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 $T_{\mathrm{eff}}\sim 4000-5000\mathrm{K}$. We develop (and publicly release) a synthetic spectral library of optically thick atmospheres with gas conditions tailored for LRDs, parameterized by effective temperature $T_{\mathrm{eff}}$ and surface gravity $g$. Given the uncertain dynamical structure of LRDs, we interpret $g$ mainly as a proxy for the photospheric density ${\rho }_{\mathrm{ph}}$. 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 continuum, the rest-$1.6\mu \mathrm{m}$ ``kink'' from ${\mathrm{H}}^{-}$ opacity, and the Ca~II triplet (CaT) absorption at rest-$8500\mathring{A}$. When compared against a local LRD, the Egg, all three features are consistent with a low photospheric density ${\rho }_{\mathrm{ph}}\sim 10^{-11}\mathrm{g}\mathrm{c}{\mathrm{m}}^{-3}$ ($g\sim 10^{-3}\mathrm{cm}{\mathrm{s}}^{-2}$ in our library), although CaT alone admits another higher-density solution. This low ${\rho }_{\mathrm{ph}}$ directly results from our radiative transfer modeling; with the additional assumption that the CaT line width traces turbulent support at the continuum photosphere in a spherical geometry, we infer a mass within the photosphere (black hole plus gas) of $\sim 10^4{M}_{\odot }$, with an Eddington ratio ${\lambda }_{\mathrm{Edd}}\gtrsim 20$. 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.

28 pages, 16 figures, accepted for publication in ApJ. Comments welcome!