Dynamical mass of a star cluster in M 83: a test of fibre-fed multi-object spectroscopy

Aims: We obtained VLT/FLAMES+UVES high-resolution, fibre-fed spectroscopy of five young massive clusters (YMCs) in M 83 (NGC 5236). This forms the basis of a pilot study testing the feasibility of using fibre-fed spectroscopy to measure the velocity dispersions of several clusters simultaneously, in order to determine their dynamical masses. In principle, this reduces the telescope time required to obtain a statistically significant sample of dynamical cluster masses. These can be used to assess the long-term survivability of YMCs by comparing their dynamical and photometric masses, which are necessary to ascertain the potential evolution of YMCs into second-generation globular clusters. Methods: We adopted two methods for determining the velocity dispersion of the star clusters: cross-correlating the cluster spectrum with the template spectra and minimising a χ² value between the cluster spectrum and the broadened template spectra. We also considered both red giant and red supergiant template stars. Cluster 805 in M 83 (following the notation of Larsen) was chosen as a control to test the reliability of the results obtained by this observational method, through a comparison with the results obtained from a standard echelle VLT/UVES spectrum obtained by Larsen & Richtler. Results: We find no dependence of the velocity dispersions measured for a cluster on the choice of red giant versus red supergiant templates, nor on the method adopted. However, we do find that the standard deviation of the results obtained with only one method may underestimate the true uncertainty. We measure a velocity dispersion of σ_los = 10.2 ± 1.1 km s^-1 for cluster 805 from our fibre-fed spectroscopy. This is in excellent agreement with the velocity dispersion of σ_los = 10.6 ± 1.4 km s^-1 determined from the standard echelle UVES spectrum of cluster 805. Our FLAMES+UVES velocity dispersion measurement gives M_vir, = (6.6 ± 1.7) × 10⁵ M_⊙, consistent with previous results. This value of the virial mass is a factor of ∼3 greater than the cluster's photometric mass, indicating a lack of virial equilibrium. However, based on its effective star formation efficiency, the cluster is likely to virialise, and may survive for a Hubble time, in the absence of external disruptive forces. Unfortunately, our observations of the other M 83 star clusters have insufficient signal-to-noise ratios to determine robust cluster velocity dispersions. Conclusions: We find that reliable velocity dispersions can be determined from high-resolution, fibre-fed spectroscopy. The advantages of observing several clusters simultaneously outweighs the difficulty of accurate galaxy background subtraction, providing that the targets are chosen to provide sufficient signal-to-noise ratios, and are much brighter than the galaxy background.