Little Red Dots as Self-gravitating Disks Accreting on Supermassive Stars: Spectral Appearance and Formation Pathway of the Progenitors to Direct Collapse Black Holes

Abstract We propose an alternative physical interpretation and formation pathway for the recently discovered “little red dots” (LRDs). We model LRDs as supermassive stars (SMSs) surrounded by massive self-gravitating accretion disks (supermassive disks, 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 active galactic nucleus, 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 Markov Chain Monte Carlo 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 disks, and the recovered SMS masses of few 10 6 M ⊙ imply the subsequent formation of massive black holes (BHs) that squarely follow the expected BH mass–galaxy mass relation, while also predicting a cutoff luminosity of the order of few 10 44 erg s −1 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.