The dark side of FIRE: predicting the population of dark matter subhaloes around Milky Way-mass galaxies

A variety of observational campaigns seek to test dark matter models by measuring dark matter subhaloes at low masses. Despite their predicted lack of stars, these subhaloes may be detectable through gravitational lensing or via their gravitational perturbations on stellar streams. To set measurable expectations for subhalo populations within Lambda cold dark matter, we examine 11 Milky Way (MW)-mass haloes from the FIRE-2 baryonic simulations, quantifying the counts and orbital fluxes for subhaloes with properties relevant to stellar stream interactions: masses down to 10(6) M-circle dot, distances 50 kpc of the galactic centre, across z = 0 - 1 (t(lookback) = 0-8 Gyr). We provide fits to our results and their dependence on subhalo mass, distance, and lookback time, for use in (semi)analytical models. A typical MW-mass halo contains 16 subhaloes > 10(7) M-circle dot (similar to 1 subhalo > 10(8) M-?) within 50 kpc at z similar to 0. We compare our results with dark matter-only versions of the same simulations: because they lack a central galaxy potential, they overpredict subhalo counts by 2-10x, more so at smaller distances. Subhalo counts around a given MW-mass galaxy declined over time, being similar to 10x higher at z = 1 than at z similar to 0. Subhaloes have nearly isotropic orbital velocity distributions at z similar to 0. Across our simulations, we also identified 4 analogues of Large Magellanic Cloud satellite passages; these analogues enhance subhalo counts by 1.4-2.1 times, significantly increasing the expected subhalo population around the MW today. Our results imply an interaction rate of similar to 5 per Gyr for a stream like GD-1, sufficient to make subhalo-stream interactions a promising method of measuring dark subhaloes.