”完全に暗い”ダークマターハローは存在するのか（Do “Completely Dark” Dark Matter Halos Exist?）

2025-04-08 カリフォルニア大学サンディエゴ校（UCSD）

＜関連情報＞

• https://today.ucsd.edu/story/dark-dark-matter-halos
• https://iopscience.iop.org/article/10.3847/2041-8213/adbc6e

水素分子の冷却が銀河形成の閾値に与える影響 The Impact of Molecular Hydrogen Cooling on the Galaxy Formation Threshold

Ethan O. Nadler
The Astrophysical Journal Letters  Published 2025 April 8
DOI:10.3847/2041-8213/adbc6e

Abstract

We study the impact of molecular (H₂) 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 ∼10⁸ M_⊙ can form stars before reionization through H₂ cooling. These halos remain dark when only H i cooling and reionization are modeled. However, less than ≈10% of halos with peak masses below ∼10⁷ M_⊙ ever exceed M_crit(z), even when H₂ 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 × 10³ M_⊙). Based on the abundance of MW satellites, these results imply that at least some known ultrafaint dwarf galaxies formed through H₂ 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.