  Far-infrared (FIR)--radio correlation is a well-established empirical
connection between continuum radio and dust emission of star-forming galaxies,
often used as a tool in determining star-formation rates. Here we expand the
point made by Murphy (2013) that in the case of some interacting star-forming
galaxies there is a non-thermal emission from the gas bridge in between them,
which might cause a dispersion in this correlation. Galactic interactions and
mergers have been known to give rise to tidal shocks and disrupt morphologies
especially in the smaller of the interacting components. Here we point out that
these shocks can also heat the gas and dust and will inevitably accelerate
particles and result in a tidal cosmic-ray population in addition to standard
galactic cosmic rays in the galaxy itself. This would result in a non-thermal
emission not only from the gas bridges of interacting systems, but from
interacting galaxies as a whole in general. Thus both tidal heating and
additional non-thermal radiation will obviously affect the FIR-radio
correlation of these systems, the only question is how much. In this scenario
the FIR-radio correlation is not stable in interacting galaxies, but rather
evolves as the interaction/merger progresses. To test this hypothesis and probe
the possible impact of tidal cosmic ray population we have analyzed a sample of
43 infrared bright star-forming interacting galaxies at different merger
stages. We have found that their FIR-radio correlation parameter and radio
emission spectral index vary noticeably over different merger stages and behave
as it would be expected from our tidal-shock scenario. Important implications
of departure of interacting galaxies from the FIR-radio correlation are
discussed.
