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

We present mid-IR images of a 15'x15' field in the south-west part of the A ndromeda galaxy M31 obtained with the ISOCAM camera (6 " pixels) on board I SO. These broad-band images complement spectre-imaging observations of smal ler 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 telescop e FOCA 1000. These images are inter-compared and also compared with HI, CO( 1-0) and Htr maps. The mid-IR emission as seen through wide-band filters ce ntered at 7 and 15 mu m is extremely well correlated with the distribution of neutral gas as shown by the HI and CO(1-0) maps, while the correlation i s poorer with the distribution of the ionized gas seen through its Her emis sion. 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 M31. The spectro-imaging observatio ns of Cesarsky et al. (1998) show that the mid-IR spectra of several region s of M31, two of which are in the presently studied area, are dominated by a strong emission band at 11.3 mu m while emission in the other classical A romatic infrared Bands (AIBs) at 6.2, 7.7 and 8.6 mu m is faint or absent. This result is precised, and we find that the mid-IR spectral variations ar e not clearly related to the UV radiation field. The present observations h ave important consequences on our understanding of excitation of the inters tellar mid-IR emission in general. In particular, we conclude that like for M31, excitation in the Galactic cirruses may not be dominated by UV photon s but rather by another mechanism which remains to be identified (visible p hotons?). The UV excitation appears to become important when the UV radiati on density is of the order of 2 times that near the Sun.