We estimate the dynamical masses of several young ( \approx 10 Myr ) massive star clusters in two interacting galaxies , NGC 4038/4039 ( ” The Antennae ” ) and NGC 1487 , under the assumption of virial equilibrium .
These are compared with photometric mass estimates from K -band photometry and assuming a standard Kroupa IMF .
The clusters were selected to have near-infrared colors dominated by red supergiants , and hence to be old enough to have survived the earliest phases of cluster evolution when the interstellar medium is rapidly swept out from the cluster , supported by the fact that there is no obvious H \alpha emission associated with the clusters .
All but one of the Antennae clusters have dynamical and photometric mass estimates which are within a factor \approx 2 of one another , implying both that standard IMFs provide a good approximation to the IMF of these clusters , and that there is no significant extra-virial motion , as would be expected if they were rapidly dispersing .
These results suggest that almost all of the Antennae clusters in our sample have survived the gas removal phase as bound or marginally bound objects .
Two of the three NGC 1487 clusters studied here have M _ { dyn } estimates which are significantly larger than the photometric mass estimates .
At least one of these two clusters , and one in the Antennae , may be actively in the process of dissolving .
The process of dissolution contributes a component of non-virial motion to the integrated velocity measurements , resulting in an estimated M _ { dyn } which is too high relative to the amount of measured stellar light .
The dissolution candidates in both galaxies are amongst the clusters with the lowest pressures/densities measured in our sample .