Wheeler et al., available on arXiv

Abstract: We study a suite of extremely high-resolution cosmological FIRE simulations of dwarf galaxies (M_halo <~ 10^10 Msun), run to z=0 with 30 Msun 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 Msun is populated by a resolved stellar galaxy, suggesting very low-mass dwarfs may be ubiquitous in the field. Our ultra-faint dwarfs (UFDs; M_star<10^5 Msun) have their star formation truncated early (z>~2), likely by reionization, while classical dwarfs (M_star>10^5 Msun) continue forming stars to z<0.5. The systems have bursty star formation (SF) histories, forming most of their stars in periods of elevated SF strongly clustered in both space and time. This allows our dwarf with M_star/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_star>10^4 Msun have half-mass radii (R_1/2) in agreement with Local Group (LG) dwarfs; dynamical mass vs. R_1/2 and the degree of rotational support 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).