X-Ray Measurements of the Dark Matter Content of Galaxy Clusters

Abstract

Accurate characterization of the cluster baryon fraction depends on measuring the temperature and gas density profile of a large sample the clusters out to the full extent of the cluster atmosphere. Simulations show that the cluster baryon fraction depends on radius and that the present measurements are biased in different ways depending on the radius to which they are calculated. A present limitation is that few clusters have accurate radial temperature profiles; this will change with XMM and Chandra observations. Even with the present data, cluster baryon fractions have become quite stable when calculated out to a standard radius of 2.5 Mpc and typically fall between 20 – 30%. The real remaining uncertainty at this point is in possible systematic errors. Hard X-ray observatories, such the Rossi X-ray Timing Explorer, can test one of the important assumptions of the model: that the cluster magnetic field is dynamically unimportant at all radii. While the observations are not yet sensitive enough to constrain a dynamically important magnetic field, several planned hard X-ray experiments will.