2019MNRAS.490.4447W

We study a suite of extremely high-resolution cosmological Feedback in Realistic Environments simulations of dwarf galaxies (M_ halo_ ≲10¹⁰ M_☉), run to z = 0 with 30 M_☉ resolution, sufficient (for the first time) to resolve the internal structure of individual supernovae remnants within the cooling radius. Every halo with M_ halo_ ≳10^8.6 M_☉ is populated by a resolved stellar galaxy, suggesting very low-mass dwarfs may be ubiquitous in the field. Our ultra-faint dwarfs (UFDs; M_*< 10⁵ M_☉) have their star formation (SF) truncated early (z >= 2), likely by reionization, while classical dwarfs (M_*> 10⁵ M_☉) continue forming stars to z < 0.5. The systems have bursty star formation histories, forming most of their stars in periods of elevated SF strongly clustered in both space and time. This allows our dwarf with M_*/M_halo > 10^–4 to form a dark matter core >200 pc, while lower mass UFDs exhibit cusps down to ≲100 pc, as expected from energetic arguments. Our dwarfs with M_*> 10⁴ M_☉ have half-mass radii (R_1/2) in agreement with Local Group (LG) dwarfs (dynamical mass versus R_1/2 and stellar rotation also resemble observations). The lowest mass UFDs are below surface brightness limits of current surveys but are potentially visible in next-generation surveys (e.g. LSST). The stellar metallicities are lower than in LG dwarfs; this may reflect pre-enrichment of the LG by the massive hosts or Pop-III stars. Consistency with lower resolution studies implies that our simulations are numerically robust (for a given physical model).