  We present results on the interstellar medium (ISM) properties of 29 galaxies
based on a comparison of {\it Spitzer} far-infrared and Westerbork Synthesis
Radio Telescope radio continuum imagery. Of these 29 galaxies, 18 are close
enough to resolve at $\la$1 kpc scales at 70 $\micron$ and 22 cm. We extend the
\citet{ejm06a,ejm06b} approach 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 $\micron$ image
into one component containing the star-forming {\it structures} and a second
one for the diffuse {\it 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 $\sim10^{8}$ 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.

Astrophysics