Is There an Ultra-Faint Missing Satellites Problem?

Due to their high relative dark matter content, ultra-faint dwarf galaxies (UFDs; M_* < 10⁵M_sun) are excellent testbeds for cosmological models. Historically, the number of low-mass galaxies around the Milky Way (MW) predicted by naively assigning a galaxy to each dark matter (DM) halo in a gravity-only simulation does not align with the observations (only ~60 observed). This is known as the classical missing satellite problem (MSP), and it initially posed a major challenge to the preferred cosmological model, Lambda-Cold Dark Matter Theory (LCDM). While the MSP has been largely solved for MW satellites above the UFD mass range, it may still exist for UFDs. In order to determine if this is the case, we need to know not only how many DM halos should host a UFD, but how many should be visible with current observational capabilities that are effective surface-brightness-limited. We redefine the simulated galaxy edge for all UFDs in our high and ultra-high resolution FIRE-2 (Hopkins et al. 2018) simulations, using several methods: we first attempt different fractions of the halo radius from 10%-40%, comparing the resulting stellar mass to that calculated by the halo finder, as well as determining the effect of this edge on predicted galaxy properties. We then employ a cut in effective surface brightness (SB), re-defining this edge for all simulated galaxies with 10 or more star particles (M_* = a few x 100 M_sun). Using the latter method, we find that most simulated UFDs have a larger redefined galaxy edge then what is currently predicted by all FIRE-2 simulations to date. This suggests that observations of UFDs may be detecting only their brighter central cores, and that observed ultra-faint MW satellites may have unobserved diffuse stellar halos. One such halo has been observed recently in Tucana II (Chiti et al. 2021), who find new stars out to nine of the UFD’s half-light radii. We make new testable predictions for the observed counts and properties of UFDs based on this SB cut, predictions that are more likely to match observations of existing observed ultra-faint galaxies.