Max Planck Institute for Astronomy - Annual Report 2005
Max Planck Institute for Astronomy - Annual Report 2005
Max Planck Institute for Astronomy - Annual Report 2005
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36 II. Highlights<br />
8.5 �m<br />
2.3 ly<br />
0 . � 02<br />
sume that the brightness distribution of the dust emission<br />
can be approximated in all projections and all wavelengths<br />
by a Gaussian function. Under this assumption, the visibility<br />
observed (Fig. II.5.7) can be converted directly<br />
into a measurement of the effective width. The points in<br />
Fig. II.5.8 represent this reconstructed effective size of<br />
the dust distribution in the Circinus galaxy <strong>for</strong> two wavelength<br />
ranges (λ � 8.5 and 13µm, cf. Fig. II.5.7). These<br />
wavelengths are nearly unaffected by absorption through<br />
silicate-dust grains.<br />
With the help of this simple image reconstruction, we<br />
find the distribution of the dust to be box-shaped both at a<br />
wavelength of 12.5µm (dominated by dust at about 300 K)<br />
and 8.5µm (hotter dust). At 12.5µm, the observed dimension<br />
of the box is 50�30mas, corresponding to a diameter<br />
of 3.2ly and a height of 1.9ly. The size and orientation<br />
match other observations of the nucleus of the Circinus<br />
� �<br />
13 �m<br />
1.5 ly<br />
Fig. II.5.8: From the measured visibility (Fig. II.5.7) the effective<br />
size of the dusty torus of the Circinus galaxy can be<br />
derived – here shown <strong>for</strong> two wavelenghts. At the larger wavelength<br />
the torus appears larger – as expected.<br />
Fig. II.5.9: The axis of symmetry of the ionized cone in the<br />
Circinus galaxy imaged by the HST is exactly perpendicular to<br />
the dursty torus, as reconstructed from the Midi data. Note the<br />
different scales.<br />
1 ly<br />
H �<br />
60 ly<br />
�<br />
3.6 ly<br />
2.3 ly<br />
galaxy (Fig. II.5.9): For example, the symmetry axis of<br />
the ionization cone observed with HST is exactly perpendicular<br />
to the maximum extension (the plane of the torus)<br />
found. In addition, the region where Maser emission of<br />
molecular H 2 O gas was found in well-ordered rotation<br />
exactly corresponds to our dust distribution. This proves<br />
that water vapor and dust are concentrated in the same<br />
regions.<br />
The actual vertical dimension of the dust distribution<br />
still has to be examined since an apparent height could be<br />
feigned by an inclined disk. Applying the most probable<br />
value <strong>for</strong> the orientation of the symmetry axis with respect<br />
to our line of sight (70 °), we find the true height of the dust<br />
torus to be just under 1 ly. This leads us to the conclusione<br />
that the torus in the Circinus galaxy is relatively thin,<br />
the ratio of height to diameter here is about h/d � 0.25.<br />
In comparison, the relative thickness in NGC 1068 is<br />
h/d � 0.6. This finding is in excellent agreement with the<br />
observation that the ionization cone of the Circinus galaxy<br />
has a much wider opening angle than that of NGC 1068<br />
(comp. Fig. II.5.9 to Fig. II.5.1b).<br />
The previous Midi-observations of the two nearest<br />
Seyfert II galaxies thus strongly confirm the picture of a<br />
central dust torus preventing a direct view of the accretion<br />
disk. In the future, comparison with high-resolution radio<br />
maps and millimeter observations with alMa will enable<br />
us to study in more detail how the gas reservoir within the<br />
torus flows inwards, thus »feeding« the accretion disk.<br />
Interferometric Observations of the Nucleus of the<br />
Radio Galaxy Centaurus A<br />
At a distance of only 12 million ly, Centaurus A is the<br />
nearest radio galaxy. We have also observed it with Midi.<br />
This data shows the mid-infrared emission to be dominated<br />
by an unresolved point source. Its diameter is apparently<br />
smaller than 0.6 ly. From comparison with observations<br />
at radio- and millimeter-wavelengths, we conclude that<br />
this emission does not originate from warm dust, but from<br />
synchrotron radiation of high-energy electrons spiraling<br />
in a magnetic field of 0.3 Gauß (Fig. II.5.10). We will not<br />
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