An Initial Look at the Far-Infrared-Radio Correlation within Nearby Star-forming Galaxies Using the Spitzer Space Telescope

We present an initial look at the far-infrared-radio correlation within the star-forming disks of four nearby, nearly face-on galaxies (NGC 2403, NGC 3031, NGC 5194, and NGC 6946). Using Spitzer MIPS imaging, observed as part of the Spitzer Infrared Nearby Galaxies Survey (SINGS), and Westerbork Synthesis Radio Telescope (WSRT) radio continuum data, taken for the WSRT SINGS radio continuum survey, we are able to probe variations in the logarithmic 24 μm/22 cm (q24) and 70 μm/22 cm (q70) surface brightness ratios across each disk at subkiloparsec scales. We find general trends of decreasing q24 and q70 with declining surface brightness and with increasing radius. The residual dispersion around the trend of q24 and q70 versus surface brightness is smaller than the residual dispersion around the trend of q24 and q70 versus radius, on average by ~0.1 dex, indicating that the distribution of star formation sites is more important in determining the infrared/radio disk appearance than the exponential profiles of disks. We have also performed preliminary phenomenological modeling of cosmic-ray electron (CR electron) diffusion using an image-smearing technique and find that smoothing the infrared maps improves their correlation with the radio maps. We find that exponential smoothing kernels work marginally better than Gaussian kernels, independent of projection for these nearly face-on galaxies. This result suggests that additional processes besides simple random walk diffusion in three dimensions must affect the evolution of CR electrons. The best-fit smoothing kernels for the two less active star-forming galaxies (NGC 2403 and NGC 3031) have much larger scale lengths than those of the more active star-forming galaxies (NGC 5194 and NGC 6946). This difference may be due to the relative deficit of recent CR electron injection into the interstellar medium for the galaxies that have largely quiescent disks.