The Cores of Dark Matter Dominated Galaxies: theory vs. observations

TL;DR

This paper addresses the core-cusp problem by comparing rotation curves of dark-matter-dominated dwarf and LSB galaxies to high-resolution CDM-like simulations. Using a Zhao-type density profile with a shallow inner slope, the authors show that observed RCs are self-similar and well described by $\rho(r) \propto r^{-\gamma}$ with $\gamma \approx 0.2-0.4$, implying $v(r) \propto r^{g}$ with $g \approx 0.8-0.9$. High-resolution ART simulations across CDM, LCDM, and CHDM produce average halo velocity profiles that agree with observations, albeit with substantial scatter in inner densities, and with a trend that smaller halos are denser. The study argues that previous claims of a discrepancy between simulated halos and dwarf galaxy dynamics may be overstated, emphasizing the importance of scatter, distance uncertainties, and the proper inner radial range in comparisons, and suggesting the inner slope $\gamma$ could probe formation history and the primordial power spectrum.

Abstract

We use the rotation curves of a sample of dark matter dominated dwarf and low-surface brightness (LSB) late-type galaxies to study their radial mass distributions. We find that the shape of the rotation curves is remarkably similar for all (both dwarf and LSB) galaxies in the sample, suggesting a self-similar density distribution of their dark matter (DM) halos. This shape can be reproduced well by a density profile with a shallow central cusp (rho(r)\propto 1/r^gamma, gamma\approx 0.2-0.4), corresponding to a steeply rising velocity curve (v(r)\propto r^g, g\approx 0.9-0.8). We further show that the observed shape of the rotation curves is well matched by the density profiles of DM halos formed in very high resolution simulations of the CDM, LCDM, and CHDM models of structure formation. This is surprising in light of several previous studies which suggested that the structure of simulated DM halos is inconsistent with the dynamics of dwarf galaxies. We discuss possible explanations for this discrepancy. We show that although the mass distribution in the hierarchically formed halos is on average consistent with the shape of rotation curves of dark matter dominated galaxies, the scatter of the individual profiles around the average is substantial and should not be neglected in comparisons with the data. Finally, we show that the DM halos in our hierarchical simulations and the real galaxies in our sample exhibit a similar decrease in their characteristic densities with increasing characteristic radial scales, and increase in their maximum rotation velocities with increasing radii at which their maximum velocity occurs. (Abridged)