Determining Inclinations of Active Galactic Nuclei Via Their Narrow-Line Region Kinematics - I. Observational Results

TL;DR

This paper introduces a new method to determine the inclinations of active galactic nuclei by analyzing the kinematics of their narrow-line regions using HST imaging and spectroscopy, providing insights into AGN orientation.

Contribution

The study develops a technique using NLR kinematics and biconical outflow models to measure AGN inclinations, which was not previously feasible for most AGN.

Findings

NLR kinematics can be modeled with simple biconical outflows.

Seyfert 1 AGN have NLRs inclined closer to our line of sight.

No correlation between NLR inclination and host galaxy disk orientation.

Abstract

Active Galactic Nuclei (AGN) are axisymmetric systems to first order; their observed properties are likely strong functions of inclination with respect to our line of sight. However, except for a few special cases, the specific inclinations of individual AGN are unknown. We have developed a promising technique for determining the inclinations of nearby AGN by mapping the kinematics of their narrow-line regions (NLRs), which are often easily resolved with Hubble Space Telescope (HST) [O III] imaging and long-slit spectra from the Space Telescope Imaging Spectrograph (STIS). Our studies indicate that NLR kinematics dominated by radial outflow can be fit with simple biconical outflow models that can be used to determine the inclination of the bicone axis, and hence the obscuring torus, with respect to our line of sight. We present NLR analysis of 53 Seyfert galaxies and resultant inclinations from models of 17 individual AGN with clear signatures of biconical outflow. Our model results agree with the unified model in that Seyfert 1 AGN have NLRs inclined further toward our line of sight (LOS) than Seyfert 2 AGN. Knowing the inclinations of these AGN NLRs, and thus their accretion disk and/or torus axes, will allow us to determine how their observed properties vary as a function of polar angle. We find no correlation between the inclinations of the AGN NLRs and the disks of their host galaxies, indicating that the orientation of the gas in the torus is independent from that of the host disk.