AChandraView of Dark Matter in Early‐Type Galaxies

We present a Chandra study of mass profiles in seven elliptical galaxies, of which three have galaxy-scale and have group-scale halos, demarcated at 1013 M. These represent the best available data for nearby objects with X-ray luminosities. We measure approximately flat mass-to-light (M/L) profiles within an optical halflight (ReA), rising by an order of magnitude at 10 ReA, which confirms the presence of dark matter (DM ). data indicate hydrostatic equilibrium, which is also supported by agreement with studies of stellar kinematics in galaxies. The data are well fitted by a model comprising an NFW DM profile and a baryonic component the optical light. The distribution of DM halo concentration parameters (c) versus Mvir agrees with CDM and our observations of bright groups. Concentrations are slightly higher than expected, which is most a selection effect. Omitting the stellar mass drastically increases c, possibly explaining large concentrations by some past observers. The stellar M/LK agree with population synthesis models, assuming a Kroupa IMF. adiabatic compression (AC ) of the DM halo by baryons made M/L more discrepant, casting some doubt AC. Our best-fitting models imply total baryon fractions 0.04Y0.09, consistent with models of galaxy formation strong feedback. The groups exhibit positive temperature gradients, consistent with the ‘‘universal’’ found in other groups and clusters, whereas the galaxies have negative gradients, suggesting a change in the history of the systems around Mvir ’ 1013 M.