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

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.

💡 Research Summary

This paper presents a comprehensive JWST near‑ and mid‑infrared spectroscopic study of eight nearby low‑luminosity active galactic nuclei (LLAGN) and the well‑studied LLAGN Cen A, covering a broad range in black‑hole mass (10⁵·⁶–10⁹·⁸ M⊙) and Eddington ratio (log Lbol/Ledd ≈ ‑6.2 to ‑2.7). Using NIRSpec (R ≈ 2700) and MIRI/MRS (R ≈ 4600 – 1300) the authors obtained continuous spectra from 1.66 μm to 28.9 μm with unprecedented spatial resolution (0.1″–0.35″). By measuring wavelength‑dependent point‑spread functions from standard stars, they defined a nuclear extraction aperture that encloses 75 % of the encircled energy at each wavelength, corresponding to physical scales of 2–58 pc. This approach cleanly isolates the AGN emission from host‑galaxy light.

Data reduction included the use of the WICKED tool to correct NIRSpec “wiggle” artifacts caused by under‑sampling, and a customized background subtraction for MIRI that combined multiple off‑target exposures, improving signal‑to‑noise by 2–15 %. Emission lines were fitted across the full wavelength range, yielding detections of high‑ionization lines such as