Although fossil galaxy systems are thought to be very old , and thus should possess above-average halo concentrations , typically fossils have unexceptional concentrations for their masses .
We revisit the massive isolated elliptical galaxy / fossil group NGC 6482 for which previous X-ray studies of a modest \chandra observation obtained a very uncertain , but also possibly very high , halo concentration .
We present new measurements of the hot gas surface brightness , temperature , and iron abundance using the modest \chandra observation and a previously unpublished \suzaku observation , the latter of which allows the measurements of the gas properties to be extended out to \sim \rtwofiveh .
By constructing hydrostatic equilibrium models of the gas with separate components for the gas , BCG stellar mass , and the dark matter ( DM ) , we measure c _ { 200 } = 32.2 \pm 7.1 and M _ { 200 } = ( 4.5 \pm 0.6 ) \times 10 ^ { 12 } M _ { \odot } using an NFW DM profile .
For a halo of this mass , the measured concentration c _ { 200 } exceeds the mean value ( 7.1 ) expected for relaxed \lcdm halos by 3.5 \sigma in terms of the observational error , and by 6 \sigma considering the intrinsic scatter in the \lcdm c - M relation , which situates NGC 6482 as the most extreme outlier known for a fossil system .
We explored several variants of adiabatic contraction ( AC ) models and , while the AC models provide fits of the same quality as the un-contracted models , they do have the following advantages : ( 1 ) smaller c _ { 200 } that is less of an outlier in the \lcdm c - M relation , and ( 2 ) baryon fractions ( f _ { b, 200 } ) that agree better with the mean cosmic value .
While the standard AC prescriptions yield a BCG stellar mass that is uncomfortably small compared to results from stellar population synthesis ( SPS ) models , a weaker AC variant that artificially shuts off cooling and star formation at z = 2 yields the same stellar mass as the un-contracted models .
For these reasons , we believe our X-ray analysis prefers this weaker AC variant applied to either an NFW or Einasto DM halo .
Finally , the stellar mass we measure for the BCG from the hydrostatic analysis strongly favors results from SPS models with a Chabrier or Kroupa IMF over a Salpeter IMF .