Forged in FIRE: Cusps, cores and baryons in low-mass dwarf galaxies

We present multiple ultrahigh resolution cosmological hydrodynamic simulations of M_* ≃ 10^4-6.3 M_⊙ dwarf galaxies that form within two M_vir = 10^9.5-10 M_⊙ dark matter halo initial conditions. Our simulations rely on the Feedback in Realistic Environments (FIRE) implementation of star formation feedback and were run with high enough force and mass resolution to directly resolve structure on the ~200 pc scales. The resultant galaxies sit on the M_* versus M_vir relation required to match the Local Group stellar mass function via abundance matching. They have bursty star formation histories and also form with half-light radii and metallicities that broadly match those observed for local dwarfs at the same stellar mass. We demonstrate that it is possible to create a large (~1 kpc) constant-density dark matter core in a cosmological simulation of an M_* ≃ 10^6.3 M_⊙ dwarf galaxy within a typical M_vir = 10¹⁰ M_⊙ halo - precisely the scale of interest for resolving the 'too big to fail' problem. However, these large cores are not ubiquitous and appear to correlate closely with the star formation histories of the dwarfs: dark matter cores are largest in systems that form their stars late (z ≲ 2), after the early epoch of cusp building mergers has ended. Our M_* ≃ 10⁴ M_⊙ dwarf retains a cuspy dark matter halo density profile that matches that of a dark-matter-only run of the same system. Though ancient, most of the stars in our ultrafaint form after reionization; the ultraviolet field acts mainly to suppress fresh gas accretion, not to boil away gas that is already present in the protodwarf.