Astronomy and Astrophysics – Astrophysics
Scientific paper
May 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995a%26a...297..311n&link_type=abstract
Astronomy and Astrophysics, v.297, p.311
Astronomy and Astrophysics
Astrophysics
18
Galaxies: Starburst, Galaxies: Individual (M 82), Infrared: Galaxies, Ism: Dust, Extinction, Galaxies: Ism
Scientific paper
We present subarcsecond infrared images in the 3.3μm PAH feature of M 82, the prototype of starburst galaxies. Observations were done using the Paris-Meudon Observatory CIRCUS IR Camera at the CFHT 3m60 telescope. The most interesting results derive from the map of line-to-continuum ratio (R=[3.3μm]/Cont) which exhibits very peculiar features: i) a strong anti-correlation, at large scale, with the continuum intensity, the line being practically lacking in the central lane where peaks the continuum emission, a clear indication of the destruction of 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 (540x240pc) south of the starburst region and coinciding tightly with the external part of the molecular disk traced by CO and HCN emission; iii) on smaller scales a rather complex and clumpy structure is found, both in the bow of enhanced R and at center, where almost total depletion of PAH (lowest value of R) is found in several bubbles, a few of them at location of strong radio SNR, and in a channel, north of the nucleus, that coincides rather precisely with the 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 3-4μm continuum emission cannot be explained by reddened starlight or by classical dust thermal emission, and is larger than predicted by models. The ratio R should give a fair estimate of the local 3.3μm line carrier abundance. Mechanisms of destruction and production of PAHs are investigated; in particular, we show that the two regimes of depletion may correspond to two different destructive processes: one, more efficient, that may correspond to electron bombardment in the SN dominated central region and one driven by UV photons in the molecular disk. For the later, we show that three competitive mechanisms are suitable: ionisation of PAH that affect the 3.3μm oscillator strength, chemisputtering by H^+^ and photo-thermo-dissociation. For the former mechanism, we compute that the electrons with energy >10 eV, associated with the superwind and SN remnants, are extremely frequent and must play an important role in the destruction. The lack of 3.3μm line appears thus as a tracer of two strong sources of energy: a) UV photons produced by hot young stars, a fact already known in the Galaxy, b) high velocity winds associated either with individual SNRs or their merging in a superwind, this last result being new. On the other hand, the strong spatial correlation between line-to-continuum ratio and molecular emission points a) to the efficient screening by dense molecular clouds that prevents PAH outside the starburst from direct destruction by hard photons and b) to the clumpy structure of the ISM that allows a significant fraction 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 sensitive to destruction.
Lacombe Francois
Normand Philippe
Rouan Daniel
Tiphene Didier
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