Spatially Resolved Spectroscopic Study of nearby Seyfert Galaxies: Implications for a Population of “Missed” Seyferts at High-z

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 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 shift by a large amount, while some others barely move or even shift in different directions. We find that those Seyfert galaxies with the fraction of nuclear H α 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 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.