The Correlation between Far-Infrared and Non-Thermal Radio Emission at 151 MHz for Galaxies: Observations and Modelling

We have compared far-infrared (FIR) data from the IRAS survey with the 6C radio survey (at 151 MHz) in a region defined approximately by |$8^\text h\leqslant\alpha\leqslant 17^\text h;\enspace 31^\circ\leqslant\delta\leqslant 68^\circ$|⁠. Comparison of these two flux-limited samples reveals (i) for |$\text {log} [f\text {(FIR)/W}\enspace\text m^{-2}]\gt - 12.6$| a population of 74 objects for which |$f\text {(FIR)}$| and |$S_\nu (151\text {MHz})$| are strongly correlated and most of the identifications arc spiral galaxies; (ii) that for |$\text {log} [f\text {(FIR)/W}\enspace\text m^{-2}]\lt - 12.6$| there are ∼ 24 identifications which are probably real, among many random coincidences indicating a population including some radio bright objects (e.g. 3C 345). At 151 MHz the radio emission is totally dominated by the non-thermal component; we find a slope for the FIR/radio correlation of 0.86±0.03, significantly different from unity and in the sense that the most luminous IR objects are more intense at 151 MHz.

We have constructed models for the FIR emission from normal spiral galaxies by assuming that (i) the observed FIR luminosities represent the bulk of the radiation from young stars still embedded inside their parent molecular clouds; (ii) the average star-formation rate (Ψ) has remained approximately constant throughout the lifetime of the massive stars. Using various analytical forms for the initial mass function and a power-law approximation for stellar lifetimes, we predict the ratios FIR-luminosity/Ψ and supernova-rate/Ψ. By including a simple model for the emission from discrete supernova remnants (SNRs) we predict the ratio |$L_\text {FIR}/L_{151}$| and find it to be greater than observed for all reasonable model parameters (including an IMF suitable for starburst galaxies). This implies that discrete SNRs cannot be responsible for the radio emission either in normal or starburst galaxies.