The Far-Infrared–Radio Correlation in Nearby Abell Clusters

Abstract

A comprehensive study of the effect of the cluster environment on the far-infrared (FIR)-radio correlation in nearby Abell clusters is presented. Using a cluster radio galaxy database, optical spectroscopy, and high-resolution radio images to remove active galactic nuclei (AGNs), we assess the FIR-radio correlation of cluster galaxies from the centers of the clusters out well past the classical Abell radius. The FIR-radio correlation is shown to hold quite well for star-forming galaxies, and the FIR and radio fluxes for cluster AGNs are also well correlated. In the case of AGNs, the relative radio-to-FIR fluxes are greater and the scatter in the correlation is larger than those seen for star-forming galaxies. We also find that there is a rare but statistically significant excess of star-forming galaxies with enhanced radio emission in the centers of the clusters and that the degree of this enhancement is typically a factor of 2 or 3. The FIR-radio correlation for cluster star-forming galaxies is also tested against the line-of-sight velocity relative to the cluster systemic velocities, but no significant correlation is found. While the radial dependence of the FIR-radio correlation is consistent with the model wherein ram pressure increases the magnetic field strengths of the cluster galaxies through compression, the velocity data do not confirm this model. Although a contribution from ram pressure cannot be ruled out, the thermal pressure due to the ICM alone is an equally viable alternative. The high-resolution radio images largely reject the hypothesis that the increased radio emission arises from an AGN component, strengthening the claim that the change in the correlation is caused by a change in the environment of the galaxies.