Little Red Dot $-$ Host Galaxy $=$ Black Hole Star: A Gas-Enshrouded Heart at the Center of Every Little Red Dot

Wendy Q. Sun, Rohan P. Naidu, Jorryt Matthee, Anna de Graaff, John Chisholm, Jenny E. Greene, Pascal A. Oesch, Alberto Torralba, Raphael E. Hviding, Gabriel Brammer, Robert A. Simcoe, Sownak Bose, Rychard Bouwens, Pratika Dayal, Anna-Christina Eilers, Qinyue Fei, Lukas J. Furtak, Rashmi Gottumukkala, Andy Goulding, Kasper E. Heintz, Michaela Hirschmann, Vasily Kokorev, Joel Leja, Zhaoran Liu, Priyamvada Natarajan, Andrew D. Santarelli, David J. Setton, Aaron Smith, Sandro Tacchella, Marta Volonteri, Fabian Walter, Andrea Weibel, Christina C. Williams

First listed 2026-01-28 | Last updated 2026-01-28

Abstract

The central engines of Little Red Dots (LRDs) may be ``black hole stars" (BH*s), early stages of black hole growth characterized by dense gas envelopes. So far, the most direct evidence for BH*s comes from a handful of sources where the host galaxy is completely outshone as suggested by their remarkably steep Balmer breaks. Here we present a novel scheme to disentangle BH*s from their host galaxies assuming that the [OIII]5008Å line arises exclusively from the host. Using a sample of 98 LRDs ($z$~$2-9$) with high quality NIRSpec/PRISM spectra, we demonstrate that the host-subtracted median stack displays a Balmer break $>2\times$ stronger than massive quiescent galaxies, with the rest-optical continuum resembling a blackbody-like SED ($T_{\rm{eff}}$~$4050$ K, $\log(L_{\rm{bol}})$~$43.9$ erg s$^{-1}$, $R_{\rm{eff}}$~$1300$ au). We measure a steep Balmer decrement (H$α$/H$β>10$) and numerous density-sensitive features (e.g., FeII, HeI, OI). These are hallmark signatures of dense gas envelopes, providing population-level evidence that BH*s indeed power LRDs. In the median LRD, BH*s account for $\sim20\%$ of the UV emission, $\sim50\%$ at the Balmer break, and $\sim90\%$ at wavelengths longer than H$α$ with the remainder arising from the host. BH*s preferentially reside in low-mass galaxies ($M_{\rm{\star}}$~$10^{8}\,{\rm M}_{\rm{\odot}}$) undergoing recent starbursts, as evidenced by extreme emission line EWs (e.g., [OIII]5008Å~$1100$Å, CIII]~$12$Å), thereby favoring BH* origins linked to star-formation. We show V-shaped LRD selections are biased to high BH*/host fractions ($\gtrsim60\%$ at 5500Å) -- less dominant BH*s may be powering JWST's blue broad-line AGN. We find BH*s are so commonplace and transient (duty cycle $\sim1\%$, lifetime $\sim10$ Myrs) that every massive black hole may have once shone as a BH*.

Short digest

Using 98 LRDs at z≈2–9 with high-quality NIRSpec/PRISM spectra, the authors introduce an [O III]5008-anchored host-subtraction to isolate a putative “black hole star” (BH*) component from its galaxy. The host-subtracted median spectrum shows a Balmer break more than twice that of massive quiescent galaxies and a blackbody-like continuum (Teff≈4050 K, log Lbol≈43.9, Reff≈1300 au), alongside a steep Balmer decrement (Halpha/Hbeta>10) and dense-gas tracers (Fe II, He I, O I), arguing that dense gas envelopes power LRDs. BH*s contribute ≈20% of the UV, ≈50% at the Balmer break, and ≈90% redward of Halpha; they preferentially sit in low-mass, recent-starburst hosts (M*≈10^8 Msun) with extreme EWs ([O III]5008≈1100 A, C III]≈12 A). V-shaped selections pick systems with large BH*/host fractions, and the inferred duty cycle (~1%) and lifetime (~10 Myr) imply a common, transient BH* phase in massive black-hole growth.

Key figures to inspect

• [O III]5008-based host-subtraction schematic and validation: how the line anchors the host continuum scaling and reveals the residual BH* continuum with the oversized Balmer break and blackbody fit (Teff≈4050 K, Reff≈1300 au).
• Median PRISM stacks before/after subtraction highlighting the deep Balmer break and series, the steep Balmer decrement (Halpha/Hbeta>10), and density-sensitive Fe II/He I/O I features—use these to assess envelope density and optical depth.
• BH*/host flux-fraction vs wavelength: the ~20% (UV) → ~50% (Balmer break) → ~90% (>Halpha) trend; check how the fraction maps onto the V-shaped color selection and to individual object scatter.
• Host demographics and line-EW distributions: M*≈10^8 Msun and extreme EWs ([O III]5008≈1100 A, C III]≈12 A) tying BH*s to recent starbursts; look for how these compare to non-LRD dwarfs.
• Occurrence and timescale inference: number densities to duty cycle (~1%) and lifetime (~10 Myr); evaluate assumptions linking BH* incidence to “every massive BH once shone as a BH*.”