2007ApJ...669..158G

We present radial mass profiles within ∼0.3r_vir for 16 relaxed galaxy groups–poor clusters (kT range 1-3 keV) selected for optimal mass constraints from the Chandra and XMM-Newton data archives. After accounting for the mass of hot gas, the resulting mass profiles are described well by a two-component model consisting of dark matter, represented by an NFW model, and stars from the central galaxy. The stellar component is required only for eight systems, for which reasonable stellar mass-to-light ratios (M/L_K) are obtained, assuming a Kroupa IMF. Modifying the NFW dark matter halo by adiabatic contraction does not improve the fit and yields systematically lower M/L_K. In contrast to previous results for massive clusters, we find that the NFW concentration parameter (c_vir) for groups decreases with increasing M_vir and is inconsistent with no variation at the 3 σ level. The normalization and slope of the c_vir-M_vir relation are consistent with the standard ΛCDM cosmological model with σ₈=0.9 (considering a 10% bias for early forming systems). The small intrinsic scatter measured about the c_vir-M_vir relation implies that the groups represent preferentially relaxed, early forming systems. The mean gas fraction (f=0.05±0.01) of the groups measured within an overdensity Δ=2500 is lower than for hot, massive clusters, but the fractional scatter (σ_f/f=0.2) for groups is larger, implying a greater impact of feedback processes on groups, as expected.