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

We present multiple ultrahigh resolution cosmological hydrodynamic simulations of M-* similar or equal to 10(4-6.3) M-circle dot dwarf galaxies that form within two M-vir = 10(9.5-10) M-circle dot 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 similar to 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 (similar to 1 kpc) constant-density dark matter core in a cosmological simulation of an M-* similar or equal to 10(6.3) M-circle dot dwarf galaxy within a typical M-vir = 10(10) M-circle dot 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 less than or similar to 2), after the early epoch of cusp building mergers has ended. Our M-* similar or equal to 10(4) M-circle dot 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.