A Black-Hole Envelope Interpretation for Cosmological Demographics of Little Red Dots

Hiroya Umeda, Kohei Inayoshi, Yuichi Harikane, Kohta Murase

First listed 2025-12-03 | Last updated 2025-12-03

Abstract

Little red dots (LRDs) newly discovered with JWST are active galactic nuclei (AGN) that may represent black hole (BH) growth at the earliest cosmic epochs. These sources show puzzling features unlike typical AGNs, including red optical continua, weak hot-dust emission, and a lack of detectable X-rays. Previously, LRDs have often been interpreted as dust-reddened AGNs, leading to severe inconsistencies with the luminosity and BH mass densities inferred for previously known AGNs over $0<z<5$. The BH-envelope (BHE) model has been proposed to explain these characteristics, in which an accreting BH is enshrouded by a dense, optically thick gaseous envelope. In this Letter, we reanalyze the SEDs of $\sim 400$ photometric LRDs in the COSMOS-Web survey using the BHE model and reassess their implications for cosmological BH evolution. We find that the optical-NIR spectra of LRDs are well reproduced by blackbody emission with an effective temperatures of $4000-6000~\K$. Within the BHE framework, the inferred bolometric luminosities decrease by $\gtrsim1-2$ orders of magnitude compared to dust-reddened AGN assumptions. As a result, the revised luminosity function, BH accretion density, and BH mass function become consistent with those of AGNs at $z<5$. The stellar masses of LRD hosts are estimated by attributing the UV excesses to star formation. Although the resulting $M_{\rm BH}/M_\star$ ratio remains higher than the local empirical value, the excess is modest. Overall, the BHE model not only resolves the spectral features of LRDs but also brings their statistical properties into agreement with the broader cosmological BH population.

Short digest

Reanalyzing 434 COSMOS-Web LRDs (148 with MIRI for the luminosity function), the authors fit SEDs with a black-hole envelope model: a 4,000–6,000 K blackbody photosphere with a Balmer break plus a UV power law. This reproduces the V-shaped optical–NIR spectra and MIRI non-detections while reducing inferred bolometric luminosities by ≥1–2 dex relative to dust-reddened AGN fits. With these revisions, the LRD luminosity function, BH accretion density, and BH mass function align with those of AGNs at z<5. UV excesses are attributed to star formation, yielding host masses that keep MBH/M⋆ modestly above but close to the local relation.

Key figures to inspect

• Example SED-fit panels for representative LRDs showing the blackbody+Balmers break+UV power-law components; check how the Balmer break location constrains redshift and how MIRI upper limits force the cool (4–6 kK) photosphere solution.
• Histogram (or corner plot) of fitted photospheric temperatures and Balmer-break amplitudes; verify that most sources cluster at 4–6 kK and that break strength tracks the red optical slope.
• Bolometric luminosity function derived from the 148-object MIRI-covered fiducial sample; compare directly to the dust-reddened assumption to see the ≥1–2 dex downward shift.
• BH accretion density and BH mass function versus redshift; look for overlap with z<5 AGN constraints to assess demographic consistency under the envelope interpretation.
• MBH/M⋆ versus M⋆ (or distribution) inferred from attributing UV excess to star formation; check that the ratio shows only a modest elevation over the local relation.