Paper: astro-ph/0601301
Date: Fri, 13 Jan 2006 20:55:03 GMT (597kb)

Title: A Chandra View of Dark Matter in Early-Type Galaxies

Authors: Philip J. Humphrey, David A. Buote, Fabio Gastaldello, Luca
Zappacosta, James S. Bullock (UC Irvine), Fabrizio Brighenti (Bologna),
William G. Mathews (UCSC)

Comments: 20 pages and 6 figures. Submitted to the Astrophysical Journal
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We present a Chandra study of mass profiles in 7 elliptical galaxies, of
which 3 have galaxy-scale and 4 group-scale halos. These represent the best
available data for nearby objects with comparable X-ray luminosities. We
measure ~flat mass-to-light (M/L) profiles within an optical half-light radius
(Reff), rising by an order of magnitude at ~10Reff, which confirms the presence
of dark matter (DM). The data indicate hydrostatic equilibrium, which is also
supported by agreement with studies of stellar kinematics in elliptical
galaxies. The data are well-fitted by a model comprising an NFW DM profile and
a baryonic component following the optical light. The distribution of DM halo
concentration parameters (c) versus Mvir agrees with LCDM predictions and our
observations of bright groups. Concentrations are slightly higher than
expected, which is most likely a selection effect. Omitting the stellar mass
drastically increases c, possibly explaining large concentrations found by some
past observers. The stellar M/LK agree with population synthesis models,
assuming a Kroupa IMF. Allowing adiabatic 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 strong feedback. The groups exhibit positive
temperature gradients, consistent with the "Universal" profiles found in other
groups and clusters, whereas the galaxies have negative gradients, suggesting a
change in the evolutionary history of the systems around Mvir=1E13 Msun.

\\ ( http://arXiv.org/abs/astro-ph/0601301 , 597kb)