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 ^ { \prime \prime } \times 25 ^ { \prime \prime } map at 6 \mu m and 170 \mu m photometry . b ) The data are complemented by a polarization measurement at 170 \mu m ( 2.5 { \pm } 0.4 % at position angle 94 \degr { \pm } 5 \degr ) 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 \geq 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 .