Integrated high-dispersion spectroscopy of massive star clusters is a promising method for abundance analysis in extragalactic young stellar populations. Using the NIRSPEC spectrograph at Keck II, we have obtained IR spectra of a few young and luminous super-star clusters (SSCs) in nearby galaxies. First results on the SSC in the spiral galaxy NGC 6946 were published in Larsen et al. (2006). This year we published the results for the massive star cluster `B' in the nearby dwarf irregular galaxy NGC 1569 (Larsen et al. 2008). From spectral synthesis and equivalent width measurements, we obtain abundances of iron, C and alpha elements and abundance patterns, a supersolar [/Fe] abundance ratios and low isotopic ratio. Using archival imaging from the Advanced Camera for Surveys onboard the Hubble Space Telescope (HST), we construct a Colour Magnitude Diagram for the cluster in which we identify about 60 red supergiant (RSG) stars, consistent with the strong RSG features seen in the H-band spectrum. From the CMD, we derive an age of 15-25 Myr, slightly older than previous estimates based on integrated broad-band colours. We derive a radial velocity of km s and a velocity dispersion of km s. In combination with an estimate of the half-light radius of arcsec from the HST data, this leads to a dynamical mass of M. The dynamical mass agrees very well with the mass predicted by simple stellar population models for a cluster of this age and luminosity, assuming a normal stellar initial mass function.
This work is in collaboration with S. Larsen (University of Utrecht, NL), J. Brodie (UCO/Lick Observatory, USA) and J. Galagher (University of Wisconsin, USA).