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

TL;DR

This paper presents a comprehensive near-infrared spectroscopic analysis of 453 AGN, revealing new diagnostics for AGN activity, correlations with X-ray luminosity, and insights into circumnuclear gas dynamics, enhancing understanding of AGN environments.

Contribution

It introduces a refined NIR diagnostic diagram for classifying AGN and star-forming sources, and demonstrates the superior correlation of NIR coronal lines with X-ray luminosity compared to optical lines.

Findings

NIR coronal lines, especially SiVI, are better tracers of AGN luminosity than optical lines.

Broad Paschen lines are detected in a significant fraction of Seyfert 1.9 and Seyfert 2 galaxies.

The hot molecular gas mass correlates moderately with X-ray luminosity, indicating diverse excitation processes.

Abstract

We present an analysis of near-infrared (NIR) emission-line properties, AGN diagnostics, and circumnuclear gas dynamics for 453 hard X-ray selected (14$-$195 keV) AGN from the BAT AGN Spectroscopic Survey (BASS) NIR Data Release 3 (DR3; $\langle z \rangle=0.036$, $z<1.0$). This dataset is the largest compilation of rest-frame NIR spectroscopic observations of hard-X-ray-selected AGN and includes the full DR2 sample. Observations were obtained with VLT X-shooter, a multiwavelength (0.3$-$2.5$\mu$m) spectrograph ($R$ = 4,000$-$18,000), using a $\geq 2 \sigma$ detection threshold, enabling broad analysis of emission features. We find that NIR coronal lines, particularly SiVI $\lambda1.964$, are more reliable tracers of AGN luminosity than optical [OIII], showing a tighter correlation with hard X-ray luminosity ($\sigma=0.25$dex) than [OIII] $\lambda5007$ ($\sigma=0.55$dex). Broad Paschen lines (Pa$\alpha$ and Pa$\beta$) are detected in 12% of Seyfert 2 and 57% of Seyfert 1.9, consistent with previous hidden BLR studies. We introduce a refined NIR diagnostic diagram (FeII $\lambda1.257$$\mu$m/Pa$\beta$ and H$_2$ $\lambda 2.122$$\mu$m/Br$\gamma$) that effectively distinguishes AGN, star-forming, and composite sources even when contamination limits individual diagnostics or only upper limits are available. Additionally, we find a moderate correlation ($p \approx 7.4 \times 10^{-3}$) between hot molecular gas mass (traced by H$_2$ 2.121$\mu$m) and X-ray luminosity, while its relation with Eddington ratio is weaker. The hot-to-cold gas mass ratio spans four orders of magnitude, averaging $\sim 3 \times 10^{-7}$, 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 AGN and their circumnuclear environments.