Gemini spectroscopic survey of young star clusters in merging/interacting galaxies. IV. Stephan's Quintet

We present a spectroscopic survey of 21 young massive clusters and complexes and one tidal dwarf galaxy (TDG) candidate in Stephan's Quintet, an interacting compact group of galaxies. All of the selected targets lie outside the main galaxies of the system and are associated with tidal debris. We find clusters with ages between a few and 125Myr and confirm the ages estimated through Hubble Space Telescope photometry by Fedotov et al., as well as their modeled 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 (125Myr), high mass (2 × 10⁶ M, and position in the extreme outer end of the young tidal tail. This cluster appears to be quite extended (R _eff 12-15pc) compared to clusters observed in galaxy disks (R _eff 3-4pc), which confirms an effect we previously found in the tidal tails of NGC3256, where clusters are similarly extended. We find that star and cluster formation can proceed at a continuous pace for at least 150Myr within the tidal debris of interacting galaxies. The spectrum of the TDG candidate is dominated by a young population (7Myr), and, assuming a single age for the entire region, has a mass of at least 10⁶ M.