Multi-wavelength probes of obscuration toward the narrow-line region in Seyfert galaxies

Abstract

We present a study of reddening and absorption toward the narrow line regions (NLRs) in active galactic nuclei (AGNs) selected from the Revised Shapley-Ames, 12 μm, and Swift/Burst Alert Telescope samples. For the sources in host galaxies with inclinations of b/a > 0.5, we find that the mean ratio of [OIII] λ5007, from ground-based observations, and [OIV] 28.59 μm, from Spitzer/Infrared Spectrograph observations, is a factor of two lower in Seyfert 2s than Seyfert 1s. The combination of low [OIII]/[OIV] and [OIII] λ4363/λ5007 ratios in Seyfert 2s suggests more extinction of emission from the NLR than in Seyfert 1s. Similar column densities of dusty gas, NH ∼ several × 1021 cm∼-2, can account for the suppression of both [OIII] λ5007 and [OIII] λ4363, as compared to those observed in Seyfert 1s. Also, we find that the X-ray line Ovii λ 22.1 Å is weaker in Seyfert 2s, consistent with absorption by the same gas that reddens the optical emission. Using a Hubble Space Telescope/Space Telescope Imaging Spectrograph slitless spectrum of the Seyfert 1 galaxy NGC 4151, we estimate that only ∼30% of the [OIII] λ5007 comes from within 30 pc of the central source, which is insufficient to account for the low [OIII]/[OIV] ratios in Seyfert 2s. If Seyfert 2 galaxies have similar intrinsic [OIII] spatial profiles, the external dusty gas must extend further out along the NLR, perhaps in the form of nuclear dust spirals that have been associated with fueling flows toward the AGN. © 2011. The American Astronomical Society.