Dark matter distribution in dwarf spheroidal galaxies

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  • 3 December 2001
Abstract
We present predictions for the line-of-sight velocity dispersion profiles of dwarf spheroidal galaxies and compare them to observations in the case of the Fornax and Draco dwarfs. The predictions are made in the framework of standard dynamical theory of two-component (stars and dark matter) spherical systems with different velocity distributions. The stars are assumed to be distributed according to Sersic laws with parameters fitted to observations. We discuss different dark matter density distributions, both cuspy and possessing flat density cores. For isotropic models the dark haloes with cores are found to fit the data better than those with cusps. Anisotropic models are studied by fitting two parameters, dark mass and velocity anisotropy, to the data. We find that the steeper the cusp of the profile, the more tangential is the velocity distribution required to fit the data, in agreement with the well-known degeneracy of density profile versus velocity anisotropy. However, only the profiles with cores yield good fits with anisotropy parameter close to the most natural, isotropic value. The results support the evidence based on studies of rotation curves of spiral galaxies which also favours density profiles with cores over those with cusps. We also show that with present quality of the data the alternative explanation of velocity dispersions in terms of Modified Newtonian Dynamics cannot yet be ruled out.

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