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Constraints on the Structure of Dark Matter Halos from the Rotation Curves of Low Surface Brightness Galaxies

We reexamine the disk-halo decompositions of the rotation curves of low surface brightness (LSB) galaxies with Vmax ≥ 80 km s-1, taking full account of the effects of beam smearing. We show that the spatial resolution of the data is not sufficient to put any meaningful constraints on the density profiles of the dark halos, or on cosmological parameters. This is in strong contrast to claims made in the literature that these LSB rotation curves are only consistent with dark matter halos with shallow central cusps, and it has important implications regarding the halos of LSB galaxies, such as the self-similarity of their rotation curves, and their inconsistency with certain cosmological models or with cold dark matter altogether. Only in one case are the data of sufficient spatial resolution to obtain reliable constraints on the slope of the central density distribution of the dark matter halo. For this single case, we find a central cusp ρ ∝ r-α with 0.55 < α < 1.26 at the 99.73% confidence level. This contrasts strongly with the results for two dwarf galaxies (Vmax < 70 km s-1) that we analyze, which yield α < 0.5 at the same level of confidence. This possibly suggests that halos with constant-density cores are restricted to low-mass systems. We show that violent outflows of baryonic matter by supernova feedback can reproduce this mass dependence of halo cusp slopes.