Ages of Star Clusters in the Tidal Tails of Merging Galaxies
  We study the stellar content in the tidal tails of three nearby merging
galaxies, NGC 520, NGC 2623, and NGC 3256, using BVI imaging taken with the
Advanced Camera for Surveys on board the Hubble Space Telescope. The tidal
tails in all three systems contain compact and fairly massive young star
clusters, embedded in a sea of diffuse, unresolved stellar light. We compare
the measured colors and luminosities with predictions from population synthesis
models to estimate cluster ages and find that clusters began forming in tidal
tails during or shortly after the formation of the tails themselves. We find a
lack of very young clusters ($\le 10$ Myr old), implying that eventually star
formation shuts off in the tails as the gas is used up or dispersed. There are
a few clusters in each tail with estimated ages that are older than the modeled
tails themselves, suggesting that these may have been stripped out from the
original galaxy disks. The luminosity function of the tail clusters can be
described by a single power-law, $dN/dL \propto L^\alpha$, with $-2.6 < \alpha
< -2.0$. We find a stellar age gradient across some of the tidal tails, which
we interpret as a superposition of 1) newly formed stars and clusters along the
dense center of the tail and 2) a sea of broadly distributed, older stellar
material ejected from the progenitor galaxies.
