars to total baryon budget increases with decreasing halo mass, such that (mass of stars+ICM)/(mass of stars+ICM+dark matter) = universal baryon fraction. Their other finding is that the ICL+BCG contribution can be dominated by the ICL (which can account for 80% of the ICL+BCG light); thus it is important to account for the diffuse ICL. The fractional (ICL+BCG)/(ICL+BCG+other galaxies) contribution decreases with radius in all halos (Fig. 5). The volume integrated ICL+BCG fraction of total stellar light decreases with increasing halo mass (Fig. 4); thus ICL is more important in groups than in clusters. This paper (http://arxiv.org/abs/0911.2230; Fig. 4) shows the baryon fraction going to much lower halo masses. The baryon fraction decreases quite rapidly with the decreasing halo mass for masses smaller than ~ 5.e13 Msun (this is the "missing baryon problem").