{
  "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 $M_{\\rm vir}$, $M_{\\rm gas}$, 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 ($M_{\\rm HI}$, AUC $=0.739$), the extent of the neutral gas ($r_{\\rm HI,50}/R_{\\rm vir}$, AUC $=0.738$), and the central HI density ($ρ_{\\rm HI,0}$, 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 $z\\simeq2$. 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.",
  "abstract_shown": true,
  "affiliations": [],
  "article_type": null,
  "arxiv": "2609.30267",
  "author_total": 10,
  "authors": [
    {
      "name": "Jorge Moreno"
    },
    {
      "name": "Defne Aluç"
    },
    {
      "name": "Eisha López"
    },
    {
      "name": "Vivienne Wood"
    },
    {
      "name": "Emmanuel Santiago"
    },
    {
      "name": "Francisco J. Mercado"
    },
    {
      "name": "Caleb R. Choban"
    },
    {
      "name": "Coral Wheeler"
    },
    {
      "name": "M. Katy Rodriguez Wimberly"
    },
    {
      "name": "Robert Feldmann"
    }
  ],
  "bibtex": "https://research.draconitic.com/paper/2w66a95c7g221.bib",
  "cited_by": 0,
  "doi": "10.48550/arxiv.2609.30267",
  "fulltext_url": null,
  "id": "2w66a95c7g221",
  "issns": [],
  "landing_url": "https://arxiv.org/abs/2609.30267",
  "language": null,
  "licence": "CC-BY-4.0",
  "licence_label": "CC BY 4.0",
  "licence_source": "article",
  "licence_url": "http://creativecommons.org/licenses/by/4.0/",
  "metadata_licence": "CC0-1.0",
  "native_subjects": [
    [
      "arxiv",
      "astro-ph.GA"
    ],
    [
      "openalex",
      "3103|Galaxies: Formation, Evolution, Phenomena"
    ]
  ],
  "openalex": "W7214303596",
  "pdf_url": "https://arxiv.org/pdf/2609.30267",
  "pmcid": null,
  "pmid": null,
  "published": "2026-09-24",
  "publisher": null,
  "ris": "https://research.draconitic.com/paper/2w66a95c7g221.ris",
  "sources": [
    {
      "first_seen": "2026-10-30T12:24:11Z",
      "native_id": "2609.30267",
      "source": "arxiv",
      "url": "https://arxiv.org/abs/2609.30267"
    }
  ],
  "subjects": [
    {
      "field": "astronomy",
      "origin": "arxiv:astro-ph.GA",
      "subfield": "galaxies"
    }
  ],
  "title": "Igniting Galaxy Formation in the Postreionization Universe, II: Nature versus Nurture",
  "title_text": "Igniting Galaxy Formation in the Postreionization Universe, II: Nature versus Nurture",
  "type": "preprint",
  "updated_at": "2026-10-30T12:24:11Z",
  "url": "https://research.draconitic.com/paper/2w66a95c7g221",
  "venue": "arXiv",
  "versions": []
}