Mid-IR Spectroscopy with the Spitzer IRS of Spiral Bridges and Tails

The Spitzer Space Telescope is providing new insight into the birth and evolution of stars in a wide variety of environments in the Universe. Tidally interacting and colliding galaxies are interesting because conditions in both the inner and outer disks can be strongly perturbed by tidal forces, or strong shocks. We present new mid-IR spectroscopy using the Spitzer IRS, of selected regions in four interacting systems-Arp 84, Arp 86, Arp 271 and Arp 280. We compare the relative strengths of various key PAH bands and atomic and/or molecular-line features across the face of these galaxies and in tidal tails. Through a comparison with numerical models of the collisions, and a large body of ancillary data, we investigate the relationship between MIR spectral-line features with the : 1) strength of the MIR and UV continuum, 2) strength of predicted large-scale shocks, 3) age of star clusters in the galaxies and tails and, 4) radial distance from center of host galaxy. The work compliments similar studies of more normal galaxies, allowing us to assess how the early stages of the merging process affects global mid-IR spectroscopic properties.