The infrared continuum radiation of NGC 1808. A PAH and polarisation study
R. Siebenmorgen, Krü, E. gel, R. J. Laureijs;
AaA, 2001, 377, 735
ABSTRACT: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 mu m; a 25''x 25'' map at 6 mu m and 170 mu m photometry.
b)
The data are complemented by a polarisation measurement at 170 mu m
(2.5+/-0.4% at position angle 94degr +/-5degr ) and a map at 6 mu m.
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
>= 13 mu 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 mu m
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 mu m
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.
Based on observations with ISO (Kessler et al.
1996), an ESA project with
instruments funded by ESA Member States (especially the PI countries: France,
Germany, the Netherlands and the UK) with the participation of ISAS and NASA.
KEYWORDS: polarisation, infrared: galaxies, galaxies: ism, galaxies: magnetic fields -, galaxies: individual: ngc 1808
PERSOKEY:polarisation, iso, dust, mir, galaxies, pah, ,
CODE: siebenmorgen2001a