Dynamical Mass Estimates for Five Young Massive Stellar Clusters 1

We have obtained high-dispersion spectra for four massive star clusters in the dwarf irregular galaxies NGC 4214 and NGC 4449, using the HIRES spectrograph on the Keck I telescope. Combining the velocity dispersions of the clusters with structural parameters and photometry from images taken with the Hubble Space Telescope, we estimate mass-to-light ratios and compare these with simple stellar population (SSP) models in order to constrain the stellar mass functions (MFs) of the clusters. For all clusters we find mass-to-light ratios which are similar to or slightly higher than for a Kroupa MF, and thereby rule out any MF which is deficient in low-mass stars compared to a Kroupa-type MF. The four clusters have virial masses ranging between 2.1×10M⊙ and 1.5×10M⊙, half-light radii between 3.0 and 5.2 pc, estimated core densities in the range 2×103 M⊙ pc −3 to 2×105 M⊙ pc −3 and ages between 200 Myr and 800 Myr. We also present new high-dispersion near-infrared spectroscopy for a luminous young (∼ 15 Myr) cluster in the nearby spiral galaxy NGC 6946, which we have previously observed with HIRES. The new measurements in the infrared agree well with previous estimates of the velocity dispersion for this cluster, yielding a mass of about 1.7 × 10M⊙. The properties of the clusters studied here are all consistent with the clusters being young versions of the old globular clusters found around all major galaxies. Subject headings: galaxies: star clusters — galaxies: irregular