We study a suite of extremely high-resolution cosmological Feedback in Realistic Environments simulations of dwarf galaxies (⁠|$M_{\rm halo} \lesssim 10^{10}\rm \, M_{\odot }$|⁠), run to z = 0 with |$30\, \mathrm{M}_{\odot }$| resolution, sufficient (for the first time) to resolve the internal structure of individual supernovae remnants within the cooling radius. Every halo with |$M_{\rm halo} \gtrsim 10^{8.6}\, \mathrm{M}_{\odot }$| is populated by a resolved stellar galaxy, suggesting very low-mass dwarfs may be ubiquitous in the field. Our ultra-faint dwarfs (UFDs; |$M_{\ast }\lt 10^{5}\, \mathrm{M}_{\odot }$|⁠) have their star formation (SF) truncated early (z ≳ 2), likely by reionization, while classical dwarfs (⁠|$M_{\ast }\gt 10^{5}\, \mathrm{M}_{\odot }$|⁠) 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*/Mhalo > 10−4 to form a dark matter core |${\gt}200\rm \, pc$|⁠, while lower mass UFDs exhibit cusps down to |${\lesssim}100\rm \, pc$|⁠, as expected from energetic arguments. Our dwarfs with |$M_{\ast }\gt 10^{4}\, \mathrm{M}_{\odot }$| have half-mass radii (R1/2) in agreement with Local Group (LG) dwarfs (dynamical mass versus R1/2 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).

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