Little Red Dots host Black Hole Stars: A unified family of gas-reddened AGN revealed by JWST/NIRSpec spectroscopy

Anna de Graaff, Raphael E. Hviding, Rohan P. Naidu, Jenny E. Greene, Tim B. Miller, Joel Leja, Jorryt Matthee, Gabriel Brammer, Harley Katz, Rachel Bezanson, Leindert A. Boogaard, Sownak Bose, John Chisholm, Nikko J. Cleri, Pratika Dayal, Robert Feldmann, Yoshinobu Fudamoto, Seiji Fujimoto, Lukas J. Furtak, Karl Glazebrook, Rashmi Gottumukkala, Kasper E. Heintz, Vasily Kokorev, Ivo Labbe, Michael V. Maseda, Ian McConachie, Themiya Nanayakkara, Erica Nelson, Przemysław Nowaczyk, Pascal A. Oesch, Hans-Walter Rix, David J. Setton, Alberto Torralba, Fabian Walter, Bingjie Wang, Andrea Weibel, Arjen van der Wel

First listed 2025-11-26 | Last updated 2025-11-26

Abstract

We use the DAWN JWST Archive to construct and characterise a sample of 116 little red dots (LRDs) across 2.3<z<9.3, selecting all sources with v-shaped UV-optical continua from NIRSpec/PRISM spectra and compact morphologies in NIRCam/F444W imaging. We show that LRD continuum spectra are ubiquitously well described by modified blackbodies across ~$0.4-1.0μ$m, with typical T~5000K or $λ_{peak}$~$0.65μ$m across 2 dex in luminosity, and a tail toward T~2000K. LRDs therefore trace a locus in the Hertzsprung-Russell diagram that is directly analogous to stars on the Hayashi track, strongly supporting the picture that LRDs are AGN embedded in thermalised dense gas envelopes in approximate hydrostatic equilibrium. Hotter LRDs with $λ_{peak}<0.65μ$m typically have strong Balmer breaks, redder UV slopes and high optical luminosities; other LRDs show weak or no Balmer breaks, and wide variety in $β_{UV}$ and $L_{5100}$. Crucially, we demonstrate that the UV-optical continuum shapes and luminosities are strongly linked to the $Hα,\ Hβ$, [OIII] and OI line properties. There is a tight linear relation between the H$α$ and optical continuum luminosities, as well as H$α$ and OI$_{8446}$, indicating that Balmer, OI and optical emission must primarily be powered by the same source. The Balmer decrement increases strongly toward higher $L_{Hα}$, $L_{5100}$ and Balmer break strength, providing key evidence for luminosity-dependent effects of collisional (de-)excitation and resonant scattering in the gaseous envelopes. In contrast, we show that [OIII] emission likely originates from star-forming host galaxies, and that its strong correlation with Balmer break strength arises naturally from variation in the AGN-to-host ratio. Our work presents an empirical description of the nature and structure of LRDs, defining a new benchmark for ongoing LRD model developments.

Short digest

From the DAWN JWST Archive, the authors assemble 116 little red dots at 2.3<z<9.3 with v‑shaped UV–optical continua and compact F444W morphologies, then show their 0.4–1.0 μm continua are ubiquitously fit by modified blackbodies with typical T≈5000 K (λpeak≈0.65 μm) and a tail down to ≈2000 K. In the T–luminosity plane, LRDs trace a Hayashi‑track–like locus, supporting a “black hole star” picture: accreting black holes embedded in dense, thermalized gas envelopes near hydrostatic equilibrium. Continuum shapes tightly couple to lines: L(Hα) scales linearly with L5100 and with OIλ8446, while the Balmer decrement rises with L(Hα), L5100, and Balmer‑break strength, consistent with collisional (de‑)excitation and resonant scattering in the envelopes. [O III] instead tracks the host, with its correlation to the Balmer break emerging from AGN‑to‑host variations, yielding an empirical blueprint for LRD structure and powering.

Key figures to inspect

• Population T–L diagram (or λpeak–luminosity plane): verify the Hayashi‑like locus and the spread from ~2000–5000 K; see where hotter LRDs (λpeak<0.65 μm) sit relative to cooler ones.
• Representative NIRSpec/PRISM SED fits: compare modified‑blackbody fits across 0.4–1.0 μm for objects with strong versus weak/no Balmer breaks to see how continuum shape, βUV, and L5100 co‑vary.
• Hα luminosity versus L5100: inspect slope and scatter of the tight linear relation and how it partitions by Balmer‑break strength or λpeak, reinforcing a shared power source.
• Hα versus OIλ8446 and Balmer decrement trends: check the Hα–OI8446 correlation and the rise of Hα/Hβ with L(Hα), L5100, and break strength as evidence for collisional and resonant processes in dense envelopes.
• [O III] strength versus Balmer‑break or AGN/host proxy: assess that [O III] likely originates in star‑forming hosts and that its correlation with the break follows AGN‑to‑host ratio changes; note any outliers.