Other
Scientific paper
Jan 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011aas...21732505v&link_type=abstract
American Astronomical Society, AAS Meeting #217, #325.05; Bulletin of the American Astronomical Society, Vol. 43, 2011
Other
Scientific paper
All galaxies without a radio-loud AGN follow a tight correlation between their global FIR and radio synchrotron luminosities, which is believed to be ultimately the result of the formation of massive stars. A much discussed but controversial physical explanation is the ``calorimeter theory'' which stipulates that galaxies are, at least approximately, calorimeters for their own UV radiation as well as for the relativistic electrons produced at the end of their stars life cycle. Interestingly, two colliding pairs of galaxies, UGC12914/5 and UGC 813/6 deviate from this correlation and show an excess of radio emission which in both cases originates to a large extent in a gas bridge connecting the two galactic disks. These objects have been interpreted as showing that the energetic electrons escape from their parent galaxies rather than loose their energy in the galactic magnetic fields. We argue here in an analytic model that the gas-dynamic interaction of the two Interstellar Media necessarily produces two strong shock waves which diffusively accelerate a new population of energetic particles. It can readily explain the excess radio emission from the gas bridge and thereby the deviation from the FIR-radio correlation for the combined system. The accompanying high-energy gamma-ray emission is too low for detection with present instruments, but might be detectable with a future gamma-ray instrument like CTA. Also the synchrotron spectral index at the present epoch agrees with the observations. The acceleration of relativistic electrons in shocks caused by an ISM collision, in the same way as described here, is likely to take place in other systems as well, as in galaxy clusters and groups or high-redshift systems.
Lisenfeld Ute
Voelk Heinz
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