Abstract
We present ultra-high resolution cosmological hydrodynamic simulations of
$M_*\simeq10^4-6M_ødot$ dwarf galaxies that form within
$M_v=10^9.5-10M_ødot$ dark matter halos. Our simulations rely on the
FIRE implementation of star formation feedback and were run with high enough
force and mass resolution to directly resolve stellar and dark matter structure
on the ~200 pc scales of interest for classical and ultra-faint dwarfs in the
Local Group. The resultant galaxies sit on the $M_*$ vs. $M_v$ relation
required to match the Local Group stellar mass function. 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. For the
first time we demonstrate that it is possible to create a large (~1 kpc) dark
matter core in a cosmological simulation of an $M_*\simeq10^6M_ødot$ dwarf
galaxy that resides within an $M_v=10^10M_ødot$ 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 ($złesssim2$), after the early epoch of cusp
building mergers has ended. Our $M_*\simeq10^4M_ødot$ dwarf retains a cuspy
dark matter halo density profile that matches almost identically that of a
dark-matter only run of the same system. Despite forming in a field
environment, this very low mass dwarf has observable properties that match
closely to those of ultra-faint satellite galaxies of the Milky Way, including
a uniformly old stellar population (>10 Gyr). Though ancient, most of the stars
in our ultra-faint form after reionization; the UV field acts mainly to
suppress fresh gas accretion, not to boil away gas that is already present in
the proto-dwarf.
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