The outer halos of elliptical galaxies
  Recent progress is summarized on the determination of the density
distributions of stars and dark matter, stellar kinematics, and stellar
population properties, in the extended, low surface brightness halo regions of
elliptical galaxies. With integral field absorption spectroscopy and with
planetary nebulae as tracers, velocity dispersion and rotation profiles have
been followed to ~4 and ~5-8 effective radii, respectively, and in M87 to the
outer edge at ~150 kpc. The results are generally consistent with the known
dichotomy of elliptical galaxy types, but some galaxies show more complex
rotation profiles in their halos and there is a higher incidence of
misalignments, indicating triaxiality. Dynamical models have shown a range of
slopes for the total mass profiles, and that the inner dark matter densities in
ellipticals are higher than in spiral galaxies, indicating earlier assembly
redshifts. Analysis of the hot X-ray emitting gas in X-ray bright ellipticals
and comparison with dynamical mass determinations indicates that non-thermal
components to the pressure may be important in the inner ~10 kpc, and that the
properties of these systems are closely related to their group environments.
First results on the outer halo stellar population properties do not yet give a
clear picture. In the halo of one bright galaxy, lower [alpha/Fe] abundances
indicate longer star formation histories pointing towards late accretion of the
halo. This is consistent with independent evidence for on-going accretion, and
suggests a connection to the observed size evolution of elliptical galaxies
with redshift.
