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_{\mathrm{halo}}\lesssim 10^{10}$$M_{\odot }$), run to $z=0$ with $30M_{\odot }$ resolution, sufficient (for the first time) to resolve the internal structure of individual supernovae remnants within the cooling radius. Every halo with $M_{\mathrm{halo}}\gtrsim 10^{8.6}{M}_{\odot }$ is populated by a resolved {\em stellar} galaxy, suggesting very low-mass dwarfs may be ubiquitous in the field. Our ultra-faint dwarfs (UFDs; $M_{\ast }<10^5{M}_{\odot }$) have their star formation truncated early ($z\gtrsim 2$), likely by reionization, while classical dwarfs ($M_{\ast }>10^5{M}_{\odot }$) 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 $M_{\ast }/M_{\mathrm{halo}}>10^{-4}$ to form a dark matter core $>200$pc, while lower-mass UFDs exhibit cusps down to $\lesssim 100$pc, as expected from energetic arguments. Our dwarfs with $M_{\ast }>10^4{M}_{\odot }$ 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).

14 pages; 9 figures; 1 table; submitted to MNRAS