Synthetic Spectral Library of Optically Thick Atmospheres for Little Red Dots
- 1Princeton University
- 2Flatiron Institute
- 3Tsinghua University
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.
Related
- SPECULATE: A synthetic spectral library, emulator and fitting package for accretion disc winds -- Infrastructure and application to CVs
- MEGARA Stellar Spectral Library. Second Release
- The MIRI/MRS Library. III. Detector-based optimal spectral extraction and high-contrast imaging applications
- Spectral Appearance of Self-gravitating Disks Powered by Stellar Objects: Universal Effective Temperature in the Optical Continuum and Application to Little Red Dots
- A Spectral Framework for Testing the Quasi-Star Hypothesis in Little Red Dots I: Weighing LRDs by Their Super-Eddington Luminosity Ratios---No Signs of Overmassive Black Holes
BibTeX
@article{刘2026synthetic,
title = {{Synthetic Spectral Library of Optically Thick Atmospheres for Little Red Dots}},
author = {Hanpu 翰溥 Liu 刘 and Yan-Fei 燕飞 Jiang 姜 and Eliot Quataert and Jenny E. Greene and Yilun 逸伦 Ma 马 and Xiaojing Lin},
journal = {The Astrophysical Journal},
year = {2026},
month = oct,
doi = {10.3847/1538-4357/ae9d72},
url = {https://iopscience.iop.org/article/10.3847/1538-4357/ae9d72},
note = {CC BY 4.0},
}
RIS
TY - JOUR
TI - Synthetic Spectral Library of Optically Thick Atmospheres for Little Red Dots
AU - Hanpu 翰溥 Liu 刘
AU - Yan-Fei 燕飞 Jiang 姜
AU - Eliot Quataert
AU - Jenny E. Greene
AU - Yilun 逸伦 Ma 马
AU - Xiaojing Lin
PY - 2026
DA - 2026/10/09
JO - The Astrophysical Journal
DO - 10.3847/1538-4357/ae9d72
UR - https://iopscience.iop.org/article/10.3847/1538-4357/ae9d72
AB - 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.
SN - 0004-637X
SN - 1538-4357
ER -