Shock acceleration of relativistic particles in galaxy-galaxy collisions
  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. Two face-on 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. The radio synchrotron
emission expected from the bridge region is calculated, assuming that the
kinetic energy liberated in the predominantly gas dynamic interaction of the
respective interstellar media (ISM) has produced shock waves that efficiently
accelerate nuclei and electrons to relativistic energies. A simple model for
the acceleration of relativistic particles in these shocks is presented
together with a calculation of the resulting radio emission, its spectral index
and the expected high-energy gamma-ray emission. This process is not related to
star formation. It is found that the nonthermal energy produced in the
collision is large enough to explain the radio emission from the bridge between
the two galaxies. The calculated spectral index at the present time also agrees
with the observed value. The expected gamma-ray emission, on the other hand, is
too low by a factor of several to be detectable even with foreseeable
instruments like CTA.
