Little red dots host black hole stars: a unified family of gas-reddened AGN revealed by JWST /NIRSpec spectroscopy

ABSTRACT We construct and characterize a sample of 146 little red dots (LRDs) across ${2.0{\lt} z{\lt} 9.3}$, selecting all sources with v-shaped ultraviolet (UV)–optical continua from James Webb Space Telescope (JWST) NIRSpec/PRISM spectra and compact morphologies in NIRCam/F444W imaging. We show that LRD continuum spectra are well described by modified blackbodies across $\sim 0.4-1.0\, \rm{\mu m}$, with typical $T\sim 5000\,$ K ($\lambda _{\rm peak}\sim 0.65\, \rm{\mu m}$) across 2 dex in luminosity, and a tail toward $T\sim 2000\,$ K. LRDs therefore trace a locus in the Hertzsprung–Russell diagram that is analogous to stars on the Hayashi track, strongly supporting the picture that LRDs are active galactic nucleus (AGN) embedded in optically thick dense gas envelopes. Hotter LRDs with $\lambda _{\rm peak}\lt 0.65\, \rm{\mu m}$ typically have strong Balmer breaks, redder UV slopes, and high optical luminosities; other LRDs show weak or no Balmer breaks, and wide variety in $\beta _{\rm UV}$ and $L_{5100}$. Crucially, we demonstrate that the UV–optical continua are strongly linked to the H$\alpha$, H$\beta$, [O iii], and O i line properties. There is a tight linear relation between the H$\alpha$ and optical continuum luminosities, as well as H$\alpha$ and $\rm {O\, {\small I} }_{\ \lambda 8446}$, indicating that Balmer, O i, and optical emission must primarily be powered by the same source. The Balmer decrement increases strongly toward higher $L_{\rm H\alpha }$, $L_{5100}$, 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 [O iii] emission likely originates from star-forming host galaxies, with strong variation in the AGN-to-host ratio among the LRD population. Our work presents an empirical description of the nature and structure of LRDs, defining a new benchmark for ongoing LRD model developments.