Max Planck Institute for Astronomy - Annual Report 2005

II.5 Dust Tori in Active Galactic Nuclei
Almost all galaxies appear to have a very massive black
hole at their center. In many cases – for instance in our
Milky Way – it is barely noticeable. But in a small percentage
of galaxies, the black hole makes itself conspicuous
through intense radiation emitted by matter in its
vicinity. Such cases are called active galactic nuclei.
For years, there has been indirect evidence that gas
and dust accumulate in the outer sphere of influence of
the black hole, forming a thick donut-shaped structure.
However, even in the nearest galaxies, these »dust
tori« are too small to be resolved with conventional single
telescopes. Using the Mid-Infrared Interferometric
Instrument (MIDI) at the VLT in Chile, astronomers at the
MPIA have for the first time succeeded in directly imaging
several dust tori. Comparisons to newly developed
models allow them to reconstruct and study the infall of
matter onto active galactic nuclei in detail. MIDI is the
achievement of a German-Dutch-French cooperation
under the leadership of the MPIA.
For many years, astronomers have suspected that many
active galactic nuclei (AGN) are embedded in a thick
torus of gas and dust. However, until 2003 it was impossible
to verify this hypothesis by direct observations.
The expected structures are too small. A simple estimate
shows that in a typical AGN, the dust can be heated to
several hundred Kelvin (300 Kelvin correspond to 27 °C)
only within a radius of 1 to 10 light years (ly) around
the black hole. At this temperature, the dust is warm enough
to emit thermal radiation in the mid-infrared range,
which can be detected best at wavelengths between 5 and
20 �m. Even in the nearest AGN of typical luminosity,
10 ly correspond to an apparent size of only 40 milliarcseconds
(mas). This can be compared to viewing
a b
Clouds with
narrow emission lines
Seyfert 1
Clouds with
broad
emission lines
a 1-Euro coin from a distance of 113 km. Even at the
short-wavelength end of the mid-infrared range, at 5 �m,
an 8 m telescope can achieve a resolution of 100 mas at
best, so every dust torus would appear as an unresolved
point source.
Thus all evidence of the presence of such dust tori had
been indirect until recently. A special type of AGN in the
local universe, so-called Seyfert galaxies, play a particular
role here. Two types are distinguished that differ
significantly in their optical properties, while their dust
emission in the mid- and far-infrared range appears to be
very similar. This is easily explained if a dust torus exists
in all Seyfert galaxies that presents a different appearance
to us, depending on our viewing angle (Fig. II.5.1a). In
Seyfert 1 galaxies, we look along the axis directly into
the core of the AGN. We then observe the radiation
emitted by the hot accretion disk and the rapidly orbiting
clouds of hot gas. The disk emits a blue continuum while
the clouds display the broad emission lines typical for
Seyfurt 1. In Seyfert 2 galaxies, we view the dust torus
edge-on, the dust absorption within the torus blocks our
direct view of the accretion disk and the fast gas clouds.
Only the slower gas clouds farther away from the black
hole are still visible, which are also above and below the
dust torus, emitting the narrow emission lines typical of
Fig. II.5.1: a) According to the unified model, the nuclei of
Seyfert galaxies are surrounded by dusty tori. Depending on
whether one is looking at the hot nuclear region or merely at
the torus (arrows), one observes Seyfert 1 or Seyfert 2 galaxies.
b) In the Seyfert 2 galaxy NGC 1068 a cone of ionized gas is
flowing outwards from the centre of the dusty torus; the torus
is shown enlarged by a factor of about 100 compared to the
ionization cone.
Seyfert 2
Ionisation cone
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