Multi-wavelength Probes of Obscuration Towards the Narrow Line Region in Seyfert Galaxies

TL;DR

This study investigates the obscuration and reddening of the Narrow Line Regions in Seyfert galaxies using multi-wavelength observations, revealing that Seyfert 2s experience more extinction due to extended dusty gas, affecting emission line ratios.

Contribution

It provides new evidence that extended dusty gas causes significant obscuration in Seyfert 2 galaxies, affecting optical and X-ray emission lines, and suggests the presence of nuclear dust spirals.

Findings

Seyfert 2s show lower [O III]/[O IV] ratios than Seyfert 1s.

Dusty gas with NH ~ several x 10^21 cm^-2 causes emission suppression.

X-ray line OVII is weaker in Seyfert 2s, indicating absorption by the same gas.

Abstract

We present a study of reddening and absorption towards the Narrow Line Regions (NLR) in active galactic nuclei (AGN) selected from the Revised Shapley-Ames, 12mu, and Swift/Burst Alert Telescope samples. For the sources in host galaxies with inclinations of b/a > 0.5, we find that mean ratio of [O III] 5007A, from ground-based observations, and [O IV] 28.59mu, from Spitzer/Infrared Spectrograph observations, is a factor of 2 lower in Seyfert 2s than Seyfert 1s. The combination of low [O III]/[O IV] and [O III] 4363/5007 ratios in Seyfert 2s suggests more extinction of emission from the NLR than in Seyfert 1s. Similar column densities of dusty gas, NH ~ several X 10^21 cm^-2, can account for the suppression of both [O III] 5007A and [O III] 4363A, as compared to those observed in Seyfert 1s. Also, we find that the X-ray line OVII 22.1A 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 [O III] 5007A comes from within 30 pc of the central source, which is insufficient to account for the low [O III]/[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 towards the AGN.