The Local Group on FIRE: dwarf galaxy populations across a suite of hydrodynamic simulations

We present a new set of high-resolution hydrodynamic cosmological zoom-in simulations that apply the Feedback In Realistic Environments physics to both Local Group (LG)-like and isolated Milky Way (MW)-like volumes (10 host systems in total with a baryonic particle mass ≃ 3500-7000 M_⊙). We study the stellar mass functions, circular velocity or mass profiles, and velocity dispersions of the dwarf galaxy populations. The simulations reproduce the stellar mass function and central densities of MW satellite dwarfs for M_\ast ≥ 10^{5.5} M_⊙ and predict the existence of ∼3 unidentified galaxies with M_\ast ∼ 10^5 M_⊙ within 300 kpc of the MW. Overall, we find no evidence for the classical missing satellites or too-big-to-fail (TBTF) problems for satellite galaxies in our sample. Among the satellites, TBTF is resolved primarily by subhalo disruption and overall mass-loss; central density profiles of subhaloes are of secondary importance. For non-satellite galaxies, our LG-like simulations predict as many as ∼10 as-of-yet unseen galaxies at distances 0.3-1 Mpc from both hosts, with M_\ast ∼eq 10^{5-6} M_⊙ (in haloes with V_max ∼ 20 km s^-1), albeit with large halo-to-halo variance. None of our simulations produces a compact, baryon-dominated, high-density dwarf elliptical-type galaxy (with V_circ ≳ 35 km s^-1 at r<1 kpc), of which six may appear in the LG (but none in the MW). It may therefore remain a challenge to reproduce the full diversity of the dwarf population, including both the highest and lowest density systems.