The Impact of Molecular Hydrogen Cooling on the Galaxy Formation Threshold

Abstract

We study the impact of molecular (${\mathrm{H}}_2$) and atomic (HI) 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, $M_{\mathrm{crit}}(z)$, as a function of their peak mass. By convolving analytic halo mass accretion histories (MAHs) with models for $M_{\mathrm{crit}}(z)$, we predict that halos with peak virial masses below $\sim 10^8{M}_{\odot }$ can form stars before reionization through ${\mathrm{H}}_2$ cooling. These halos remain dark when only HI cooling and reionization are modeled. However, less than $\approx 10\%$ of halos with peak masses below $\sim 10^7{M}_{\odot }$ ever exceed $M_{\mathrm{crit}}(z)$, even when ${\mathrm{H}}_2$ 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 $6.3\times 10^3{M}_{\odot }$). Based on the abundance of MW satellites, these results imply that at least some known ultra-faint dwarf galaxies formed through ${\mathrm{H}}_2$ 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.

10 pages, 7 figures. Updated to published version

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• Impact of baryons on the population of Galactic subhalos and implications for dark matter searches
• Simulation based inference of the ionization history from the 2D 21 cm power spectrum
• Warm, not Fuzzy: Generalized Ultralight Dark Matter Limits from Milky Way Satellites