Synthetic Spectral Library of Optically Thick Atmospheres for Little Red Dots

TL;DR

This paper introduces a synthetic spectral library for optically thick atmospheres in Little Red Dots, revealing spectral diagnostics that challenge simple blackbody models and provide insights into their physical properties and central black hole masses.

Contribution

We developed and publicly released a spectral library modeling optically thick atmospheres for LRDs, enabling detailed diagnostics of their physical conditions and mass estimates.

Findings

Spectral features indicate low photospheric density in LRDs.

Blackbody approximation is insufficient for LRD spectral modeling.

Spectral diagnostics suggest high Eddington ratios and specific black hole masses.

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.