P. Le Coupanec 1, D. Rouan 1, F. Lacombe 1, D. Tiphene 1, P. Gallais 2, A. Léger 3, & M. Giard 4
1 Observatoire de Paris-Meudon
2 Sap CEA-SACLAY
3 IAS-Orsay
4 CESR-Toulouse
Detection of the Unidentified Infrared Bands (UIB) in the diffuse interstellar medium of external normal spiral galaxies and then mapping of their carriers, taken by hypothesis as PAH molecules, were the purpose of an ISO program. The aim was to confirm the universal presence of a PAH component and to study the abundance and conditions of excitation of these carriers at a large spatial scale in the interstellar medium of external galaxies. The specific idea to look at edge-on galaxies has the advantage that the column density in this case is much larger than in face-on galaxies. In this context, we observed with ISOCAM the ``normal'' edge-on galaxy NGC891 which is a good target because of its proximity. The galaxy was mapped (a) in a raster scan mode which covered the whole plane of the galaxy and through the large filters from LW1 (4.5 um) to LW9 (15 um) providing both spatial and spectral information and, (b) with a CVF scan centered on the galaxy nucleus. The detection of the UIR bands (6.2, 7.7, 8.6 and 11.3 um) is clearly done with a high signal to noise ratio in the whole galactic plane, providing the distribution of PAH molecules in the ISM of this galaxy. The bands are detected at far longitude position as well as at high galactic latitude associated with cirrus clouds. It confirms that the excitation of the infrared bands is possible even in low irradiated medium. Along the galactic plane, the shape of the spectra is similar to the one observed for diffuse regions of the interstellar medium in our galaxy. Correlation between the distribution of PAHs (in terms of abundance and excitation state), mapped by the unidentified infrared bands ratios, and the galactic structure (distribution of gas and radiation field) can be made, showing peculiar regions such as medium with a lower rate of UIB carriers or star forming regions. Finally, a comparison is made with what is observed in the diffuse interstellar medium of the Milky Way.