Correlation of the radio continuum intensity with the FIR luminosity and its implication for dust heating sources and physical processes in galaxies

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The tight correlation between the far-infrared (FIR) luminosities and the radio continuum intensities of late-type galaxies can be shown to be not only a mass-scaling (or “richness”) effect. It rather depends on intrinsic properties like star-formation rate per unit mass, connecting différent physical processes in a galaxy. While the FIR emission is thermal radiation of dust grains heated by stellar UV and optical light, the radio continuum consists of thermal Bremsstrahlung, and nonthermal synchrotron radiation from relativistic electrons. The dominance of the so-called cool component of the total FIR radiation can be understood by the absorption of non-ionizing UV emission from intermediate massive stars (5–20 M.) which also contribute dominantly to the galaxian supernova rates. The relativistic electrons are therefore generated as a consequence of supernova explosions whose dynamical influence on galaxian gas motions (“turbulence”) in turn affects the generation of the magnetic fields in which the synchrotron emission occurs. Most galaxian disks are optically thick for their own UV emission, ionizing and non-ionizing. Similarly, energetic electrons lose most of their energy by Inverse Compton and synchrotron losses in galaxian disks and their halos. Therefore the independence of morphology, size, color, etc. of the FIR/radio correlation is basically explained by a “calorimeter theory”. However a residual “radio-quiet” FIR emission due to dust-absorbed optical emission from old, low-mass stars appears necessary to explain the non-linearity of the correlation. The abnormal FIR-to-radio ratios of clustered galaxies are interpreted by an active interaction between these galaxies and presumably existing dense intracluster fragments.