Connecting Far-Infrared and Radio Morphologies of Disk Galaxies: Cosmic-Ray Electron Diffusion After Star Formation Episodes

We present results on the ISM properties of 29 galaxies based on a comparison of Spitzer far-infrared and Westerbork Synthesis Radio Telescope radio continuum imagery. Of these 29 galaxies, 18 are close enough to resolve at lesssim1 kpc scales at 70 μm and 22 cm. We extend the approach of our earlier work of smoothing infrared images to approximate cosmic-ray (CR) electron spreading and thus largely reproduce the appearance of radio images. Using a wavelet analysis, we decompose each 70 μm image into one component containing the star-forming structures and a second one for the diffuse disk. The components are smoothed separately, and their combination compared to a free-free corrected 22 cm radio image; the scale lengths are then varied to best match the radio and smoothed infrared images. We find that late-type spirals having high amounts of ongoing star formation benefit most from the two-component method. We also find that the disk component dominates for galaxies having low star formation activity, whereas the structure component dominates at high star formation activity. We propose that this result arises from an age effect rather than from differences in CR electron diffusion due to varying ISM parameters. The bulk of the CR electron population in actively star-forming galaxies is significantly younger than that in less active galaxies due to recent episodes of enhanced star formation; these galaxies are observed within ~10⁸ yr since the onset of the most recent star formation episode. The sample irregulars have anomalously low best-fit scale lengths for their surface brightnesses compared to the rest of the sample spirals, which we attribute to enhanced CR electron escape.