Dynamical masses of young star clusters in interacting galaxies

Using ISAAC on VLT-ANTU and UVES on VLT-KUEYEN we have begun a program to measure stellar velocity dispersions of young star clusters in merging and interacting galaxies. In this contribution, we present results for clusters in two interacting galaxies - NGC 4038/39 and NGC 1487. Combining the measured velocity dispersions with sizes of the clusters estimated from Hubble Space Telescope imaging data resulted in the first determinations of dynamical masses of stellar clusters in galaxy mergers. Due to the faintness of the clusters and the high spectral resolution required, these results could only be obtained in with 10m class telescopes. Our results suggest that masses, sizes and concentrations of the light distributions are comparable to those of globular clusters, supporting the idea that part of the globular cluster population in elliptical galaxies is formed as a result of a merger event between to gas-rich spiral galaxies. However, the initial mass function (IMF) of the stars in the clusters seems to vary with environment: In some regions (dust-rich?), the IMF is more biased towards low-mass stars than in other (dust-poor) regions. There is a long-standing and substantial controversy in the literature whether or not their exists a "universal IMF". Our results for clusters in merging galaxies support the notion that the IMF depends on the birth environment of the cluster or perhaps some other variable. The relative content of low mass stars also influences the survival probability of stellar clusters. For their masses and light concentrations, some of the clusters have sufficiently shallow IMFs that it is likely that they will dissolve within a Hubble time, while for others, the IMF is sufficiently steep that they are likely to survive but undergo significant mass loss during their evolution.