THE SPATIAL VARIATION OF THE INFRARED-TO-RADIO RATIO IN SPIRAL GALAXIES

We have produced two-dimensional maps of the intensity ratio, Q(60), o f 60 mu m infrared to 20 cm radio continuum emission, for a set of 25 nearby galaxies, mostly spirals. The ratio maps were obtained from inf rared images made using IRAS data with the maximum correlation method, and radio images made using VLA data. Before taking the ratio, the ra dio images were processed so as to have the same resolution properties as the infrared images; the final spatial resolution in all cases is approximately 1', corresponding to 1-2 kpc for most galaxies. This res olution represents a significant improvement over previous studies. Ou r new high-resolution maps confirm the slow decrease of Q(60) with inc reasing radial distance from the nucleus, but show additional structur e which is probably associated with separate sites of active star form ation in the spiral arms. The maps show Q(60) to be more closely relat ed to infrared surface brightness than to the radial distance r in the galaxy disk. We note also that the Q(60) gradients are absent (or at least reduced) for the edge-on galaxies, a property which can be attri buted to the dilution of contrast due to the averaging of the addition al structure along the line of sight. The results are all in qualitati ve agreement with the suggestion that the radio image represents a sme ared version of the infrared image, as would be expected on the basis of current models in which the infrared-radio correlation is driven by the formation of massive stars, and the intensity distribution of rad io emission is smeared as a result of the propagation of energetic ele ctrons accelerated during the supernova phase.