Abstract
High resolution cosmological N-body simulations of four galaxy-scale dark
matter halos are compared to corresponding N-body/hydrodynamical simulations
containing dark matter, stars and gas. The simulations without baryons share
features with others described in the literature in that the dark matter
density slope continuously decreases towards the center, with a density
\rho~r^-1.3+/-0.2, at about 1% of the virial radius for our Milky Way sized
galaxies. The central cusps in the simulations which also contain baryons
steepen significantly, to \rho~r^-1.9+/-0.2, with an indication of the inner
logarithmic slope converging. Models of adiabatic contraction of dark matter
halos due to the central build-up of stellar/gaseous galaxies are examined. The
simplest and most commonly used model, by Blumenthal et al., is shown to
overestimate the central dark matter density considerably. A modified model
proposed by Gnedin et al. is shown to be a considerable improvement, but not
perfect. Moreover it is found that the contraction parameters not only depend
on the orbital structure of the dark-matter-only halos but also on the stellar
feedback prescription which is most relevant for the baryonic distribution.
Implications for dark matter annihilation at the galactic center are discussed
and it is found that although our simulations show a considerable reduced halo
contraction as compared to the Blumenthal et al. model, the fluxes from dark
matter annihilation is still expected to be enhanced by at least a factor of a
hundred as compared to dark-matter-only halos. Finally, it is shown that while
dark-matter-only halos are typically prolate, the dark matter halos containing
baryons are mildly oblate with minor-to-major axis ratios of c/a=0.73+/-0.11,
with their flattening aligned with the central baryonic disks.
Description
Baryonic Pinching of Galactic Dark Matter Halos
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