|Annu. Rev. Astron. Astrophys. 2002. 40:
Copyright © 2002 by Annual Reviews. All rights reserved
Two interesting issues have arisen in the study of cluster gas and gravitational masses. First is the fact that total gravitating masses derived from weak gravitational lensing are a factor of a few higher than those derived from X-ray observations of cluster atmospheres assuming hydrostatic equilibrium of isothermal atmospheres (Loeb & Mao 1994, Miralda-Escude & Babul 1995). And second is the `baryon crisis', in which the baryonic mass of a cluster, which is dominated by the mass of gas in the hot cluster atmosphere, corresponds to roughly 5% of the gravitational mass derived assuming hydrostatic equilibrium for an isothermal cluster atmosphere. This baryon fraction is a factor of three to five larger than the baryon fraction dictated by big bang nucleosynthesis in inflationary world models (White et al. 1993).
A possible solution to both these problems is to invoke non-thermal pressure support for the cluster atmosphere, thereby allowing for larger gravitating masses relative to those derived assuming hydrostatic equilibrium. A number of authors have investigated the possibility of magnetic pressure support for cluster atmospheres (Loeb & Mao 1994, Miralda-Escude & Babul 1995, Dolag & Schindler 2000). The required fields are about 50µG, which is an order of magnitude, or more, larger than the measured fields in most cases, except perhaps in the inner 10's of kpc of cooling flow clusters. For most relaxed clusters Dolag & Schindler (2000) find that magnetic pressure affects hydrostatic mass estimates by at most 15%.
Other mechanisms for non-thermal pressure support of cluster atmospheres involve motions of the cluster gas other than thermal, such as turbulent or bulk motions due to a recent cluster merger (Mazzotta et al. 2001, Wu 2000). For relaxed clusters a number of groups have shown that the lensing and X-ray mass estimates can be reconciled by using non-isothermal models for the cluster atmospheres, i.e., by allowing for radial temperature gradients (Allen, Ettori, & Fabian 2001b, Markevitch et al. 1999).