Synthetic Spectral Library of Optically Thick Atmospheres for Little Red Dots

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 $\rho_{\rm\!\,ph}$. 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-$1.6{\rm~\mu m}$ 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 $\rho_{\rm\!\,ph}\sim 10^{-11}{\rm~g~cm^{-3}}$ ($g\sim10^{-3}{\rm~cm~s^{-2}}$ 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 $\lambda_{\rm Edd}\gtrsim20$, 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.