Mid - infrared and far - ultraviolet observations of the star - forming ring of M 31 ?

We present mid–IR images of a 15′×15′ field in the south–west part of the Andromeda galaxy M 31 obtained with the ISOCAM camera (6′′ pixels) on board ISO. These broad–band images complement spectro–imaging observations of smaller fields (Cesarsky et al. 1998). We also present a 20′′ resolution far–UV image of a larger field at 200 nm obtained with the balloon–borne telescope FOCA 1000. These images are inter–compared and also compared with H i, CO(1–0) and Hα maps. The mid–IR emission as seen through wide–band filters centered at 7 and 15 μm is extremely well correlated with the distribution of neutral gas as shown by the H i and CO(1–0) maps, while the correlation is poorer with the distribution of the ionized gas seen through its Hα emission. There is some correlation with the UV radiation, but it appears that the contribution of UV photons to the excitation of the carriers of the mid–IR emission is not dominant in most of M 31. The spectro– imaging observations of Cesarsky et al. (1998) show that the mid–IR spectra of several regions of M 31, two of which are in the presently studied area, are dominated by a strong emission band at 11.3 μm while emission in the other classical Aromatic Infrared Bands (AIBs) at 6.2, 7.7 and 8.6 μm is faint or absent. This result is precised, and we find that the mid-IR spectral variations are not clearly related to the UV radiation field. The present observations have important consequences on our understanding of excitation of the interstellar mid–IR emission in general. In particular, we conclude that like for M 31, excitation in the Galactic cirruses may not be dominated by UV photons but rather by another mechanism which remains to be identified (visible photons?). The UV excitation appears to become important when the UV radiation density is of the order of 2 times that near the Sun.