Overmassive No More: The Case for Little Red Dots Hosting Black Hole Seeds as Massive as Single Supermassive Stars
Abstract
Little Red Dots (LRDs) display singular properties unlike any known class of AGN or galaxies, motivating novel mass estimators for their central engines. Inspired by their similarities to stellar phenomena, here we interpret the LRD continuum as being produced by a pseudo-photosphere. We fit tailored stellar atmosphere models to host-subtracted LRD central engines ("black hole stars," BH*s) represented by stacks of $117$ objects. Typical BH* continuum spectra are well fit by models in a narrow range of temperatures (Teff≈4200−4800 K), with bolometric luminosities ≈1043−45 erg s−1, implying pseudo-photospheric radii ≈700−2000 au. Based on these parameters, we explore four different approaches to deriving BH* masses: 1) using the surface gravity from atmosphere models; 2) appealing to the resemblance to super-Eddington phenomena; 3) approximating the escape velocity from the outflowing material; and 4) exploiting the lack of variability to bound the dynamical time. For the typical BH*, all of these methods yield remarkably consistent masses of ≈104−5 M⊙, implying a highly super-Eddington luminosity of Lbol/LEdd∼5−50. These mass estimates place BH*s within the scatter of the local scaling relation between black hole mass and host galaxy stellar mass, providing a self-consistent alternative to "overmassive" black holes that lie 2−3 dex above it. Crucially, our derived masses are consistent with BH*s arising from single supermassive stars (SMSs), whose masses cannot exceed ≈105−6 M⊙ due to general relativistic instabilities. Furthermore, for our derived Lbol/LEdd, the sharp cutoff of the LRD luminosity function matches the maximum theoretical mass of an SMS. With LRDs, we may therefore be directly observing the birth of heavy black hole seeds.
Related
- Halo-driven Origin and Evolution of Overmassive Black Holes and Little Red Dots
- Overmassive supermassive black holes in SDSS close galaxy pairs
- A Spectral Framework for Testing the Quasi-Star Hypothesis in Little Red Dots I: Weighing LRDs by Their Super-Eddington Luminosity Ratios---No Signs of Overmassive Black Holes
- From ASTRID to BRAHMA – The role of overmassive black holes in little red dots in cosmological simulations
- Inflated Supermassive Stars as Little Red Dots and Progenitors of Supermassive Black Holes
BibTeX
@misc{sun2026overmassive,
title = {{Overmassive No More: The Case for Little Red Dots Hosting Black Hole Seeds as Massive as Single Supermassive Stars}},
author = {Wendy Q. Sun and Rohan P. Naidu and Hanpu Liu and Anna de Graaff and Jenny E. Greene and Jorryt Matthee and Chris Ashall and John Chisholm and Anna-Christina Eilers and Qinyue Fei and Kasper E. Heintz and Daichi Hiramatsu and Vasily Kokorev and Joel Leja and Zhaoran Liu and Priyamvada Natarajan and Pascal A. Oesch and Robert A. Simcoe and Alberto Torralba and Andrea Weibel},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.09274},
eprint = {2609.09274},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.09274},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Overmassive No More: The Case for Little Red Dots Hosting Black Hole Seeds as Massive as Single Supermassive Stars
AU - Wendy Q. Sun
AU - Rohan P. Naidu
AU - Hanpu Liu
AU - Anna de Graaff
AU - Jenny E. Greene
AU - Jorryt Matthee
AU - Chris Ashall
AU - John Chisholm
AU - Anna-Christina Eilers
AU - Qinyue Fei
AU - Kasper E. Heintz
AU - Daichi Hiramatsu
AU - Vasily Kokorev
AU - Joel Leja
AU - Zhaoran Liu
AU - Priyamvada Natarajan
AU - Pascal A. Oesch
AU - Robert A. Simcoe
AU - Alberto Torralba
AU - Andrea Weibel
PY - 2026
DA - 2026/09/08
PB - arXiv
DO - 10.48550/arxiv.2609.09274
UR - https://arxiv.org/abs/2609.09274
AB - Little Red Dots (LRDs) display singular properties unlike any known class of AGN or galaxies, motivating novel mass estimators for their central engines. Inspired by their similarities to stellar phenomena, here we interpret the LRD continuum as being produced by a pseudo-photosphere. We fit tailored stellar atmosphere models to host-subtracted LRD central engines ("black hole stars," BH*s) represented by stacks of $117$ objects. Typical BH* continuum spectra are well fit by models in a narrow range of temperatures (T_eff≈4200−4800 K), with bolometric luminosities ≈10^43−45 erg s^−1, implying pseudo-photospheric radii ≈700−2000 au. Based on these parameters, we explore four different approaches to deriving BH* masses: 1) using the surface gravity from atmosphere models; 2) appealing to the resemblance to super-Eddington phenomena; 3) approximating the escape velocity from the outflowing material; and 4) exploiting the lack of variability to bound the dynamical time. For the typical BH*, all of these methods yield remarkably consistent masses of ≈10^4−5 M_⊙, implying a highly super-Eddington luminosity of L_bol/L_Edd∼5−50. These mass estimates place BH*s within the scatter of the local scaling relation between black hole mass and host galaxy stellar mass, providing a self-consistent alternative to "overmassive" black holes that lie 2−3 dex above it. Crucially, our derived masses are consistent with BH*s arising from single supermassive stars (SMSs), whose masses cannot exceed ≈10^5−6 M_⊙ due to general relativistic instabilities. Furthermore, for our derived L_bol/L_Edd, the sharp cutoff of the LRD luminosity function matches the maximum theoretical mass of an SMS. With LRDs, we may therefore be directly observing the birth of heavy black hole seeds.
ER -