Probing the dark matter and gas fraction in relaxed galaxy groups with x-ray observations from Chandra and XMM-Newton

We present radial mass profiles within similar to 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/ LK) 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 sigma level. The normalization and slope of the c(vir)-Mvir relation are consistent with the standard Lambda CDM cosmological model with sigma(8) = 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 Delta = 2500 is lower than for hot, massive clusters, but the fractional scatter (sigma(f) /f = 0.2) for groups is larger, implying a greater impact of feedback processes on groups, as expected.