2010A&A...524A..27L

All galaxies without a radio-loud AGN follow a tight correlation between their global far-infrared (FIR) and radio synchrotron luminosities, which is believed to be ultimately the result of the formation of massive stars. Two colliding pairs of galaxies, UGC 12914/5 and UGC 813/6 deviate from this correlation and show an excess of radio emission that in both cases originates to a large extent in a gas bridge connecting the two galactic disks. We are aiming to clarify the origin of the radio continuum emission from the bridge. The radio synchrotron emission expected from the bridge regions 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. We present a model for the acceleration of relativistic particles in these shocks and calculate the resulting radio emission, its spectral index and the expected high-energy γ-ray emission. We found that the nonthermal energy produced in the collision is high 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 γ-ray emission is not detectable with present-day instruments, but might be observable with the next generation of γ-ray telescopes. The deviation of these two interacting galaxy systems from the standard FIR-radio correlation is consistent with the acceleration of an additional population of electrons in large-scale shock waves resulting from the gas dynamic interaction of the two ISM. This process is not related to star formation, and therefore it is expected that the systems do not follow the FIR-radio correlation. In particular, the radio emissions of these systems do not represent an argument against the calorimeter theory. 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, such as galaxy clusters and groups or high-redshift systems.