Mid - IR imaging of Toomre ’ s Merger Sequence

We present results on mid-IR (5–16µm) spectral imaging of a sequence of interacting galaxies, observed by ISOCAM. The galaxies are part of the well known Toomre's " merger sequence " which was defined as a sample of galaxies depicting progressive snapshots in the time evolution of a merging event. To trace the intensity of the radiation field in a starburst, we use the ratio of the 15µm to 7µm flux. Our analysis indicates that this ratio increases as galaxies move from the pre-starburst to the starburst phase and goes again down to ∼ 1 in the post-starburst phase, a value typical of normal star forming regions in galactic disks. Moreover, we find that the variation of this ratio is well correlated with the one of the IRAS 25µm/12µm and 60µm/100µm flux ratios.