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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IOP Publishing
2025-01-01
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| Series: | The Astrophysical Journal Letters |
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| Online Access: | https://doi.org/10.3847/2041-8213/adbc6e |
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| author | Ethan O. Nadler |
| author_facet | Ethan O. Nadler |
| author_sort | Ethan O. Nadler |
| collection | DOAJ |
| description | 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. |
| format | Article |
| id | doaj-art-e4961ea55b1e48c7a9f2e3c9821bec9d |
| institution | DOAJ |
| issn | 2041-8205 |
| language | English |
| publishDate | 2025-01-01 |
| publisher | IOP Publishing |
| record_format | Article |
| series | The Astrophysical Journal Letters |
| spelling | doaj-art-e4961ea55b1e48c7a9f2e3c9821bec9d2025-08-20T03:17:02ZengIOP PublishingThe Astrophysical Journal Letters2041-82052025-01-019831L2310.3847/2041-8213/adbc6eThe Impact of Molecular Hydrogen Cooling on the Galaxy Formation ThresholdEthan O. Nadler0https://orcid.org/0000-0002-1182-3825Department of Astronomy & Astrophysics, University of California , San Diego, La Jolla, CA 92093, USAWe 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.https://doi.org/10.3847/2041-8213/adbc6eGalaxy formationStar formationGalaxy dark matter halos |
| spellingShingle | Ethan O. Nadler The Impact of Molecular Hydrogen Cooling on the Galaxy Formation Threshold The Astrophysical Journal Letters Galaxy formation Star formation Galaxy dark matter halos |
| title | The Impact of Molecular Hydrogen Cooling on the Galaxy Formation Threshold |
| title_full | The Impact of Molecular Hydrogen Cooling on the Galaxy Formation Threshold |
| title_fullStr | The Impact of Molecular Hydrogen Cooling on the Galaxy Formation Threshold |
| title_full_unstemmed | The Impact of Molecular Hydrogen Cooling on the Galaxy Formation Threshold |
| title_short | The Impact of Molecular Hydrogen Cooling on the Galaxy Formation Threshold |
| title_sort | impact of molecular hydrogen cooling on the galaxy formation threshold |
| topic | Galaxy formation Star formation Galaxy dark matter halos |
| url | https://doi.org/10.3847/2041-8213/adbc6e |
| work_keys_str_mv | AT ethanonadler theimpactofmolecularhydrogencoolingonthegalaxyformationthreshold AT ethanonadler impactofmolecularhydrogencoolingonthegalaxyformationthreshold |