Probing the Dark Matter and Gas Fraction in Relaxed Galaxy Groups with X‐Ray Observations fromChandraandXMM‐Newton

We present radial mass profiles within 0: 3rvir for 16 relaxed galaxy groups—poor clusters (kT range 1Y3 keV) for optimal mass constraints from the Chandra and XMM-Newton data archives. After accounting for the mass hot gas, the resulting mass profiles are described well by a two-component model consisting of dark matter, represented an NFW model, and stars from the central galaxy. The stellar component is required only for eight systems, which reasonable stellar mass-to-light ratios (M/LK) are obtained, assuming a Kroupa IMF. Modifying the dark matter halo by adiabatic contraction does not improve the fit and yields systematically lower M/LK. contrast to previous results for massive clusters, we find that the NFW concentration parameter (cvir) for groups with increasing Mvir and is inconsistent with no variation at the 3 level. The normalization and slope of the -Mvir relation are consistent with the standard CDM cosmological model with 8 1⁄4 0: 9 (considering a 10% bias early forming systems). The small intrinsic scatter measured about the cvir-Mvir relation implies that the groups preferentially relaxed, early forming systems. The mean gas fraction (f 1⁄4 0: 05 0: 01) of the groups measured an overdensity 1⁄4 2500 is lower than for hot, massive clusters, but the fractional scatter (f /f 1⁄4 0: 2) groups is larger, implying a greater impact of feedback processes on groups, as expected.