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

TL;DR

This study uses JWST infrared spectra to analyze low-luminosity AGN, revealing UV-deficient SEDs, elevated H$_2$ temperatures, and nuclear silicate emission, advancing understanding of their interstellar medium and emission properties.

Contribution

First comprehensive JWST-based characterization of infrared emission lines in the nuclear regions of low-luminosity AGN, highlighting spectral and physical distinctions from higher-luminosity AGN.

Findings

Detection of faint high-ionization lines with JWST's high resolution.

Identification of a spectral transition at log($L_{bol}/L_{Edd}$) ~ -3.5.

Elevated H$_2$ excitation temperatures (~500 K higher) compared to luminous AGN.

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$_{11.3 \mu m}$ 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 $\mu$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.