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

Abstract

We re-examine the disk-halo decompositions of the rotation curves of low surface brightness (LSB) galaxies with V_max > 80 km/s, taking full account of the effects of beam smearing. We show that the rotation curves of these LSB galaxies are, contrary to previous claims, consistent with dark halos with steep central cusps in their density distribution. In fact, the spatial resolution of the data is not sufficient to put any meaningful constraints on the density profiles of their dark halos, or on cosmological parameters. This has important implications for numerous claims made in the literature 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 is 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 density cusp with a slope 0.55 < alpha < 1.26 at the 99.73 percent confidence levels. This contrasts strongly with the results for two dwarf galaxies (V_max < 70 km/s) that we analyze, which yield alpha < 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 supernovae feedback can reproduce this mass-dependence of halo cusp slopes.