H{\alpha} Imaging of Nearby Seyfert Host Galaxies

TL;DR

This study used Hα imaging to analyze star formation and AGN activity in nearby Seyfert galaxies, revealing correlations between star formation rates, AGN luminosity, and emission properties, with implications for high-redshift observations.

Contribution

First detailed Hα imaging of nearby Seyfert galaxies separating nuclear and host galaxy emission, linking star formation with AGN activity and predicting high-redshift observational appearances.

Findings

Extended Hα emission correlates with star formation indicators.

Radio excess suggests additional processes beyond star formation.

Nuclear Hα luminosity correlates with hard X-ray luminosity.

Abstract

We used narrowband interference filters with the CCD imaging camera on the Nickel 1.0 meter telescope at Lick Observatory to observe 31 nearby (z < 0.03) Seyfert galaxies in the 12 {\mu}m Active Galaxy Sample. We obtained pure emission line images of each galaxy in order to separate H{\alpha} emission from the nucleus from that of the host galaxy. The extended H{\alpha} emission is expected to be powered by newly formed hot stars, and correlates well with other indicators of current star formation in these galaxies: 7.7 {\mu}m PAH, far-infrared, and radio luminosity. Relative to what would be expected from recent star formation, there is a 0.8 dex excess of radio emission in our Seyfert galaxies. The nuclear H{\alpha} luminosity is dominated by the AGN, and is correlated with the hard X-ray luminosity. There is an upward offset of 1 dex in this correlation for the Seyfert 1s due to a strong contribution from the Broad Line Region. We found a correlation between star formation rate and AGN luminosity. In spite of selection effects, we concluded that the absence of bright Seyfert nuclei in galaxies with low SFRs is real, albeit only weakly significant. We used our measured spatial distributions of H{\alpha} emission to determine what these Seyfert galaxies would look like when observed through fixed apertures at high redshifts. Although all would be detectable emission line galaxies at any redshift, most would appear dominated by HII region emission. Only the most luminous AGN would still be identified at z~0.3.