Cold collapse and the core catastrophe

TL;DR

The paper tests whether cold dark matter (CDM) halos can reproduce rotation curves of dark matter–dominated galaxies using high-resolution cosmological N-body simulations. It finds that CDM halos develop a cuspy inner density profile, well described by ρ(r) ∝ [ (r/r_s)^{1.5}(1+(r/r_s)^{1.5}) ]^{-1}, leading to rotation curves that rise too steeply for low surface brightness galaxies. Importantly, the inner cusp persists regardless of merger history or small-scale power cutoff, and warm dark matter does not resolve the discrepancy. The authors conclude that CDM in its current form cannot account for observed core-like central densities and suggest new dark matter physics, such as annihilation-driven cores, along with follow-up stellar-kinematic tests to verify the results.

Abstract

We show that a universe dominated by cold dark matter fails to reproduce the rotation curves of dark matter dominated galaxies, one of the key problems that it was designed to resolve. We perform numerical simulations of the formation of dark matter halos, each containing \gsim 10^6 particles and resolved to 0.003 times the virial radius, allowing an accurate comparison with rotation curve data. A good fit to both galactic and cluster sized halos can be achieved using the density profile rho(r) \propto [(r/r_s)^1.5(1+(r/r_s)^1.5)]^-1, where r_s is a scale radius. This profile has a steeper asymptotic slope, rho(r) \propto r^-1.5, and a sharper turnover than found by lower resolution studies. The central structure of relaxed halos that form within a hierarchical universe has a remarkably small scatter (unrelaxed halos would not host disks). We compare the results with a sample of dark matter dominated, low surface brightness (LSB) galaxies with circular velocities in the range 100-300 km/s. The rotation curves of disks within cold dark matter halos rise too steeply to match these data which require a constant mass density in the central regions. The same conclusion is reached if we compare the scale free shape of observed rotation curves with the simulation data. It is important to confirm these results using stellar rather than HI rotation curves for LSB galaxies. We test the effects of introducing a cut-off in the power spectrum that may occur in a universe dominated by warm dark matter. In this case halos form by a monolithic collapse but the final density profile hardly changes, demonstrating that the merger history does not play a role in determining the halo structure.