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We present radial mass profiles within 0.3 r_vir for 16 relaxed galaxygroups-poor clusters (kT range 1-3 keV) selected for optimal mass constraintsfrom the Chandra and XMM data archives. After accounting for the mass of hotgas, the resulting mass profiles are described well by a two-component modelconsisting of dark matter (DM), represented by an NFW model, and stars from thecentral galaxy. The stellar component is required only for 8 systems, for whichreasonable stellar mass-to-light ratios (M/L_K) are obtained, assuming a KroupaIMF. Modifying the NFW dark matter halo by adiabatic contraction does notimprove the fit and yields systematically lower M/L_K. In contrast to previousresults for massive clusters, we find that the NFW concentration parameter(c_vir) for groups decreases with increasing M_vir and is inconsistent with novariation at the 3 sigma level. The normalization and slope of the c_vir-M_virrelation are consistent with the standard LambdaCDM cosmological model withsigma_8 = 0.9. The small intrinsic scatter measured about the c_vir-M_virrelation implies the groups represent preferentially relaxed, early formingsystems. The mean gas fraction (f =0.05 +/- 0.01) of the groups measured withinan overdensity Delta=2500 is lower than for hot, massive clusters, but thefractional scatter (sigma_f/f=0.2) for groups is larger, implying a greaterimpact of feedback processes on groups, as expected.Comment: Accepted for Publication in ApJ; 30 pages, 9 figures. No changes from  previous versio

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