Dynamical Mass Estimates for Five Young Massive Stellar Clusters

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 (M/L) ratios and compare these with simple stellar population models in order to constrain the stellar mass functions (MFs) of the clusters. For all clusters we find M/L values that are similar to or slightly higher than for a Kroupa MF and thereby rule out any MF that is deficient in low-mass stars compared with a Kroupa-type MF. The four clusters have virial masses ranging between 2.1×10⁵ and 1.5×10⁶M_solar, half-light radii between 3.0 and 5.2 pc, estimated core densities in the range 2×10³ to 2×10⁵M_solarpc^-3, and ages between 200 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 1.7×10⁶M_solar. Including an improved estimate of the reddening toward this cluster, the new data yield an M/L in excellent agreement with a Kroupa-type MF also for this cluster. 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.

Based on data obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. Also based on observations with the Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555.