SPECTER-IMAGING OF M-82 AT 3.3-MU-M - EVIDENCE FOR DISSOCIATION OF CARRIERS IN THE STARBURST

We present subarcsecond infrared images in the 3.3 mu m PAH feature of M 82, the prototype of starburst galaxies. Observations were done usi ng the Paris-Meudon Observatory CIRCUS IR Camera at the CFHT 3m60 tele scope. The most interesting results derive from the map of line-to-con tinuum ratio (R = [3.3 mu m]/Cont) which exhibits very peculiar featur es: i) a strong anti-correlation, at large scale, with the continuum i ntensity, the line being practically lacking in the central lane where peaks the continuum emission, a clear indication of the destruction o f the carrier of the line by some process associated to the starburst phenomenon; ii) the region where R is maximum is found essentially in an extended bow (540 pc x 240 pc) south of the starburst region and co inciding tightly with the external part of the molecular disk traced b y CO and HCN emission; iii) on smaller scales a rather complex and clu mpy structure is found, both in the bow of enhanced R and at center, w here almost total depletion of PAH (lowest value of R) is found in sev eral bubbles, a few of them at location of strong radio SNR, and in a channel, north of the nucleus, that coincides rather precisely with th e superwind direction. There is indication of two different regimes of depletion: a partial depletion and a quasi-complete depletion in the central lane around the nucleus. It is shown that the 3-4 mu m continu um emission cannot be explained by reddened starlight or by classical dust thermal emission, and is larger than predicted by models. The rat io R should give a fair estimate of the local 3.3 mu m line carrier ab undance. Mechanisms of destruction and production of PAHs are investig ated; in particular, we show that the two regimes of depletion may cor respond to two different destructive processes: one, more efficient, t hat may correspond to electron bombardment in the SN dominated central region and one driven by UV photons in the molecular disk. For the la ter, we show that three competitive mechanisms are suitable: ionisatio n of PAH that affect the 3.3 mu m oscillator strength, chemisputtering by H+ and photothermo-dissociation. For the former mechanism, we comp ute that the electrons with energy > 10eV, associated with the superwi nd and SN remnants, are extremely frequent and must play an important role in the destruction. The lack of 3.3 mu m line appears thus as a t racer of two strong sources of energy: a) UV photons produced by hot y oung stars, a fact already known in the Galaxy, b) high velocity winds associated either with individual SNRs or their merging in a superwin d, this last result being new. On the other hand, the strong spatial c orrelation between line-to-continuum ratio and molecular emission poin ts a) to the efficient screening by dense molecular clouds that preven ts PAH outside the starburst from direct destruction by hard photons a nd b) to the clumpy structure of the ISM that allows a significant fra ction of those photons to reach and excite PAH. Finally, we show that the continuum emission at those wavelength is probably emitted by the same family of very small particles, but that the carrier is less sens itive to destruction.