THE ENTIRE VIRIAL RADIUS OF THE FOSSIL CLUSTER RXJ 1159 + 5531. II. DARK MATTER AND BARYON FRACTION

Abstract

ABSTRACT In this second paper on the entire virial region of the relaxed fossil cluster RXJ 1159+5531, we present a hydrostatic analysis of the azimuthally averaged hot intracluster medium (ICM) using the results of Su et al. For a model consisting of ICM, stellar mass from the central galaxy (BCG), and an NFW dark matter (DM) halo, we obtain a good description of the projected radial profiles of ICM emissivity and temperature that yield precise constraints on the total mass profile. The BCG stellar mass component is clearly detected with a K-band stellar mass-to-light ratio, , consistent with stellar population synthesis models for a Milky Way initial mass function. We obtain a halo concentration, , and virial mass, . For its mass, the inferred concentration is larger than most relaxed halos produced in cosmological simulations with Planck parameters, consistent with RXJ 1159+5531 forming earlier than the general halo population. The baryon fraction at r 200, , is slightly below the Planck value (0.155) for the universe. However, when we take into account the additional stellar baryons associated with non-central galaxies and the uncertain intracluster light (ICL), increases by , consistent with the cosmic value and therefore no significant baryon loss from the system. The total mass profile is nearly a power law over a large radial range (∼0.2–10 R e ), where the corresponding density slope α obeys the scaling relation for massive early-type galaxies. Performing our analysis in the context of MOND still requires a large DM fraction ( at r = 100 kpc) similar to that obtained using the standard Newtonian approach. The detection of a plausible stellar BCG mass component distinct from the NFW DM halo in the total gravitational potential suggests that represents the mass scale above which dissipation is unimportant in the formation of the central regions of galaxy clusters.