Synthetic Spectral Library of Optically Thick Atmospheres for Little Red Dots

Hanpu Liu, Yan-Fei Jiang, Eliot Quataert, Jenny E. Greene, Yilun Ma, Xiaojing Lin

First listed 2026-03-02 | Last updated 2026-03-02

Abstract

Little Red Dots (LRDs) challenge conventional models of active galactic nuclei. At rest-optical-to-near-infrared (IR) wavelengths, these compact extragalactic objects show blackbody-like continuum emission and spectral features reminiscent of stars, motivating models with an optically thick atmosphere at $T_{\rm\!\,eff}\sim4000-5000{\rm~K}$. We develop (and publicly release) a synthetic spectral library of optically thick atmospheres with gas conditions tailored for LRDs, parameterized by effective temperature $T_{\rm\!\,eff}$ and surface gravity $g$. Given the uncertain dynamical structure of LRDs, we interpret $g$ most directly as a photospheric density $ρ_{\rm\!\,ph}$. We show that blackbodies are only crude approximations to the emission from LRD-like atmospheres. Spectral features are abundant, many of which are sensitive diagnostics of photospheric density, including the overall curvature of the spectral energy distribution, the rest-$1.6{\rm~μm}$ spectral ''kink'' from $\rm H^-$ opacity, and the Ca II triplet (CaT) absorption at rest-8500 $\unicode{x212B}$. When compared against a local LRD, \egg, all three features consistently indicate a low photospheric density of $ρ_{\rm\!\,ph}\sim 10^{-11}{\rm~g~cm^{-3}}$ ($g\sim10^{-3}{\rm~cm~s^{-2}}$ in our library). This disfavors hydrostatic configurations and suggests a mass within the photosphere (black hole plus gas) of $10^4~M_\odot$, with an Eddington ratio $λ_{\rm Edd}\gtrsim20$, if the CaT width traces turbulent support at the photosphere in spherical symmetry; the inferred mass could be higher depending on the geometry and the radius probed by CaT. For higher redshift LRDs, we advocate for rest-near-IR spectroscopic surveys and high-resolution spectra of potential absorption lines as a test of the optically thick atmosphere scenario and as a unique probe of the central engine mass.

Short digest

Builds and releases a TLUSTY/SYNSPEC-based spectral library for optically thick, LRD-like atmospheres (Teff ≈ 4–5 kK) parameterized by Teff and a surface gravity interpreted as photospheric density. Blackbodies underpredict structure: the SED curvature, the rest-1.6 μm H− “kink,” and Ca II triplet absorption at 8500 Å emerge as density-sensitive diagnostics. Applied to the local LRD “the Egg,” all three indicators favor ρ_ph ≈ 10^-11 g cm^-3 (g ≈ 10^-3 cm s^-2), disfavoring hydrostatic configurations and implying Mwithin,ph ≈ 10^4 M⊙ with λ_Edd ≳ 20 if CaT width traces turbulent support. The work motivates rest-near-IR surveys and high-resolution absorption-line spectroscopy at higher z to test the optically thick atmosphere picture and to weigh LRD central engines.

Key figures to inspect

• Model grid overview: synthetic spectra across Teff ≈ 4000–5000 K and decreasing g (increasing ρ_ph), showing the SED curvature and deviations from a blackbody; inspect how the H−-driven continuum shape strengthens with lower ρ_ph.
• Zoom on the rest-1.6 μm region: amplitude and sharpness of the H− “kink” versus photospheric density and metallicity; learn which ρ_ph values reproduce the blue near-IR plus red optical seen in LRDs.
• Ca II triplet window (~8400–8700 Å): predicted EW and line widths versus ρ_ph and microturbulence, with overplot of the Egg; see how the best-fit lines imply ρ_ph ≈ 10^-11 g cm^-3 and translate to Mwithin,ph and λ_Edd under spherical, turbulence-supported assumptions.
• Atmosphere structure and contribution functions: T(τ), ρ(τ), and line/continuum contribution for CaT and 1.6 μm; understand where the features form and why low g (low ρ_ph) drives the observed continuum curvature and line depths.
• Ca H&K transfer test: profiles with and without partial frequency redistribution at low densities; assess how PFR alters cores and why it matters for using resonance lines as diagnostics in LRD regimes.