The Impact of Molecular Hydrogen Cooling on the Galaxy Formation Threshold
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 ma...
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| Main Author: | |
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| Format: | Article |
| Language: | English |
| Published: |
IOP Publishing
2025-01-01
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| Series: | The Astrophysical Journal Letters |
| Subjects: | |
| Online Access: | https://doi.org/10.3847/2041-8213/adbc6e |
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| Summary: | 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. |
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| ISSN: | 2041-8205 |