Optically Thick Outflow Driven by Supercritical Accretion May Explain Little Red Dots

Abstract

Recent JWST observations have revealed a population of compact, optically red sources known as Little Red Dots (LRDs). A popular interpretation is that LRDs host massive black holes embedded in dense gaseous envelopes, yet the physical origin of such envelopes remains unclear. We propose that the optically thick outflow driven by supercritical accretion onto black holes may explain the envelope. Based on an analytic radiative hydrodynamic outflow model, we relate the outflow properties to the black hole mass $M$ and dimensionless accretion rate $\dot{m}$. With $M\sim10^5-10^7\,M_\odot$ and $\dot m \sim 1500-5000$, the outflow photosphere reaches luminosities of $10^{43}-10^{45}\,\rm erg\,s^{-1}$ and temperatures of $\sim 3000-6000$ K, effectively matching the red optical continua observed in LRDs. The presence of a thick scattering region beyond the photosphere is central to deciphering the distinctive properties of LRDs. For each object, if one derives $M$ and $\dot{m}$ from the observed luminosity and temperature, while accounting for both kinematic broadening and scattering effects, the model predicts an emission line FWHM consistent with observations within a factor of 1.5 for more than 80\% objects. Furthermore, with Cloudy simulations, we find that the partially ionized gas beyond the photosphere produces Balmer breaks broadly consistent with measurements.