Star Cluster Formation in Extreme Starburst Environments

The currently available empirical evidence on the star formation processes in the extreme, high-pressure environments induced by galaxy encounters, mostly based on high-resolution Hubble Space Telescope imaging observations, strongly suggests that star cluster formation is an important and perhaps even the dominant mode of star formation in the starburst events associated with galaxy interactions. 1. Gravitational Interaction Induced Star (Cluster) Formation Close encounters between gas-rich galaxies often have devastating effects on the state of their interstellar medium (ISM), in particular of the ISM of either the smaller or of the most gas-rich of the interacting galaxies (e.g., de Grijs et al. 2001, 2003). The ram pressure caused by the time-varying gravitational potential induced by the interaction causes free-floating giant molecular clouds in the affected galaxy’s ISM to collapse and enter a phase of violent star formation. This process is particularly well illustrated by the ongoing interaction between NGC 6745 and its small northern companion galaxy, “NGC 6745c” (cf. de Grijs et al. 2003), where dynamical and kinematic evidence from Hi observations obtained with the Very Large Array support the scenario that the small companion galaxy has traveled from the south east across the eastern edge of the main galaxy to its current location north of the main galaxy, in its wake leaving a frenzy of very violent and active star, and in particular star cluster, formation. In de Grijs et al. (2003b) we used the ages, masses and metallicities of the rich young star cluster system in NGC 6745, based on the analysis of multipassband archival Hubble Space Telescope (HST) observations, to derive its cluster formation history and subsequent evolution. We derive a median age, and thus an estimate of the starburst duration, of ∼ 10 Myr. NGC 6745 contains a significant population of high-mass “super star clusters (SSCs)”, with masses in the range 6.5 ∼ < log(Mcl/M⊙) ∼< 8.0. We caution, however, that these massive SSC candidates may not be gravitationally bound objects, but more diffuse star forming regions or aggregates of multiple unresolved clusters instead. Nevertheless, we measure an effective radius for the most massive object (Mcl ≃ 5.9 × 10 M⊙) of only Reff ∼ 16 pc. However, this object appears very elongated, or may in fact be a double cluster. We should keep in mind, of course, that this high mass estimate is a strong function of the (low) metallicity assumed; if we had assumed solar metallicity for this object, the derived age would have been significantly smaller (∼ 10 − 20 Myr vs. ∼ 1 Gyr), and the