The Impact of Molecular Hydrogen Cooling on the Galaxy Formation Threshold

Abstract We study the impact of molecular (H 2 ) and atomic (H i ) hydrogen cooling on the galaxy formation threshold. We calculate the fraction of dark matter (DM) halos that exceeds a critical mass required for star formation, M crit ( z ), as a function of their peak mass. By convolving analytic halo mass accretion histories (MAHs) with models for M crit ( z ), we predict that halos with peak virial masses below ∼10 8 M ⊙ can form stars before reionization through H 2 cooling. These halos remain dark when only H i cooling and reionization are modeled. However, less than ≈10% of halos with peak masses below ∼10 7 M ⊙ ever exceed M crit ( z ), even when H 2 cooling is included; this threshold is primarily set by relative streaming motion between DM and baryons imprinted at recombination. We obtain similar results using subhalo MAHs from an extremely high-resolution cosmological DM-only zoom-in simulation of a Milky Way (MW) analog (particle mass 6.3 × 10 3 M ⊙ ). Based on the abundance of MW satellites, these results imply that at least some known ultrafaint dwarf galaxies formed through H 2 cooling. This work sharpens predictions for the galaxy formation threshold and demonstrates how its essential features emerge from the underlying distribution of halo growth histories.