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

Jun-Rong Liu, Hua Feng, Luis C. Ho

First listed 2026-07-21 | Last updated 2026-07-21

Abstract

Short digest

Liu, Feng, and Ho argue that the dense red envelope inferred around Little Red Dots can arise naturally as the photosphere of an optically thick, radiation-driven outflow from supercritical black-hole accretion. Applying an analytic outflow model to 36 LRDs plus the local analogue The Egg, they find that black holes of roughly 10^5–10^7 solar masses accreting at dimensionless rates of about 1500–5000 reproduce the observed red-envelope luminosities and 3000–6000 K continua. A scattering region outside the photosphere then helps explain the broad Balmer lines: model FWHMs agree within a factor of 1.5 for over 80% of objects, while Cloudy calculations show that partially ionized outflow gas can also yield broadly observed Balmer-break strengths. The work supplies a physically motivated origin for the otherwise phenomenological dense-envelope picture of LRDs and links their continua, line widths, and Balmer features to a single supercritical-accretion framework.

Key figures to inspect

• Figure 2. This is the primary population-level test of the proposed photospheric-outflow interpretation: the measured luminosity-temperature locus of 36 LRD envelopes and The Egg is compared directly with model tracks spanning black-hole mass and accretion rate.
• Figure 3. This figure shows the key line-diagnostic result, comparing observed broad-line FWHMs with predictions that combine outflow kinematics and electron-scattering broadening. Its proximity to the one-to-one relation demonstrates why the extended scattering region is central to the model.
• Figure 4. This distribution-level comparison tests whether partially ionized gas beyond the photosphere can produce the observed Balmer breaks. It connects the continuum-fitting outflow solution to a spectral feature that is especially characteristic of LRDs.
• Figure 5. The Balmer-decrement distribution provides a complementary nebular diagnostic using object-by-object outflow parameters inferred from luminosity and temperature. It tests whether the same model that matches the red continuum and line widths remains broadly consistent with the hydrogen-line ratios.