Does the fir-radio correlation evolve with redshift in irregular and disk galaxies?

Abstract

It was confirmed that there is a strong linear correlation between the thermal far-infrared (FIR) and non-thermal radio emission of the star-forming galaxies. Recent works based on this correlation over large redshifts have shown that the correlation is evolving towards higher redshifts. In this paper, possible physical causes that lead to the evolution of this correlation are explored. One possible cause is that the interaction between galaxies is responsible for this behavior. We used the morphology of galaxies, as an indicator of past or present interactions, because it is generally known that the irregular morphology of galaxies is a consequence of collisions or close approaches. To test this hypothesis, a sample of dusty star-forming galaxies up to a redshift z = 3:5 from the COSMOS field has been selected. The sample has been divided, according to the morphological type, into two subsamples (disk and irregular galaxies), and the evolution of the correlation with redshift has been analyzed separately for both of them. It was found that in both subsamples there is no indication for the redshift evolution of the FIR-radio correlation. However, it was also found that the mean correlation parameter, qFIR, is lower in irregular galaxies, which may indicate that they can still affect the evolution of the correlation if their abundance in the sample increases towards higher redshifts. Disk galaxies, which statistically dominate the sample, may be responsible for the lack of this evolution. On the other hand, a fundamental problem with optically determined morphology is the dust obscuration in massive galaxies at z > 2. To test the idea that interacting galaxies are responsible for redshift evolution of the FIR-radio correlation, it is, necessary to analyze a much larger sample for which the morphology has been determined, taking into account the VLA and ALMA imaging in addition to optical images, and which contains a higher fraction of irregular galaxies. Finally, it was also found that the qFIR parameter and its evolution are very sensitive to the radio spectral index above z > 1 and that its misinterpretation and taking a constant value of ? = 0:7 may be responsible for the observed evolution of the correlation.