VISIR/VLT mid-infrared imaging of Seyfert nuclei: Nuclear dust emission and the Seyfert-2 dichotomy

TL;DR

This study compares nuclear mid-infrared and [OIII] emissions in Seyfert galaxies to investigate the Seyfert-2 dichotomy, finding no evidence for a distinct population of dust-free Seyfert-2s and supporting unification models.

Contribution

First comparison of nuclear mid-infrared and [OIII] imaging at matching resolution to study Seyfert-2 classification.

Findings

No difference in nuclear mid-infrared/[OIII] ratios between Sy1s and Sy2s.

No evidence for dust-free Seyfert-2 nuclei.

Seyfert-2 dichotomy largely due to observational biases.

Abstract

Half of the Seyfert-2 galaxies escaped detection of broad lines in their polarised spectra observed so far. Some authors have suspected that these non-HBLRs contain real Sy2 nuclei without intrinsic broad line region hidden behind a dust torus. If this were true, then their nuclear structure would fundamentally differ from that of Sy2s with polarised broad lines: in particular, they would not be explained by orientation-based AGN unification. Further arguments for two physically different Sy2 populations have been derived from the warm and cool IRAS F25/F60 ratios. These ratios, however, refer to the entire host galaxies and are unsuitable to conclusively establish the absence of a nuclear dust torus. Instead, a study of the Seyfert-2 dichotomy should be performed on the basis of nuclear properties only. Here we present the first comparison between [OIII] 5007A and mid-infrared imaging at matching spatial resolution. Exploring the Seyfert-2 dichotomy we find that the distributions of nuclear mid-infrared/[OIII] luminosity ratios are indistinguishable for Sy1s and Sy2s with and without detected polarised broad lines and irrespective of having warm or cool IRAS F25/F60 ratios. We find no evidence for the existence of a population of real Sy2s with a deficit of nuclear dust emission. Our results suggest 1) that all Seyfert nuclei possess the same physical structure including the putative dust torus and 2) that the cool IRAS colours are caused by a low contrast of AGN to host galaxy. Then the Seyfert-2 dichotomy is explained in part by unification of non-HBLRs with narrow-line Sy1s and to a larger rate by observational biases caused by a low AGN/host contrast and/or an unfavourable scattering geometry.