Mass Density Profiles of Low Surface Brightness Galaxies

Abstract

We derive the mass density profiles of dark matter halos that are implied by high spatial resolution rotation curves of low surface brightness galaxies. We find that, at small radii, the mass density distribution is dominated by a nearly constant density core with a core radius of a few kiloparsecs. For ρ(r) ~ r^α, the distribution of inner slopes α is strongly peaked around α = -0.2. This is significantly shallower than the cuspy α ≤ -1 halos found in cold dark matter simulations. While the observed distribution of α does have a tail toward such extreme values, the derived value of α is found to depend on the spatial resolution of the rotation curves: α ≈ -1 is found only for the least well resolved galaxies. Even for these galaxies, our data are also consistent with constant-density cores (α = 0) of modest (~1 kpc) core radius, which can give the illusion of steep cusps when insufficiently resolved. Consequently, there is no clear evidence for a cuspy halo in any of the low surface brightness galaxies observed.