Issue |
A&A
Volume 473, Number 2, October II 2007
|
|
---|---|---|
Page(s) | 369 - 376 | |
Section | Astrophysical processes | |
DOI | https://doi.org/10.1051/0004-6361:20077520 | |
Published online | 23 July 2007 |
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
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.
Key words: galaxies: nuclei / galaxies: active / galaxies: Seyfert / infrared: galaxies
© ESO, 2007
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