AChandraView of Dark Matter in Early‐Type Galaxies

We present a Chandra study of mass profiles in 7 elliptical galaxies, ofwhich 3 have galaxy-scale and 4 group-scale halos demarcated at 1E13Msun. Theserepresent the best available data for nearby objects with comparable X-rayluminosities. We measure ~flat mass-to-light (M/L) profiles within an opticalhalf-light radius (Reff), rising by an order of magnitude at ~10Reff, whichconfirms the presence of dark matter (DM). The data indicate hydrostaticequilibrium, which is also supported by agreement with studies of stellarkinematics in elliptical galaxies. The data are well-fitted by a modelcomprising an NFW DM profile and a baryonic component following the opticallight. The distribution of DM halo concentration parameters (c) versus Mviragrees with LCDM predictions and our observations of bright groups.Concentrations are slightly higher than expected, which is most likely aselection effect. Omitting the stellar mass drastically increases c, possiblyexplaining large concentrations found by some past observers. The stellar M/LKagree with population synthesis models, assuming a Kroupa IMF. Allowingadiabatic compression (AC) of the DM halo by baryons made M/L more discrepant,casting some doubt on AC. Our best-fitting models imply total baryon fractions\~0.04--0.09, consistent with models of galaxy formation incorporating strongfeedback. The groups exhibit positive temperature gradients, consistent withthe "Universal" profiles found in other groups and clusters, whereas thegalaxies have negative gradients, suggesting a change in the evolutionaryhistory of the systems around Mvir=1E13 Msun.Comment: 22 pages and 6 figures. Accepted for publication in the Astrophysical Journal. Minor changes to match published version. Expanded galaxy/group discussion. Results unchange