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_∗/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, c₂₀₀ = 8.4 ± 1.0, and virial mass, M₂₀₀ = (7.9±0.6)×10¹³M_⊙. 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, ∫_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), ∫_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 obeys the - R_e scaling relation for massive early-type galaxies. Performing our analysis in the context of MOND still requires a large DM fraction (82.0% ± 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 ∼10¹⁴ M_⊙ represents the mass scale above which dissipation is unimportant in the formation of the central regions of galaxy clusters.