The quasi-star model for little red dots: Potential and challenges

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Abstract
Little red dots (LRDs) are a class of sources discovered by the James Webb Space Telescope observationally defined by a V-shaped rest-frame UV-optical spectral energy distribution, a compact or unresolved morphology, and, frequently, broad hydrogen emission lines. How these characteristics translate into physical and structural properties remains unclear. Among various models, those involving a quasi-star interpret LRDs as an intermediate stage in the evolution of a super-massive black hole (SMBH) seed into a classic active galactic nucleus. In this paper, we employ the radiative-transfer code Cloudy to study whether this model is able to reproduce the spectral features commonly observed in LRDs. The model consists of an accreting SMBH ( M BH ∼ 10 5-6 M ⊙ ) surrounded by a convective layer where a black-body (BB) spectrum with T ∼ 5000 K and L ∼ 10 44.4 erg s −1 is produced. This BB spectrum is then reprocessed by a concentric thick (∆ R ∼ 1000 AU) shell of dense ( n H ∼ 10 11 cm −3 ) gas partially ionised by thermal collisions. The emerging radiation is further reprocessed by a diffuse clumpy medium surrounding the quasi-star. We fit this model to JWST/NIRSpec spectra of LRDs from the literature, deriving the main physical parameters and the SMBH masses. Once coupled with the UV emission from a host galaxy, this model is able to reproduce the shape of the UV-to-NIR continuum, including the presence of a Balmer break, as well as the luminosity of the hydrogen emission lines. However, this quasi-star model does not natively account for the presence of broad helium lines and for the possible presence of hot dust, needing additional components to match these observables. Our main result is to show how some LRDs can be modelled as quasi-stars, highlighting that a significant degeneracy exists among different LRD models. This has important consequences for our understanding of the mechanisms driving black hole growth in the early Universe.

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