SIMBAD references

This paper investigates the physical reasons for the existence, the tightness and the near universality of the FIR/radio correlation for late-type field galaxies whose emission is not dominated by an active nucleus. We develop theoretical models for the radio and far-infrared (FIR) emission of such normal galaxies and study the influence of their main parameters on the ratio of the two emissions. In addition, data are used from a sample of 114 late-type galaxies which allow an estimate of the mean energy densities of the radiation field and the magnetic field, the latter crudely calculated from the synchrotron luminosity using the minimum energy condition, and of the dust opacity. These data reveal, for the first time, a reasonably good, linear correlation between the energy density of the radiation field and the energy density of the magnetic field. Interestingly this implies that the two most important energy loss rates for electrons, synchrotron and Inverse Compton losses, are proportional to each other. As a consequence the radio synchrotron emission is proportional to the total flux of radiative energy loss from the nonthermal Cosmic Ray electrons of a given energy. Furthermore we find that on average the galaxies are marginally optically thick for the non-ionizing UV light. Including their extended magnetic halos, galaxies are also found to be on average marginally optically thick regarding the radiative energy losses of the radio synchrotron emitting, non-thermal Cosmic Ray electrons. Exceptions may be galaxies with a very low (compared to our Galaxy) present star formation rate. Combining these semi-empirical results with the theoretical emission models we show first of all that in the optically thick case the linear correlation between the energy densities of the radiation field and the magnetic field is a necessary condition for the existence of the observed FIR/radio correlation. Secondly we show that the individual dispersion of the FIR to radio continuum ratio caused by uncertainties of any single one of the parameters considered is significantly less than the empirical dispersion of the FIR/radio correlation. The radio and the FIR emission are mainly determined by their sources, i.e. massive Supernova precursor stars, because galaxies act in a reasonable approximation as calorimeters for the stellar UV radiation and for the energy flux of the Cosmic Ray electrons produced in their disks. This constitutes a very general theoretical explanation for the FIR/radio correlation. In principle one can now go a significant step further and use this observed correlation plus the theory and the parameters of one galaxy as a normalization to deduce in turn the mean magnetic field strength of other normal galaxies simply from their total mean radiation energy density.