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

Abstract

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⋆ ≃ 10^(4.5) M_⊙⁠) to the largest spirals (⁠M⋆ ≃ 10¹¹ M_⊙⁠) 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⋆ and M⋆/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⋆/M_(halo) ≃ 5 × 10⁻³, or M⋆ ∼ 10⁹ M_⊙⁠, with cores that are roughly the size of the galaxy half-light radius, r_c ≃ 1−5 kpc. Furthermore, we find no evidence for core formation at radii ≳100 pc in galaxies with M⋆/M_(halo) < 5 × 10⁻⁴ or M⋆ ≲ 10⁶ M_⊙⁠. 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 ≃ 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.

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