Hα IMAGING OF NEARBY SEYFERT HOST GALAXIES

Abstract

ABSTRACT We used narrowband (Δλ = 70 Å) interference filters with the CCD imaging camera on the Nickel 1.0 m 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, which reach down to a flux limit of 7.3 × 10−15 erg cm−2 s−1 arcsec−2, and corrected these images for [N ii] emission and extinction. We separated the Hα emission line of the “nucleus” (central 100–1000 pc) from that of the host galaxy. The extended Hα emission is expected to be powered by newly formed hot stars, and indeed correlates well with other indicators of current star formation rates (SFRs) in these galaxies: extended 7.7 μm polycyclic aromatic hydrocarbon, total 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 Hα luminosity we measured in the centers of our galaxies is dominated by the active galactic nucleus (AGN), and is linearly correlated with the hard X-ray luminosity. There is, however, an upward offset of 1 dex in this correlation for the Seyfert 1s, because their nuclear Hα emission includes a strong additional contribution from the broad-line region. We found a correlation between SFR 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. Finally, we used our measured spatial distributions of Hα emission to determine what these Seyfert galaxies would look like when observed through fixed apertures (e.g., a spectroscopic fiber) at high redshifts. We found that although all of these Seyfert galaxies would be detectable emission-line galaxies at any redshift, most of them would appear to be dominated by (>67%) their H ii region emission. Only the most luminous AGNs (log(L Hα /erg s−1) > 41.5) would still be identified as such at z ∼ 0.3.