Are we missing baryons in galaxy clusters?

Abstract

The recent constraints on cosmological parameters obtained from the observations of the WMAP satellite limit the cosmic baryon fraction in a range that is larger than, and marginally consistent with, what is measured in galaxy clusters. This raises the question of whether or not we are considering all the ingredients of the cluster baryonic budget. Carefully weighing the baryons in X-ray-emitting plasma and stars in cluster galaxies, I conclude that the cluster baryonic pie consists of 13 (with a 1σ range of 8-19) per cent of stars, 70 (56-89) per cent of intracluster hot medium and 17 (0-33) per cent (and a probability of 73 per cent of being larger than 0) of 'other' baryons, presumably in the form of warm (105-107 K) material.