Star Formation History in Merging Galaxies

Galaxy interactions are known to trigger starbursts. Young massive star clusters formed in interacting galaxies and mergers may become young globular clusters. The ages of these clusters can provide clues about the timing of interaction-triggered events, and thus provide an important way to reconstruct the star formation history of merging galaxies. Numerical simulations of galaxy mergers can implement different star formation rules. For instance, star formation dependent on gas density or triggered by shocks predicts significantly different star formation histories. To test the validity of these models, multi-object spectroscopy was used to map the ages of young star clusters throughout the bodies and tails of a series of galaxy mergers at different merger stages (Arp 256, NGC 7469, NGC 4676, Arp 299, IC883, and NGC 2623). We found that the cumulative distribution of ages becomes shallower as the stage of merger advances. This result suggests a trend of cluster ages as a function of merger stage. In NGC 4676 we found that two clusters have ages of about 170 Myr, suggesting that they likely formed during its first passage. Their locations in the tidal tails are consistent with the spatial distribution of star formation predicted by shock-induced models. When comparing the ages and spatial distribution of clusters in NGC 7252 to our model, we found that some clusters are likely to form during the prompt starburst at first passage, as predicted by simulations with shock-induced star formation. These simulations show that the shock-induced mechanism is an important trigger of star formation and that using the ages of clusters formed in the starbursts can effectively determine the star formation history of merging galaxies.