Paper detail

Hα Imaging of Nearby Seyfert Host Galaxies

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 μm Active Galaxy Sample. We obtained pure emission line images of each galaxy in order to separate Hα emission from the nucleus from that of the host galaxy. The extended Hα 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 μm 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 Hα 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 Hα 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.