Issue |
A&A
Volume 377, Number 3, October III 2001
|
|
---|---|---|
Page(s) | 735 - 744 | |
Section | Cosmology (including clusters of galaxies) | |
DOI | https://doi.org/10.1051/0004-6361:20010969 | |
Published online | 15 October 2001 |
The infrared continuum radiation of NGC 1808*
A PAH and polarisation study
1
European Southern Observatory, Karl-Schwarzschildstr. 2, 85748 Garching b. München, Germany
2
Max-Planck-Institut for Radioastronomy, Auf dem Hügel 69, Postfach 2024, 53010 Bonn, Germany
3
ISO Data Centre, Villafranca del Castillo, PO Box 50727, 28080 Madrid, Spain
Corresponding author: R. Siebenmorgen, rsiebenm@eso.org
Received:
1
June
2001
Accepted:
29
June
2001
The paper is devoted to the understanding of the infrared
emission of nuclear regions in galaxies. a) ISO data of
NGC 1808 are presented: spectro-photometry from 5.1 to 16.4 μm;
a map at 6 μm and 170 μm photometry. b)
The data are complemented by a polarisation measurement at 170 μm
(
at position angle
) and a map at
6 μm. In the map, the degree of polarisation goes up to 20% in
the outer regions. We argue that the polarisation is produced by
emission of big grains and exclude very small grains and PAHs or
scattering and extinction. c) The mid infrared spectrum shows,
beside the main emission bands, a so far unknown plateau of PAH
features in the
m region. d) The total spectrum
can be fit under the assumption of optically thin emission. However,
such a model fails to reproduce the 25 μm point and implies that
the mid infrared is due to very small grains and PAHs. These
particles would then also have to be responsible for the 6 μm polarisation, which is unlikely. e) To avoid these
difficulties, we successfully turn to a radiative transfer model whose
major feature is the existence of hot spots produced by the dust
clouds around OB stars. We demonstrate the decisive influence on the
mid infrared spectrum of both the PAHs and the hot spots.
Key words: polarization / infrared: galaxies / galaxies: ISM / galaxies: magnetic fields / galaxies: individual: NGC 1808
© ESO, 2001
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