Dynamical Masses of Young Star Clusters: Constraints on the Stellar IMF and Star-Formation Efficiency

Many recent works have attempted to constrain the stellar initial mass function (IMF) inside massive clusters by comparing their dynamical mass estimates (found through measuring the velocity dispersion and effective radius) to the measured light. These studies have come to different conclusions, with some claiming standard Kroupa-type [11] IMFs (e.g. [14], [13]) while others have claimed extreme non-standard IMFs (e.g. the top or bottom of the IMF is over-populated with respect to a Kroupa IMF [17]). However, the results appear to be correlated with the age of the clusters, as older clusters (>80 Myr) all appear to be well fit by a Kroupa-type IMF whereas younger clusters display significant scatter in their best fitting IMF [3]. This has led to the suggestion that the younger clusters are out of Virial equilibrium, thus undercutting the fundamental assumption which is necessary to derive dynamical masses. We will return to this point in § 2 and § 3. Focusing on the older clusters, we see that they all have standard IMFs (see Fig 2), arguing that at least in massive clusters the IMF does not vary significantly.