Abstract
Observations of strong gravitational lensing, stellar kinematics, and mass
tracers on larger scales enable accurate measures of the distribution of dark
matter and baryons in massive early-type galaxies (ETGs). While such techniques
have previously been applied to galaxy-scale and cluster-scale lenses, the
paucity of intermediate-mass systems with high-quality data has precluded a
uniform analysis of mass-dependent trends. With the aim of bridging this gap,
we present new observations and analyses of 10 group-scale lenses at <z>=0.36
characterized by Einstein radii 2.5"-5.1" and a mean halo mass of M_200 =
10^14.0 Msol. For these groups, we find a mean halo concentration c_200 = 5.0
+- 0.8 consistent with unmodified cold dark matter halos and recent simulations
of galaxy formation. By combining our data with other lens samples in the
literature, we analyze the mass structure of ETGs in halos spanning the mass
range 10^13-10^15 Msol using homogeneous methods and data. We show that the
slope of the total density profile gamma_tot within the effective radius
depends on the stellar surface density, as demonstrated previously, but also on
the halo mass. We analyze these trends using halo occupation models and
resolved stellar kinematics with the goal of testing the universality of the
dark matter profile within ETGs of various masses. Whereas the central galaxies
of clusters require a shallow inner dark matter density profile, group-scale
lenses are consistent with an unmodified Navarro-Frenk-White profile or one
that is slightly contracted. We conclude that the net effect of baryons on the
dark matter distribution may not be universal, but more likely varies with halo
mass due to underlying trends in star formation efficiency and assembly
history.
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