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

@article{sun2026little,
  title = {{Little Red Dot − Host Galaxy = Black Hole Star: A Gas-Enshrouded Heart at the Center of Every Little Red Dot}},
  author = {Wendy Q. Sun and Rohan P. Naidu and Jorryt Matthee and Anna de Graaff and John Chisholm and Jenny E. Greene and Pascal A. Oesch and Alberto Torralba and Raphael E. Hviding and Gabriel Brammer and Robert A. Simcoe and Sownak Bose and Rychard Bouwens and Pratika Dayal and Anna–Christina Eilers and Qinyue Fei and Lukas J. Furtak and Rashmi Gottumukkala and Andy D. Goulding and Kasper E. Heintz and Michaela Hirschmann and Vasily Kokorev and Joel Leja and Zhaoran Liu and Priyamvada Natarajan and Andrew D. Santarelli and David J. Setton and Aaron Smith and Sandro Tacchella and Marta Volonteri and Fabian Walter and Andrea Weibel and Christina C. Williams},
  journal = {The Open Journal of Astrophysics},
  year = {2026},
  month = may,
  doi = {10.33232/001c.162505},
  url = {https://doi.org/10.33232/001c.162505},
  note = {CC BY},
}

TY  - JOUR
TI  - Little Red Dot − Host Galaxy = Black Hole Star: A Gas-Enshrouded Heart at the Center of Every Little Red Dot
AU  - Wendy Q. Sun
AU  - Rohan P. Naidu
AU  - Jorryt Matthee
AU  - Anna de Graaff
AU  - John Chisholm
AU  - Jenny E. Greene
AU  - Pascal A. Oesch
AU  - Alberto Torralba
AU  - Raphael E. Hviding
AU  - Gabriel Brammer
AU  - Robert A. Simcoe
AU  - Sownak Bose
AU  - Rychard Bouwens
AU  - Pratika Dayal
AU  - Anna–Christina Eilers
AU  - Qinyue Fei
AU  - Lukas J. Furtak
AU  - Rashmi Gottumukkala
AU  - Andy D. Goulding
AU  - Kasper E. Heintz
AU  - Michaela Hirschmann
AU  - Vasily Kokorev
AU  - Joel Leja
AU  - Zhaoran Liu
AU  - Priyamvada Natarajan
AU  - Andrew D. Santarelli
AU  - David J. Setton
AU  - Aaron Smith
AU  - Sandro Tacchella
AU  - Marta Volonteri
AU  - Fabian Walter
AU  - Andrea Weibel
AU  - Christina C. Williams
PY  - 2026
DA  - 2026/05/25
JO  - The Open Journal of Astrophysics
DO  - 10.33232/001c.162505
UR  - https://doi.org/10.33232/001c.162505
AB  - 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 × stronger than massive quiescent galaxies, with the rest-optical continuum resembling a blackbody-like SED ( K, erg s, 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 ∼ 20 % of the UV emission, ∼ 50 % at the Balmer break, and ∼ 90 % at wavelengths longer than H α with the remainder arising from the host. BH*s preferentially reside in low-mass galaxies () 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 ( ≳ 60 % 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 ∼ 1 % , lifetime ∼ 10 Myrs) that every massive black hole may have once shone as a BH*.
SN  - 2565-6120
ER  -