BASS. L. Near-infrared Data Release 3: A Spectral Atlas and Characterization of Active Galactic Nuclei

Abstract We present an analysis of near-infrared (NIR) emission-line properties, active galactic nucleus (AGN) diagnostics, and circumnuclear gas dynamics for 453 hard X-ray selected (14–195 keV) AGNs from the BAT AGN Spectroscopic Survey NIR Data Release 3 (DR3; 〈 z 〉 = 0.036, z < 1.0). This dataset is the largest compilation of rest-frame NIR spectroscopic observations of hard-X-ray-selected AGNs and includes the full DR2 sample. Observations were obtained with the Very Large Telescope X-Shooter, a multiwavelength (0.3–2.5 μ m) spectrograph ( R = 4000–18,000), using a ≥ 2 σ detection threshold, enabling broad analysis of emission features. We find that NIR coronal lines, particularly [Si vi ] λ 1.964, are more reliable tracers of AGN luminosity than optical O III , showing a tighter correlation with hard X-ray luminosity ( σ = 0.25 dex) than O III λ 5007 ( σ = 0.55 dex). Broad Paschen lines (Pa α and Pa β ) are detected in 12% of Seyfert 2 and 57% of Seyfert 1.9 galaxies, consistent with previous hidden broad-line region (BLR) studies. We introduce a refined NIR diagnostic diagram ([Fe ii ] λ 1.257 μ m/Pa β and H 2 λ 2.122 μ m/Br γ ) that effectively distinguishes AGN, star-forming, and composite sources even when contamination limits individual diagnostics or only upper limits are available. Additionally, we find a moderate correlation ( p ≈ 7.4 × 10 −3 ) between hot molecular gas mass (traced by H 2 2.121 μ m) and X-ray luminosity, while its relation with Eddington ratio is weaker. The hot-to-cold gas mass ratio spans 4 orders of magnitude, averaging ∼3 × 10 −7 , indicating diverse molecular gas excitation processes likely driven by star formation and AGN feedback. Our results underscore the value of NIR spectroscopy in probing AGN activity, obscured BLRs, and the complex interactions between AGNs and their circumnuclear environments.