We present the results of combining Hubble Space Telescope optical photometry with ground-based Ks-band photometry from the Gemini imagers NIRI and FLAMINGOS-I to study the globular-cluster populations in four early-type galaxies that are candidate r
emnants of recent mergers (NGC1700, NGC2865, NGC4382, and NGC7727). These galaxies were chosen based on their blue colors and fine structure, such as shells and ripples that are indicative of past interactions. We fit the combined VIKs globular-cluster data with simple toy models of mixed cluster populations that contain three subpopulations of different age and metallicity. The fits, done via Chi-square mapping of the parameter space, yield clear evidence for the presence of intermediate-age clusters in each galaxy. We find that the ages of 1-2 Gyr for these globular-cluster subpopulations are consistent with the previously estimated merger ages for the host galaxies.
We present a spectroscopic survey of 21 young massive clusters and complexes and one tidal dwarf galaxy candidate (TDG) in Stephans Quintet, an interacting compact group of galaxies. All of the selected targets lie outside the main galaxies of the sy
stem and are associated with tidal debris. We find clusters with ages between a few and 125 Myr and confirm the ages estimated through HST photometry by Fedotov et al. (2011), as well as their modelled interaction history of the Quintet. Many of the clusters are found to be relatively long-lived, given their spectrosopically derived ages, while their high masses suggest that they will likely evolve to eventually become intergalactic clusters. One cluster, T118, is particularly interesting, given its age (sim 125 Myr), high mass (sim 2times10^6 Modot) and position in the extreme outer end of the young tidal tail. This cluster appears to be quite extended (Reff sim 12 - 15 pc) compared to clusters observed in galaxy disks (Reff sim 3 - 4 pc), which confirms an effect we previously found in the tidal tails of NGC 3256, where clusters are similarly extended. We find that star and cluster formation can proceed at a continuous pace for at least sim 150 Myr within the tidal debris of interacting galaxies. The spectrum of the TDG candidate is dominated by a young population (sim 7 Myr), and assuming a single age for the entire region, has a mass of at least 10^6 Modot.
We present Gemini optical spectroscopy of 23 young star clusters in NGC3256. We find that the cluster ages range are from few Myr to ~150 Myr. All these clusters are relatively massive (2--40)x 10^{5} msun$ and appear to be of roughly 1.5 zo metallic
ity. The majority of the clusters in our sample follow the same rotation curve as the gas and hence were presumably formed in the molecular-gas disk. However, a western subsample of five clusters has velocities that deviate significantly from the gas rotation curve. These clusters may either belong to the second spiral galaxy of the merger or may have formed in tidal-tail gas falling back into the system. We discuss our observations in light of other known cluster populations in merging galaxies, and suggest that NGC 3256 is similar to Arp 220, and hence may become an Ultra-luminous Infrared Galaxy as the merger progresses and the star-formation rate increases. Some of the clusters which appeared as isolated in our ground-based images are clearly resolved into multiple sub-components in the HST-ACS images. The same effect has been observed in the Antennae galaxies, showing that clusters are often not formed in isolation, but instead tend to form in larger groups or cluster complexes.
