Probing the Dark Matter and Gas Fraction in Relaxed Galaxy Groups with X‐Ray Observations from<i>Chandra</i>and<i>XMM‐Newton</i>

Abstract

We present radial mass profiles within 0:3rvir for 16 relaxed galaxy groups—poor clusters (kT range 1Y3 keV )&#13;\nselected for optimal mass constraints from the Chandra and XMM-Newton data archives. After accounting for the mass&#13;\nof hot gas, the resulting mass profiles are described well by a two-component model consisting of dark matter, represented&#13;\nby an NFW model, and stars from the central galaxy. The stellar component is required only for eight systems,&#13;\nfor which reasonable stellar mass-to-light ratios (M/LK) are obtained, assuming a Kroupa IMF. Modifying the&#13;\nNFW dark matter halo by adiabatic contraction does not improve the fit and yields systematically lower M/LK.&#13;\nIn contrast to previous results for massive clusters, we find that the NFW concentration parameter (cvir) for groups&#13;\ndecreases with increasing Mvir and is inconsistent with no variation at the 3 level. The normalization and slope of the&#13;\ncvir-Mvir relation are consistent with the standard CDM cosmological model with 8 1⁄4 0:9 (considering a 10% bias&#13;\nfor early forming systems). The small intrinsic scatter measured about the cvir-Mvir relation implies that the groups&#13;\nrepresent preferentially relaxed, early forming systems. The mean gas fraction ( f 1⁄4 0:05 0:01) of the groups measured&#13;\nwithin an overdensity 1⁄4 2500 is lower than for hot, massive clusters, but the fractional scatter (f /f 1⁄4 0:2)&#13;\nfor groups is larger, implying a greater impact of feedback processes on groups, as expected.