Radio continuum and far-infrared emission from spiral galaxies : a close correlation.

Observations at 6.3 cm of a total of 91, mostly spiral, galaxies for which 60- and 100-micron densities had been published are the bases for a plotting of far-IR flux densities against radio ones which indicates that these quantities are closely correlated. This correlation has been further explored by plotting flux density ratio distributions for two sample galaxy subsets: optically and IR-bright nearby galaxies, and a complete IR subset of more distant galaxies. The radio/IR flux density ratio is the same for both subsets while the IR/blue flux density ratio differs by a factor of 3, implying that the relative intensity of the radio continuum and the far-IR emission is the same despite differences in present star formation activity. The predominantly nonthermal radio emission of spiral galaxies is therefore connected with the young stellar population.