2019MNRAS.488.4585G

A standard prediction of galaxy formation theory is that the ionizing background suppresses galaxy formation in haloes with peak circular velocities smaller than V_ peak_≃20 km s^–1, rendering the majority of haloes below this scale completely dark. We use a suite of cosmological zoom simulations of Milky Way-like haloes that include central Milky Way disc galaxy potentials to investigate the relationship between subhaloes and ultrafaint galaxies. We find that there are far too few subhaloes within 50 kpc of the Milky Way that had V_ peak_≳20 km s^–1 to account for the number of ultrafaint galaxies already known within that volume today. In order to match the observed count, we must populate subhaloes down to V_ peak_≃6 km s^–1 with ultrafaint dwarfs. The required haloes have peak virial temperatures as low as 1500 K, well below the atomic hydrogen cooling limit of 10⁴ K. Allowing for the possibility that the Large Magellanic Cloud contributes several of the satellites within 50 kpc could potentially raise this threshold to 10 km s^–1 (4000 K), still below the atomic cooling limit and far below the nominal reionization threshold.