We present radial mass profiles within \sim 0.3 r _ { vir } for 16 relaxed galaxy groups-poor clusters ( kT range 1-3 keV ) selected for optimal mass constraints from the Chandra and XMM 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 ( DM ) , represented by an NFW model , and stars from the central galaxy .
The stellar component is required only for 8 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 \sigma level .
The normalization and slope of the c _ { vir } - M _ { vir } 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 the groups represent preferentially relaxed , early forming systems .
The mean gas fraction ( f = 0.05 \pm 0.01 ) of the groups measured within an over-density \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 .