VISIR / VLT mid-infrared imaging of Seyfert nuclei: nuclear dust emission and the Seyfert-2 dichotomy ^*,**

¹ Astronomisches Institut, Ruhr-Universität Bochum, Universitätsstraße 150 / NA7, 44801 Bochum, Germany e-mail: haas@astro.rub.de
² European Southern Observatory, Karl-Schwarzschildstr. 2, 85748 Garching b. München, Germany
³ DSM/DAPNIA/Service d'Astrophysique, CEA/Saclay, 91191 Gif-sur-Yvette, France
⁴ Institut für Theoretische Physik und Astrophysik, Universität zu Kiel, Leibnizstr. 15, 24098 Kiel, Germany
⁵ Zentrum für Astronomie Heidelberg, Institut für Theoretische Astrophysik, Albert-Überle-Str. 2, 69120 Heidelberg, Germany
⁶ European Southern Observatory, Casilla 19001, Santiago 19, Chile

Received: 21 March 2007
Accepted: 17 July 2007

Abstract

Aims. 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 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]_{λ 5007 Å} and mid-infrared imaging at matching spatial resolution. The aim is to check whether the nuclear dust emission scales with AGN luminosity as traced by [OIII].

Methods.During the scientific verification phase of the VISIR instrument at the ESO Very Large Telescope we observed 16 Sy1 and Sy2 nuclei at 11.25 μm with 035 spatial resolution (FWHM). We supplement our observations with high-resolution 10–12 μm literature data of 58 Seyfert galaxies, for which spectroscopic or spectropolarimetric information and far-infrared data are available.

Results. Twelve of the 15 detected sources are unresolved and three sources show a dominant unresolved core surrounded by some faint knots in an area smaller than 1–2″ radius. Our VISIR photometry agrees to better than 15% with published data obtained at 15–5″ 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.