A<i>Chandra</i>View of Dark Matter in Early‐Type Galaxies

Abstract

We present a Chandra study of mass profiles in seven elliptical galaxies, of which three have galaxy-scale and&#13;\nfour have group-scale halos, demarcated at 1013 M. These represent the best available data for nearby objects with&#13;\ncomparable X-ray luminosities. We measure approximately flat mass-to-light (M/L) profiles within an optical halflight&#13;\nradius (ReA), rising by an order of magnitude at 10 ReA, which confirms the presence of dark matter (DM ).&#13;\nThe data indicate hydrostatic equilibrium, which is also supported by agreement with studies of stellar kinematics in&#13;\nelliptical galaxies. The data are well fitted by a model comprising an NFW DM profile and a baryonic component&#13;\nfollowing the optical light. The distribution of DM halo concentration parameters (c) versus Mvir agrees with CDM&#13;\npredictions and our observations of bright groups. Concentrations are slightly higher than expected, which is most&#13;\nlikely a selection effect. Omitting the stellar mass drastically increases c, possibly explaining large concentrations&#13;\nfound by some past observers. The stellar M/LK agree with population synthesis models, assuming a Kroupa IMF.&#13;\nAllowing adiabatic compression (AC ) of the DM halo by baryons made M/L more discrepant, casting some doubt&#13;\non AC. Our best-fitting models imply total baryon fractions 0.04Y0.09, consistent with models of galaxy formation&#13;\nincorporating strong feedback. The groups exhibit positive temperature gradients, consistent with the ‘‘universal’’&#13;\nprofiles found in other groups and clusters, whereas the galaxies have negative gradients, suggesting a change in the&#13;\nevolutionary history of the systems around Mvir ’ 1013 M.