D. Rouan 1, P. Le Coupanec 1, A. Léger 2, F. Boulanger 2, & P. Gallais 3
1 Observatoire de Paris-Meudon
2 IAS - Orsay
3 SAp CEA-Saclay
Observation of the 3.3 micron PAH feature in the diffuse, moderately UV irradiated component of the ISM is difficult because the emission is faint and the transient effects of detectors very painful to disentangle. We present several CAM-SW observations of this type, obtained either through long integrations in the dedicated filters of CAM or through CVF scans. Three different regions were observed : a) the large scale diffuse component in the edge-on galaxy NGXC 891, b) the reflection nebula NGC 7023 and c) a ``normal'' line of sight in the Galactic Plane, chosen to be typical of the diffuse ISM in the Milky Way. The detection of the 3.3 micron feature is clearly done in all three cases. The measured intensity of the line is well within the expectation and strongly supports the idea that only very small particles of typically 100 atoms, transiently heated, can explain it (the so-called PAH hypothesis). The spatial distribution of the emission presents, however, rather unexpected characteristics that we discuss. In the case of the reflection nebula NGC 7023, the emission in the band appears clearly linked to the small scale H2 filamentary structures previously found and points to actual density structures; in the case of the line 0f sight within the Milky Way , the emission appears essentially located around the stars found in the field : this effect, if real, may suggest that even rather cool stars can be responsible of the transient heating. We finally discuss the distribution of the 3.3 micron feature relative to the galactic structure and to other tracers of the PAH component in NGC 891.