Galaxies are surrounded by dark matter halos roughly ten times as massive as the luminous material. The regularity in the shape of dark matter halos is one of the most robust predictions of hierarchical structure formation in cosmological theory. To draw meaningful comparison between N-body simulations (that simulate only the dark matter) and observations, we must understanding how baryonic physics affects the distribution of dark matter in galaxies. Accurately modeling the shape of galaxy profiles is also very important for estimating event rates for direct and indirect detection of dark matter.
It is generally believed that the cooling of gaseous baryons early in the galaxy's history steepens the central density profile, and causes the dark matter to respond by contracting adiabatically to the center. Whether this steepened profile persists to late times remains controversial. In this talk I will discuss the physical processes in galaxy formation that are responsible for expanding and contracting the shape of the inner profile. I will present observational evidence for the persistence of adiabatic contraction in early type galaxies, using a sample of 70,000 ellipticals from the Sloan Digital Sky Survey. I will discuss sources of systematic error, particularly the distribution of stellar masses in the galaxy. Finally I will contrast elliptical galaxies to spirals, and discuss how their formation mechanisms may generate qualitatively different inner profile shapes.
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