Spatially Resolved [O III] Emission Line Kinematics of Reverberation-Mapped AGNs with the Keck Cosmic Web Imager

TL;DR

This study uses integral-field spectroscopy to analyze the kinematics of ionized gas and stars in nearby Seyfert galaxies, revealing complex motions, the unreliability of [O III] line width as a stellar velocity dispersion proxy, and larger-than-expected narrow-line region sizes.

Contribution

First spatially resolved kinematic analysis of [O III] emission in nearby AGNs, highlighting the limitations of using [O III] line width as a stellar velocity proxy and providing refined NLR size measurements.

Findings

Gas motions are mainly rotational and aligned with stars.

[O III] line width underestimates stellar velocity dispersion.

NLR sizes are larger than previous imaging estimates.

Abstract

We present optical integral-field spectroscopic data of ten nearby ($0.02\leq z \leq 0.05$) Seyfert 1 galaxies taken with the Keck Cosmic Web Imager (KCWI). We map the spatially resolved kinematics of the [O III] gas and stars, and investigate the alignments between their global kinematic position angles (PA). Large-scale gas motions are primarily dominated by rotation, and are kinematically aligned with the stars ($\Delta\text{PA}\leq 30$ deg). However, eight galaxies exhibit non-rotational kinematic signatures (e.g., kinematic twists, possible outflows) in their ionized gas velocity fields near the nucleus. We compare aperture-wide measurements of the gas and stellar velocity dispersions ($\sigma_{\text{gas}}$ and $\sigma_\star$) to test the use of the width of the [O III] line core as a surrogate for $\sigma_\star$. Direct comparisons between $\sigma_{\text{gas}}$ and $\sigma_\star$ show that $\sigma_{\text{gas}}$ tends to underestimate $\sigma_\star$, and thus is not a reliable tracer of $\sigma_\star$ for our selected galaxies. We measure the extent of the narrow-line region (NLR) using several definitions, resulting in sizes of $\sim0.1$-$10$ kpc. For a given [O III] luminosity, our NLR sizes derived from the [O III]/H$\beta$ flux ratio or an [O III] isophotal radius are an order of magnitude larger than those measured from past imaging data.