Dynamical Mass Estimates for Two Luminous Young Stellar Clusters in Messier 83

Abstract

Using new data from the UVES spectrograph on the ESO Very Large Telescope and archive images from the Hubble Space Telescope, we have measured projected velocity dispersions and structural parameters for two bright young star clusters in the nearby spiral galaxy NGC 5236. One cluster is located near the nuclear starburst of NGC 5236, at a projected distance of 440 pc from the centre, while the other is located in the disk of the galaxy at a projected galactocentric distance of 2.3 kpc. We estimate virial masses for the two clusters of (4.2+/-0.7)x10^5 Msun and (5.2+/-0.8)x10^5 Msun and ages (from broad-band photometry) of 10^(7.1+/-0.2} years and 10^(8.0+/-0.1) years, respectively. Comparing the observed mass-to-light (M/L) ratios with simple stellar population models, we find that the data for both clusters are consistent with a Kroupa-type stellar mass function (MF). In particular, we rule out any MF with a significantly lower M/L ratio than the Kroupa MF, such as a Salpeter-like MF truncated at a mass of 1 Msun or higher. These clusters provide a good illustration of the fact that massive, globular cluster-like objects ("super star clusters") can form at the present epoch even in the disks of seemingly normal, undisturbed spiral galaxies.