  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 stages (Arp 256, NGC 7469, NGC 4676, Arp 299, IC 883 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.

Cosmology and Nongalactic Astrophysics;Astrophysics of Galaxies