Abstract:
The emission mechanisms of Little Red Dots (LRDs)—compact extragalactic sources with red rest-optical continua and broad Balmer lines—remain elusive. The redness of LRDs is likely intrinsic, with a characteristic photosphere temperature of ~5000 K. Meanwhile, many LRD spectra exhibit a Balmer break, with the leading interpretation being a dense absorbing gas shell covering an AGN. Using semi-analytical atmosphere models and radiation transport calculations, we show that a super-Eddington accretion system gives rise to a Balmer break and a red optical color simultaneously without requiring an external absorber. 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 g cm^-3, implies a significant opacity contrast even at a cool 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 accretion flow implies an effective temperature 4000 K <~ T_eff <~ 6000 K that is very insensitive to the accretion rate. The continuum spectra from the latter scenario align with the Balmer break and optical color of currently known LRDs.
