Be it therefore resolved: Cosmological Simulations of Dwarf Galaxies with Extreme Resolution

Abstract

We study a suite of extremely high-resolution cosmological FIRE simulations of dwarf galaxies (M_(halo) ≲ 10^(10) 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 {\em stellar} galaxy, suggesting very low-mass dwarfs may be ubiquitous in the field. Our ultra-faint dwarfs (UFDs; M∗ < 10^5 M_⊙) have their star formation truncated early (z ≳ 2), likely by reionization, while classical dwarfs (M∗ > 10^5 M_⊙) 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∗/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∗ > 10^4 M_⊙ 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).