The infrared continuum radiation of NGC 1808 -: A PAH and polarisation study

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 mum; a 25 x 25 map at 6 mum and 170 mum photometry. b) The data are complemented by a polarisation measurement at 170 mum (2.5 +/- 0.4% at position angle 94 degrees +/- 5 degrees) and a map at 6 mum. 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 greater than or equal to 13 mum region. d) The total spectrum can be fit under the assumption of optically thin emission. However, such a model fails to reproduce the 25 mum 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 mum 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.