Studying the [OIII]$\lambda$5007A emission-line width in a sample of $\sim$80 local active galaxies: A surrogate for $\sigma_{\star}$?

TL;DR

This study evaluates whether the [OIII] emission-line width can reliably substitute stellar velocity dispersion for estimating black hole masses in local active galaxies, finding significant scatter and cautioning against its use on individual objects.

Contribution

It provides a detailed analysis of the [OIII] line profile in a sizable sample, demonstrating the limitations of using [OIII] width as a surrogate for stellar velocity dispersion in active galaxies.

Findings

Single Gaussian fits overestimate $\sigma_{ m [OIII]}$ by 50-100%.

Core [OIII] width correlates with $\sigma_{ m ext{star}}$ but with large scatter.

Radio-detected AGNs show slightly larger [OIII] widths, indicating possible influence of radio activity.

Abstract

For a sample of $\sim$80 local ($0.02 \leq z \leq 0.1$) Seyfert-1 galaxies with high-quality long-slit Keck spectra and spatially-resolved stellar-velocity dispersion ($\sigma_{\star}$) measurements, we study the profile of the [OIII]$\lambda$5007A emission line to test the validity of using its width as a surrogate for $\sigma_{\star}$. Such an approach has often been used in the literature, since it is difficult to measure $\sigma_{\star}$ for type-1 active galactic nuclei (AGNs) due to the AGN continuum outshining the stellar-absorption lines. Fitting the [OIII] line with a single Gaussian or Gauss-Hermite polynomials overestimates $\sigma_{\star}$ by 50-100%. When line asymmetries from non-gravitational gas motion are excluded in a double Gaussian fit, the average ratio between the core [OIII] width ($\sigma_{\rm {[OIII],D}}$) and $\sigma_{\star}$ is $\sim$1, but with individual data points off by up to a factor of two. The resulting black-hole-mass-$\sigma_{\rm {[OIII],D}}$ relation scatters around that of quiescent galaxies and reverberation-mapped AGNs. However, a direct comparison between $\sigma_{\star}$ and $\sigma_{\rm {[OIII],D}}$ shows no close correlation, only that both quantities have the same range, average and standard deviation, probably because they feel the same gravitational potential. The large scatter is likely due to the fact that line profiles are a luminosity-weighted average, dependent on the light distribution and underlying kinematic field. Within the range probed by our sample (80-260 km s$^{-1}$), our results strongly caution against the use of [OIII] width as a surrogate for $\sigma_{\star}$ on an individual basis. Even though our sample consists of radio-quiet AGNs, FIRST radio-detected objects have, on average, a $\sim$10% larger [OIII] core width.