ISOCAM Observations of a Galaxy Merging Sequence

Abstract

We present mid-IR (5–16μm) 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 and having no detected contribution from an active galactic nucleus (AGN) in their mid-IR spectrum. To trace the intensity of the global star formation in those galaxies, we use the ratio of the 15μm to 7μm 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 25μm/12μm and 60μm/100μm flux ratios. Improving upon these results using the Infrared Spectrograph (IRS) on board the Space Infrared Telescope Facility (SIRTF) is briefly discussed.