Sweating the small stuff: simulating dwarf galaxies, ultra-faint dwarf galaxies, and their own tiny satellites

We present Feedback in Realistic Environment (FIRE)/GIZMO hydrodynamic zoom-in simulations of isolated dark matter haloes, two each at the mass of classical dwarf galaxies (M-vir similar or equal to 10(10) M-circle dot) and ultra-faint galaxies (M-vir similar or equal to 10(9) M-circle dot), and with two feedback implementations. The resulting central galaxies lie on an extrapolated abundance matching relation from M-star similar or equal to 10(6) to 10(4) M-circle dot without a break. Every host is filled with subhaloes, many of which form stars. Each of our dwarfs with M-star similar or equal to 10(6) M-circle dot has 1-2 well-resolved satellites with M-star = 3-200 x 10(3) M-circle dot. Even our isolated ultra-faint galaxies have star-forming subhaloes. If this is representative, dwarf galaxies throughout the Universe should commonly host tiny satellite galaxies of their own. We combine our results with the Exploring the Local Volume in Simulations (ELVIS) simulations to show that targeting similar to 50 kpc regions around nearby isolated dwarfs could increase the chances of discovering ultra-faint galaxies by similar to 35 per cent compared to random pointings, and specifically identify the region around the Phoenix dwarf galaxy as a good potential target. The well-resolved ultra-faint galaxies in our simulations (M-star similar or equal to 3-30 x 10(3) M-circle dot) form within M-peak similar or equal to 0.5-3 x 10(9) M-circle dot haloes. Each has a uniformly ancient stellar population (> 10 Gyr) owing to reionization-related quenching. More massive systems, in contrast, all have late-time star formation. Our results suggest that M-halo similar or equal to 5 x 10(9) M circle dot is a probable dividing line between haloes hosting reionization 'fossils' and those hosting dwarfs that can continue to form stars in isolation after reionization.