A dark matter profile to model diverse feedback-induced core sizes of ΛCDM haloes

We analyse the cold dark matter density profiles of 54 galaxy haloes simulated with Feedback In Realistic Environments (FIRE)-2 galaxy formation physics, each resolved within 0.5 per cent of the halo virial radius. These haloes contain galaxies with masses that range from ultrafaint dwarfs (M-star similar to 10(4.5) M-circle dot) to the largest spirals (M-star similar or equal to 10(11) M-circle dot) and have density profiles that are both cored and cuspy. We characterize our results using a new, analytic density profile that extends the standard two-parameter Einasto form to allow for a pronounced constant density core in the resolved innermost radius. With one additional core-radius parameter, r(c), this three-parameter core-Einasto profile is able to characterize our feedback-impacted dark matter haloes more accurately than other three-parameter profiles proposed in the literature. To enable comparisons with observations, we provide fitting functions for r(c) and other profile parameters as a function of both M-star and M-star/M-halo. In agreement with past studies, we find that dark matter core formation is most efficient at the characteristic stellar-to-halo mass ratio M-star/M-halo similar or equal to 5 x 10(-3), or M-star similar to 10(9) M-circle dot, with cores that are roughly the size of the galaxy half-light radius, r(c) similar or equal to 1-5 kpc. Furthermore, we find no evidence for core formation at radii greater than or similar to 100 pc in galaxies with M-star/M-halo < 5 x 10(-4) or M-star less than or similar to 10(6) M-circle dot. For Milky Way-size galaxies, baryonic contraction often makes haloes significantly more concentrated and dense at the stellar half-light radius than DMO runs. However, even at the Milky Way scale, FIRE-2 galaxy formation still produces small dark matter cores of similar or equal to 0.5-2 kpc in size. Recent evidence for a similar to 2 kpc core in the Milky Way's dark matter halo is consistent with this expectation.