Little Red Dots as self-gravitating discs accreting on supermassive stars: Spectral appearance and formation pathway of the progenitors to direct collapse black holes

TL;DR

This paper models 'little red dots' as super-massive stars with accretion discs formed from galaxy mergers, explaining their spectral features and linking their evolution to black hole formation.

Contribution

It introduces a novel model of LRDs as super-massive stars with self-gravitating discs, fitting observed spectra without extra components, and predicts black hole formation consistent with galaxy relations.

Findings

Spectral features explained by superposition of black bodies and rotation.

Model parameters consistent with sub-Eddington accretion and observed luminosities.

Redshift distribution aligns with galaxy merger-driven formation scenarios.

Abstract

We propose an alternative physical interpretation and formation pathway for the recently discovered "little red dots" (LRDs). We model LRDs as super-massive stars (SMSs) surrounded by massive self-gravitating accretion discs (SMDs) that form as a consequence of gas-rich major galaxy mergers. The model provides an excellent match for numerous spectral features of LRDs, where the V-shape arises from the superposition of two black bodies, and Balmer line broadening is sourced by the intrinsic rotation of the SMD. No additional AGN, stellar, dust, or broadening component is strictly required. This results in a model with physically motivated parameters that are robust to variations in observed LRD properties. We perform MCMC fits for two representative LRD spectra, for which the full parameter posterior distributions are determined. Allowing for a compressed SMS mass-radius relation, the recovered parameters are compatible with sub-Eddington accretion in self-gravitating discs, and the recovered SMS masses of few $ 10^6$ M$_{\odot}$ imply the subsequent formation of massive black holes (BH) that squarely follow the expected BH mass--galaxy mass relation, while also predicting a cut-off luminosity of order few $10^{44}$ erg/s in quantitative agreement with current observations. While matching the abundance of LRDs is challenging, the association to galaxy mergers produces a redshift distribution that reflects observations.