The mid-infrared view of star formation in collisional ring galaxies

We present mid-infrared images and spectra of Arp 10 and Arp 118, two colli sional ring galaxies observed as part of the ISOCAM GT program CAMACTIV (P. I.: I.F. Mirabel). The observations reveal the distribution of hot dust in the galaxies and enable us to probe the mechanisms responsible for the heat ing of the ISM. Unlike the peculiar mid-infrared colours recently discovere d in the Cartwheel, the prime example of a collisional ring, Arp 10 has col ours typical of those found in spiral arms of late type spirals. Similarly for Arp 118 (NGC 1144), the mid-infrared emission is associated with the re gions of star formation in the ring. Moreover, a hot continuum in the 5.1-6 .7 mum range, which is a typical mid-infrared signature of an AGN, is clear ly detected from the Seyfert 2 nucleus of the galaxy.