A two-temperature model for the infrared and radio emission from late-type galaxies

Abstract

The tight correlation between far-infrared (FIR) and non-thermal radio luminosities for late-type galaxies is now well established. Using data from the IRAS Point Source Catalog and the 6C radio survey at 151 MHz, we show that this correlation is well-fitted by a power law of the form |$L_\text{FIR}\propto P_{151\text{MHz}}^\gamma \enspace\text {with}\enspace \gamma=0.86\pm0.03$|⁠, which is consistent with other published FIR/radio correlations. We interpret this result in the light of proposals that far-infrared emission is due partly to recently formed massive stars and partly to an older population of stars. By means of a simple model, we remove this latter component from the FIR fluxes and find the remaining emission to be consistent with |$L_\text{FIR}\propto P_{151\text{MHz}}^\gamma \enspace\text {where}\enspace \gamma=0.97\pm0.06$|⁠, a result which is only weakly dependent on the model parameters. This strongly supports the hypothesis that disc radio emission is related to the formation rate of massive stars, and not to the old stellar population. For moderately quiescent spirals, the disc radio emission may thus prove to be a more reliable guide to the recent star formation rate than is the FIR luminosity.