Abstract
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 Hα emission lower than 0.2 of the host galaxy, 2–10 keV hard X-ray luminosity lower than 1043 erg s−1, and the observed nuclear [O iii] luminosity lower than 1040.5 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.
Highlights
Most galaxies are thought to host a central super massive black hole, only a minority of them are currently accreting at sufficient rates to reveal their presence as active galactic nuclei (AGNs)
These are produced in the so-called broad-line region in Type 1 AGNs, which–we know from reverberation studies—is within a parsec of the massive black hole and strongly influenced by its gravity (Wandel et al 1999)
To study the aperture effect on Seyfert classification, we spatially separated the emission lines coming from the center of the galaxy from the emission lines produced in the host galaxy by hot young stars
Summary
Most galaxies are thought to host a central super massive black hole, only a minority of them are currently accreting at sufficient rates to reveal their presence as active galactic nuclei (AGNs). More AGNs at high redshifts would mistakenly be classified as normal star-forming galaxies This effect has been described by Barger et al (2001a, 2001b) to explain why 50% of their hard X-ray sources do not have high-ionization signatures of AGNs. it is desirable to study the spatial distribution of the emission line ratios from Seyfert nuclei and their host galaxies, in a representative sample. To study the aperture effect on Seyfert classification, we spatially separated the emission lines coming from the center of the galaxy (produced by its active nucleus) from the emission lines produced in the host galaxy by hot young stars This allows us to plot separately the line ratios from the two respective contributions in the well-established BPT diagnostic diagrams. This local sample will help to predict how many Seyfert galaxies may be misclassified at higher redshift, where it is not possible to isolate the nucleus from the host galaxy
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