Lazar et al., available on arXiv

Abstract: We analyze the cold dark matter density profiles of 54 galaxy halos simulated with FIRE-2 galaxy formation physics, each resolved within 0.5% of the halo virial radius. These halos contain galaxies with masses that range from ultra-faint dwarfs (M*~10^4.5 Msun) to the largest spirals (M*~10^11Msun) and have density profiles that are both cored and cuspy. We characterize our results using a new analytic density profile that extends the standard Einasto form to allow for a pronounced constant-density core in the resolved innermost radius. With one additional core-radius parameter, rc, this “core-Einasto” profile is able to characterize the shape and normalization of our feedback-impacted dark matter halos. In order to enable comparisons with observations, we provide fitting functions for rc and other profile parameters as a function of both M* and M(/Mhalo. In agreement with similar studies done in the literature, we find that dark matter core formation is most efficient at the characteristic stellar-mass to halo-mass ratio M*/Mhalo≃5*10^−3, or M*~10^9 Msun, with cores that are roughly the size of the galaxy half-light radius, rc~1−5 kpc. Furthermore, we find no evidence for core formation at radii >~100 pc in galaxies with M*/Mhalo<5*10^−4 or M*<~10^6 Msun. For Milky Way-size galaxies, baryonic contraction often makes halos significantly more concentrated and dense at the stellar half-light radius than dark matter only runs. However, even at the Milky Way scale, FIRE-2 galaxy formation still produces small dark matter cores of ~0.5-2 kpc in size. Recent evidence for a ~2 kpc core in the Milky Way's dark matter halo is consistent with this expectation.