[Paper Review] ReveaLLAGN 1: JWST Emission-Line Spectra Reveal Low-Luminosity AGN with UV-Deficient SEDs and Warm Molecular Gas

Kameron Goold, Anil Seth+21 more|arXiv (Cornell University)|Jan 23, 2026

Astrophysical Phenomena and Observations0 citations

TL;DR

JWST NIRSpec and MIRI/MRS observations of seven nearby LLAGN reveal UV-deficient SEDs and warm molecular gas, enabling clean nuclear emission-line measurements and insights into LLAGN ionizing continua and feedback.

ABSTRACT

We present near- and mid-infrared spectra of eight Low-Luminosity Active Galactic Nuclei (LLAGN), spanning nearly four orders of magnitude in black hole mass and Eddington ratio, obtained with JWST/NIRSpec and MIRI as part of the ReveaLLAGN program along with identical archival data of Cen A. The high spatial resolution of JWST cleanly separates AGN emission from host-galaxy contamination, enabling detections of high-ionization potential lines more than an order of magnitude fainter than previously measured. Emission-line diagnostics reveal a transition at log($L_{bol}/L_{Edd}$) ~ -3.5, where the spectral energy distribution becomes increasingly deficient in ultraviolet photons. We find that rotational H$_2$ excitation temperatures are elevated (~500 K higher) compared to both higher-luminosity AGN and star-forming galaxies, while the H$_2$(0-0)S(3)/PAH$_{11.3 μm}$ ratios are consistent with those observed in the AGN population. We discuss the possible roles of outflows, jets, and X-ray dominated regions in shaping the interstellar medium surrounding LLAGN. Silicate emission at ~10 $μ$m, localized to the nuclear region, is detected in most ReveaLLAGN targets. This dataset offers the first comprehensive JWST-based characterization of infrared emission lines in the nuclear regions of LLAGN.

Motivation & Objective

• Motivate understanding of how LLAGN accretion modes affect infrared emission and ionizing continua.
• Demonstrate JWST's ability to spatially separate nuclear AGN emission from host-galaxy light in LLAGN.
• Characterize infrared emission lines and molecular gas properties in LLAGN nuclei across a range of black hole masses and Eddington ratios.
• Investigate the role of feedback processes (outflows, jets, X-ray dominated regions) in shaping the circumnuclear ISM of LLAGN.

Proposed method

• Obtain near- and mid-infrared spectra of eight LLAGN (seven ReveaLLAGN targets plus Cen A) using JWST NIRSpec and MIRI/MRS.
• Extract nuclear spectra with wavelength-dependent apertures sized to capture the PSF at each wavelength.
• Identify and measure 131 emission lines spanning NIR and MIR using a multi-Gaussian line-fitting approach with continuum subtraction and Monte Carlo uncertainty estimates.
• De-blend blended lines by constraining secondary components with reference lines and fit complex line profiles with up to ten Gaussian components.
• Compute H2 excitation temperatures from pure rotational lines using Boltzmann statistics and normalize fluxes to a common aperture for fair comparisons.

Experimental results

Research questions

• RQ1 What is the ionizing continuum in LLAGN as traced by high-ionization emission lines?
• RQ2 How do LLAGN emission lines and molecular gas properties reflect the accretion state and feedback mechanisms (outflows, jets, X-ray dominated regions)?
• RQ3 Do UV deficiencies in LLAGN SEDs correlate with changes in infrared line diagnostics and molecular gas excitation?
• RQ4 How do LLAGN compare to higher-luminosity AGN and star-forming systems in MIR line ratios and H2 excitation?

Key findings

• A transition at log(Lbol/Ledd) ~ -3.5 where the SED becomes increasingly deficient in UV photons.
• Rotational H2 excitation temperatures are elevated by ~500 K relative to higher-luminosity AGN and star-forming galaxies.
• H2 (0-0) S(3)/PAH 11.3 μm ratios are consistent with those observed in the AGN population.
• Nuclear silicate emission at ~10 μm is detected in most targets, with silicate emission localized to the nucleus in several cases; Cen A shows absorption.
• High-ionization potential lines are detected thanks to JWST's sensitivity, enabling robust diagnostics of the ionizing continuum and nuclear environment.