Spatially Resolved Spectroscopic Study of nearby Seyfert Galaxies: Implications for a Population of "Missed" Seyferts at High-$\textit{z}$

TL;DR

This study uses spatially resolved spectroscopy of nearby active galaxies to understand how aperture size affects AGN classification, revealing many Seyferts could be misclassified in high-redshift observations due to host galaxy contamination.

Contribution

It provides the first detailed analysis of aperture effects on AGN spectral classification using mosaicked spectral maps of nearby galaxies.

Findings

Aperture size significantly influences AGN classification.

Many Seyferts shift from Seyfert to H II region classification with larger apertures.

Approximately 18% of studied galaxies change their spectral type with large apertures.

Abstract

We present mosaicked long-slit spectral maps of 18 nearby Active Galactic Nuclei (AGNs), 2 LINERs, and 4 star-forming galaxies. With the resulting data cubes taken using the Kast dual spectrograph on the 3 m Shane telescope of the Lick Observatory, we measure the aperture effects on the spectroscopic classification of AGNs. With more starlight included in a larger aperture, the nuclear spectrum that is Seyfert-like may become contaminated. We generated standard spectroscopic classification diagrams in different observing apertures. These show quantitatively how the ensemble of Seyferts migrates toward the H $\scriptsize{\textrm{II}}$ region classification when being observed with increasing aperture sizes. But the effect ranges widely in individual active galaxies. Some of the less luminous Seyferts shfit by a large amount, while some other barely move or even shift in different directions. We find that those Seyfert galaxies with the fraction of nuclear H$\alpha$ emission lower than 0.2 of the host galaxy, 2-10 keV hard X-ray luminosity lower than $10^{43}$ erg s$^{-1}$, and the observed nuclear [O $\scriptsize{\textrm{III}}$] luminosity lower than $10^{40.5}$ erg s$^{-1}$, are more likely to change activity classification type when the entire host galaxy is included. Overall, 4 of our 24 galaxies (18 Seyferts) change their spectral activity classification type when observed with a very large aperture.