The Balmer Break and Optical Continuum of Little Red Dots from Super-Eddington Accretion

TL;DR

This paper demonstrates that super-Eddington accretion models can naturally produce the observed Balmer break and red optical continuum of Little Red Dots without requiring external absorption or dust reddening, offering a new explanation for their spectral features.

Contribution

The study introduces a semi-analytical model showing super-Eddington accretion can explain LRD spectral features, challenging previous absorption-based interpretations.

Findings

Super-Eddington accretion produces a Balmer break similar to stellar atmospheres.

Optically thick, spherical accretion flows yield effective temperatures matching LRD observations.

The model's spectra align with observed optical colors and Balmer breaks of LRDs.

Abstract

The physical origin of Little Red Dots (LRDs)--compact extragalactic sources with red rest-optical continua and broad Balmer lines--remains elusive. The redness of LRDs is likely intrinsic, suggesting optically thick gas emitting at a characteristic effective temperature of $\sim5000{\rm~K}$. Meanwhile, many LRD spectra exhibit a Balmer break, often attributed to absorption by a dense gas shell surrounding an AGN. Using semi-analytical atmosphere models and radiation transport calculations, we show that a super-Eddington accretion system can give rise to a Balmer break and a red optical color simultaneously, without invoking external gas absorption for the break or dust reddening. The break originates from a discontinuity in opacity across the Balmer limit, similar to that of early-type stars, but the lower photosphere density of super-Eddington systems, $\rho<10^{-9}{\rm~g~cm^{-3}}$, implies a significant opacity contrast even at a cool photosphere temperature of $\sim5000{\rm~K}$. Furthermore, while accretion in the form of a standard thin disk requires fine tuning to match the optical color of LRDs, an alternative scenario of a geometrically thick, roughly spherical accretion flow implies an effective temperature $4000{\rm~K}\lesssim T_{\rm eff}\lesssim6000{\rm~K}$ that is very insensitive to the accretion rate (analogous to the Hayashi line in stellar models). The continuum spectra from the latter scenario align with the Balmer break and optical color of currently known LRDs. We discuss predictions of our model and the prospects for more realistic spectra based on super-Eddington accretion simulations.