The Mid-Infrared Spectra of Interacting Galaxies: From ISO to SIRTF

We present mid-infrared (5-16 μm) images and spectra of a sequence of interacting galaxies, observed by ISOCAM. The galaxies were selected as being at progressive stages in the time evolution of a merging event, following what is known as Toomre's ``merger sequence'', and having no detected contribution from an active galactic nucleus (AGN) in their mid-infrared spectrum. To trace the intensity of the global star formation in those galaxies, we use the ratio of the 15 μm to 7 μm flux. Our analysis indicates that this ratio increases from ~ 1 to ~ 5 as galaxies move from the pre-starburst to the merging/starburst phase only to decrease to ~ 1 again in the post-starburst phase of the evolved merger remnants. Moreover, we find that the variation of this ratio is well correlated with the one of the IRAS 25 μm/12 μm and 60 μm/100 μm flux ratios. The potential to test and improve upon these results using the Infrared Spectrograph (IRS) on board SIRTF is discussed.