Year: 2026
Type: article
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 eff ∼ 4000–5000 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 eff and surface gravity g . Given the uncertain dynamical structure of LRDs, we interpret g mainly as a proxy for the photospheric density ρ 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 μ m “kink” from H − opacity, and the Ca II triplet (CaT) absorption at rest-8500 Å. When compared against a local LRD, the Egg , all three features are consistent with a low photospheric density ρ ph ∼ 10 −11 g cm −3 ( g ∼ 10 −3 cm s −2 in our library); although CaT alone admits another higher-density solution. This low ρ 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 ∼10 4 M ⊙ , with an Eddington ratio λ Edd ≳ 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.
Source: The Astrophysical Journal
Institutions: Princeton University, Flatiron Institute, Tsinghua University
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: gold
Estimated APC paid: $4,499
Funders: Princeton University, Flatiron Health
OpenAlex ID: W7221135873