Van Der Kruit to Spitzer: a New Look at the Fir-radio Correlation

We present an initial look at the far infrared-radio correlation within the starforming disks of four nearby, nearly face-on galaxies (NGC 2403, NGC 3031, NGC 5194, and NGC 6946). Using Spitzer MIPS imaging and WSRT radio continuum data, we are able to probe variations in the logarithmic 70 μm/22 cm (q70) flux density ratios across each disk at sub-kpc scales. We find general trends of decreasing q70 with declining surface brightness and with increasing radius. We also find that the dispersion in q70 within galaxies is comparable to what is measured globally among galaxies at around 0.2 dex. We have also performed preliminary phenomenological modeling of cosmic ray electron (CRe−) diffusion using an image-smearing technique, and find that smoothing the infrared maps improves their correlation with the radio maps. The best fit smoothing kernels for the two less active star-forming galaxies (NGC 2403 and NGC 3031) have much larger scale-lengths than that of the more active starforming galaxies (NGC 5194 and NGC 6946). This difference may be due to the relative deficit of recent CRe− injection into the interstellar medium (ISM) for the galaxies having largely quiescent disks. A more complete discussion of this proceedings article can be found in Murphy et al. (2005).