Igniting Galaxy Formation in the Postreionization Universe, II: Nature versus Nurture
Abstract
Although the ultraviolet background from reionization is thought to conspire against the smallest halos, some gas-rich halos in the post-reionization Universe still form stars. We ask which properties, environmental or intrinsic, determine ignition. Using the FIREbox cosmological volume, we compare recently-ignited halos to a mass- and gas-matched sample of gas-resolved starless halos at z=2--6. We test three families of "third axis" properties beyond Mvir, Mgas, and z: geometric environment (the Perturbation Index and local galaxy density ρ5), hydrodynamic environment (the $1−−2,R_{\rm vir}$ gas-supply shell), and intrinsic halo properties (concentration and the neutral-gas reservoir). A nested random-forest classification finds that geometric environment adds little, while the hydrodynamic shell is modestly informative. Three complementary properties each discriminate ignited from starless halos beyond the mass--gas--redshift baseline (AUC =0.635, where $0.5$ is chance): total HI mass (MHI, AUC =0.739), the extent of the neutral gas (rHI,50/Rvir, AUC =0.738), and the central HI density (ρHI,0, AUC =0.700); jointly they reach AUC =0.823, the highest of any combination we test. At fixed halo mass, gas mass and redshift, recently-ignited halos harbor more HI over a larger radius than their matched controls, and these two enhancements are uncorrelated. The same reservoir sets when ignition can happen: from z=6 to z=0 the median starless halo keeps its gas (+0.05 dex) while its neutral fraction falls by $3.9$ dex, so the reservoir is ionized rather than exhausted and the ignition window closes below z≃2. In sum, a halo's own neutral-gas reservoir discriminates ignited from starless halos more sharply than its location in the cosmic web -- nature over nurture.
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BibTeX
@misc{moreno2026igniting,
title = {{Igniting Galaxy Formation in the Postreionization Universe, II: Nature versus Nurture}},
author = {Jorge Moreno and Defne Aluç and Eisha López and Vivienne Wood and Emmanuel Santiago and Francisco J. Mercado and Caleb R. Choban and Coral Wheeler and M. Katy Rodriguez Wimberly and Robert Feldmann},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.30267},
eprint = {2609.30267},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.30267},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Igniting Galaxy Formation in the Postreionization Universe, II: Nature versus Nurture
AU - Jorge Moreno
AU - Defne Aluç
AU - Eisha López
AU - Vivienne Wood
AU - Emmanuel Santiago
AU - Francisco J. Mercado
AU - Caleb R. Choban
AU - Coral Wheeler
AU - M. Katy Rodriguez Wimberly
AU - Robert Feldmann
PY - 2026
DA - 2026/09/24
PB - arXiv
DO - 10.48550/arxiv.2609.30267
UR - https://arxiv.org/abs/2609.30267
AB - Although the ultraviolet background from reionization is thought to conspire against the smallest halos, some gas-rich halos in the post-reionization Universe still form stars. We ask which properties, environmental or intrinsic, determine ignition. Using the FIREbox cosmological volume, we compare recently-ignited halos to a mass- and gas-matched sample of gas-resolved starless halos at z=2--6. We test three families of "third axis" properties beyond M_vir, M_gas, and z: geometric environment (the Perturbation Index and local galaxy density ρ_5), hydrodynamic environment (the $1−−2,R_{\rm vir}$ gas-supply shell), and intrinsic halo properties (concentration and the neutral-gas reservoir). A nested random-forest classification finds that geometric environment adds little, while the hydrodynamic shell is modestly informative. Three complementary properties each discriminate ignited from starless halos beyond the mass--gas--redshift baseline (AUC =0.635, where $0.5$ is chance): total HI mass (M_HI, AUC =0.739), the extent of the neutral gas (r_HI,50/R_vir, AUC =0.738), and the central HI density (ρ_HI,0, AUC =0.700); jointly they reach AUC =0.823, the highest of any combination we test. At fixed halo mass, gas mass and redshift, recently-ignited halos harbor more HI over a larger radius than their matched controls, and these two enhancements are uncorrelated. The same reservoir sets when ignition can happen: from z=6 to z=0 the median starless halo keeps its gas (+0.05 dex) while its neutral fraction falls by $3.9$ dex, so the reservoir is ionized rather than exhausted and the ignition window closes below z≃2. In sum, a halo's own neutral-gas reservoir discriminates ignited from starless halos more sharply than its location in the cosmic web -- nature over nurture.
ER -