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

Lorenz Zwick, Christopher Tiede, Lucio Mayer

First listed 2025-07-29 | Last updated 2026-03-03

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.

Short digest

Models little red dots as supermassive stars fed by massive self‑gravitating discs assembled in gas‑rich major mergers, reproducing the V‑shaped SED as two blackbodies and attributing Balmer broadening to disc rotation—without invoking extra AGN, stellar, or dust components. MCMC fits to two representative LRDs (J0647‑1045 and COS‑756434) yield sub‑Eddington disc accretion and SMS masses of a few ×10^6 Msun with physically motivated parameters. The framework predicts a luminosity cut‑off of order few ×10^44 erg/s and ties the observed redshift distribution of LRDs to merger rates, matching current trends. Acknowledged caveat: reproducing the absolute abundance of LRDs remains challenging, though the fits are robust to observed diversity.

Key figures to inspect

• Figure 2 — Use the inclination/temperature/size experiments to see how the SMS+SMD two‑BB superposition shapes the V‑spectrum; note which parameters most strongly tilt the red optical slope and the UV peak, clarifying degeneracies broken by physical constraints in Section 5.
• Figure 3 — Inspect the MCMC posterior panels for correlations between SMS/SMD temperatures and radii, then check the overplotted fits to J0647‑1045 and COS‑756434 against the observed V‑shape and Balmer features, and verify consistency with the quoted X‑ray/IR limits without any added AGN/dust component.
• Figure 4 — Compare gas‑mass vs stellar‑mass merger‑rate curves to the observed LRD redshift histogram; the gas‑weighted major‑merger track reproduces the full distribution, supporting a merger‑driven LRD formation path and indicating when LRD incidence should peak.
• Figure 1 — Schematic pathway from gas‑rich major mergers to a sub‑pc self‑gravitating disc feeding a hot SMS; use it to map which structure sources the blue/UV vs red/optical blackbody components and where intrinsic rotation broadens Balmer lines.