The Entire Virial Radius of the Fossil Cluster RXJ 1159 + 5531. II. Dark Matter and Baryon Fraction

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, {M}\star /{L}K=0.61+/- 0.11 {M}⊙ /{L}⊙ , consistent with stellar population synthesis models for a Milky Way initial mass function. We obtain a halo concentration, {c}200=8.4+/- 1.0, and virial mass, {M}200=(7.9+/- 0.6)× {10}13 {M}⊙ . 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, {f}{{b,200}}=0.134+/- 0.007, 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), {f}{{b,200}} increases by ≈ 0.015, 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 α -{R}e scaling relation for massive early-type galaxies. Performing our analysis in the context of MOND still requires a large DM fraction (85.0 % +/- 2.5 % 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 ˜ {10}14 {M}⊙ represents the mass scale above which dissipation is unimportant in the formation of the central regions of galaxy clusters.