2 Stellar Populations and Star Cluster Formation in Interacting Galaxies with the Advanced Camera for Surveys 1 Richard

Pixel-by-pixel colour-magnitude and colour-colour diagrams – based on a subset of the Hubble Space Telescope Advanced Camera for Surveys Early Release Observations – provide a powerful technique to explore and deduce the star and star cluster formation histories of the Mice and the Tadpole interacting galaxies. In each interacting system we find some 40 bright young star clusters (20 . F606W(mag) . 25, with a characteristic mass of ∼ 3×10M⊙), which are spatially coincident with blue regions of active star formation in their tidal tails and spiral arms. We estimate that the main events triggering the formation of these clusters occurred ∼ (1.5 − 2.0) × 108 yr ago. We show that star cluster formation is a major mode of star formation in galaxy interactions, with & 35% of the active star formation in encounters occurring in star clusters. This is the first time that young star clusters have been detected along the tidal tails in interacting galaxies. The tidal tail of the Tadpole system is dominated by blue star forming regions, which occupy some 60% of the total area covered by the tail and contribute ∼ 70% of the total flux in the F475W filter (decreasing to∼ 40% in F814W). The remaining pixels in the tail have colours consistent with those of the main disk. The tidally triggered burst of star formation in the Mice is of similar strength in both interacting galaxies, but it has affected only relatively small, spatially coherent areas.