CORRELATION OF THE RADIO-CONTINUUM INTENSITY WITH THE FIR LUMINOSITY AND ITS IMPLICATION FOR DUST HEATING SOURCES AND PHYSICAL PROCESSES INGALAXIES

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, conne cting different 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 non thermal synchrotron radiation from relativistic electrons. The dominan ce of the so-called cool component of the total FIR radiation can be u nderstood by the absorption of non-ionizing UV emission from intermedi ate massive stars (5-20 M.) which also contribute dominantly to the ga laxian supernova rates. The relativistic electrons are therefore gener ated as a consequence of supernova explosions whose dynamical influenc e on galaxian gas motions (''turbulence'') in tum affects the generati on of the magnetic fields in which the synchrotron emission occurs. Mo st galaxian disks are optically thick for their own UV emission, ioniz ing and non-ionizing. Similarly, energetic electrons lose most of thei r energy by Inverse Compton and synchrotron losses in galaxian disks a nd their halos. Therefore the independence of morphology, size, color, etc. of the FIR/radio correlation is basically explained by a ''calor imeter theory''. However a residual ''radio-quiet'' FIR emission due t o dust-absorbed optical emission from old, low-mass stars appears nece ssary to explain the non-linearity of the correlation. The abnormal FI R-to-radio ratios of clustered galaxies are interpreted by an active i nteraction between these galaxies and presumably existing dense intrac luster fragments.