A limit on the cosmological mass density and power spectrum from the rotation curves of low surface brightness galaxies

The concentrations of the cuspy dark matter halos predicted by simulations of cold dark matter are related to the cosmology in which the halos form. Observational constraints on halo concentration therefore map into constraints on cosmological parameters. In order to explain the observed concentrations of dark matter dominated low surface brightness galaxies, we require a cosmology with rather little power on galaxy scales. Formally, we require sigma(8) Gamma(0.6) < 0.23, where Gamma(0.6) is a modified shape parameter appropriate to this problem. Practically, this means that either Omega(m) < 0.2 or sigma(8) < 0.8. These limits apply as long as we insist that the cuspy halos found in simulations must describe the halos of low surface brightness galaxies. A low-density cosmology helps with the low observed concentrations, but it offers no explanation of the many cases where the shape of the density profile deviates from the predicted cuspy form. These cases must have suffered very extensive mass redistribution if the current halo formation picture is not to fail outright. It is far from clear whether any of the mass redistribution mechanisms that have been suggested (e.g., feedback) are viable.