Star formation, dust heating, and cosmic ray electron cooling: A far-infrared and radio study of nearby galaxies

We compare the gaseous and relativistic phases of the interstellar medium (ISM) within nearby star-forming galaxies using far-infrared (FIR) and radio continuum imagery. Infrared images were taken on board the Spitzer Space Telescope, largely as part of the Spitzer Infrared Nearby Galaxies Survey (SINGS). The radio continuum data were largely collected using the Westerbork Radio Synthesis Telescope (WSRT) and included in the WSRT-SINGS program.;We find that the dispersion in the FIR/radio ratios within galaxies is similar to that found among galaxies. By comparing the residual dispersion around FIR/radio ratios with other physically motivated parameters we conclude that the FIR-radio correlation within galaxies is most sensitive to star formation activity. We also find that the dispersion in the FIR/radio ratios within galaxies is significantly reduced using a phenomeno-logical image-smearing model; the infrared images are convolved with a parameterized kernel to approximate the time-dependent diffusion of cosmic-ray (CR) electrons and thus reproduce the appearance of the radio images.;Using this technique we have been able to characterize the diffusion of CR electrons in galaxies other than the Milky Way. We find that the mean distance traveled by CR electrons is most sensitive to the mean age of the CR electron population rather than ISM parameters which may inhibit their propagation such as density, radiation field energy density, and magnetic field strength. We also find that the global CR electron population transitions abruptly from being dominated by old to recently accelerated CR electrons with increasing star formation intensity. Irregular galaxies are not well described by this phenomenology which we attribute to enhanced CR electron escape.;By comparing FIR/radio ratios within cluster galaxies to those in the field we find systematic differences. Since their FIR disks seem largely unperturbed, we apply our image-smearing analysis to create model radio continuum images for comparison with the observed radio data. We find radio deficit regions along edges thought to be experiencing intracluster medium (ICM)-ISM effects. We conclude that the ICM wind has swept up the low density relativistic ISM and created shocks that have re-accelerated CR electrons and sheared the magnetic field.