Max Planck Institute for Astronomy - Annual Report 2005

Direct Detection of Dust Tori in Seyfert 2 Galaxies
There are two good arguments for first searching for
direct evidence for the existence of dust tori in Seyfert 2
galaxies. Firstly, the unified scheme of Seyfert galaxies
rests on the hypothesis that the dust in Seyfert 2 galaxies
is distributed in a donut-shaped structure. Secondly, here
we expect the torus to clearly dominate the mid-infrared
radiation, whereas in Seyfert 1 galaxies it will most likely
be be outshone by the long-wavelength end of the thermal
spectrum of the hot accretion disk.
Our first successful observations with Midi were made
in 2003. Based on observations of the archetypical and
brightest Seyfert 2 galaxy, NGC 1068, carried out with only
two telescope combinations, we were able to show that
a dust structure whose warm component is heated by the
AGN actually exists in its core. At a temperature of about
300 Kelvin, it has a diameter of 11 ly and an overall height
of 6.7 ly. In addition, we detected a hot dust component 3
ly across at most, lying deeply embedded within the warm
dust. We interpret this component as the signature of the
dust located at the inner surface of the torus – the axial
channel – which is heated to temperatures near the evaporation
temperature for the dust particles of 1500 K.
These findings unexpectedly closely match the prediction
of our simple model (Fig. II.5.4). Though this should
be qualified by emphasizing that with only two telescope
combinations, it is impossible to determine the orientation
of the dust structure independently. It was necessary
to assume its symmetry axis based on the orientation of
the known outflow phenomena in NGC 1068.
During four observing campaigns, we meanwhile
succeeded in observing the nearest Seyfert 2 galaxy – the
Fig. II.5.6: Aperture coverage for the observations of the
Circinus galaxy with Midi. Since each combination of telescopes
is represented symmetrically with respect to the origin,
the points cover twice the diameter of the entire telescope
(see Fig. II.5.2).
0 10 30 50 m
v
u
Total Flux [Jansky]
Corellated Flux [Jansky]
Visibility
15
10
5
0
1.5
1.0
0.5
0
0.2
0.1
0.0
8
II.5 Dust Tori in Active Galactic Nuclei 35
9 10 11 12 13
Wavelength [�m]
Fig. II.5.7: Midi observations of the Circinus galaxy. Above:
Total flux, measured with a single telescope. Center: correlated
flux from two telescopes during one night, during which the
projection on the sky of the baseline connecting the telescopes
rotated from northeast-southwest (yellow) to southeast-northwest
(pink). Below: the visibility – the quotient of correlated
and total flux – is a coarse measure for the size of the torus.
Circinus galaxy lying at a distance of just under 13 million
ly – sufficiently often to be close to the ideal case of a
well-filled aperture plane (Fig. II.5.6). For the first time,
we are now able to reconstruct an image of the dust distribution
without relying on model-dependent assumptions.
The observed sequence of combinations of two telescopes
with a total of 15 different orientations, each
containing about 20 spectrally resolved, independent
measuring points (Fig. II.5.7), in principle allows us to
reconstruct a very detailed model of the two-dimensional
(projected on the celestial plane) distribution of the dust
(including the temperature). However, we decided to do
simple image reconstruction first, which is independent
of specific model assumptions as far as possible. We as