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

Abstract

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 ∼5000 K. Meanwhile, many LRD spectra exhibit a Balmer break, often attributed to absorption by a dense gas shell surrounding an active galactic nucleus. Using semianalytical 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, ρ < 10 −9 g cm −3 , implies a significant opacity contrast even at a cool photosphere temperature of ∼5000 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 of 4000 K ≲ T eff ≲ 6000 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 the predictions of our model and the prospects for more realistic spectra based on super-Eddington accretion simulations.