The Stellar Content of the Super–Star Clusters in NGC 1569

We discuss HST FOS ultraviolet spectroscopy and NICMOS near-infraredphotometry of four young super star clusters in the central region of theirregular starburst galaxy NGC 1569. The new observations coupled with previousHST WFPC2 photometry and ground-based optical spectroscopy allow us to isolateand age-date the hot and cool stellar components of these massive clusters. Weanalyze the two components A1 and A2 of the brightest super star cluster NGC1569-A. This cluster received previous attention due to the simultaneouspresence of Wolf-Rayet stars and red supergiants. The FOS spectra provide thefirst evidence for O-stars in NGC 1569-A, indicating a young (5 Myr) stellarcomponent in A1 and/or A2. Comparison with other high-mass star-forming regionssuggests that the O- and Wolf-Rayet stars are spatially coincident. If so,cluster A2 could be the host of the very young O- and Wolf-Rayet population,and the somewhat older red supergiants could be predominantly located in A1.The mass-to-light ratio of NGC 1569-A1 is analyzed in five optical and infraredphotometric bands and compared to evolutionary synthesis models. No indicationsfor an anomalous initial mass function are found, consistent with a scenariowhere this cluster is the progenitor of present-day globular clusters. Theclusters A1 and A2 are compared to clusters B and #30. The latter two clustersare older and fully dominated by red supergiants. All four super star clustersprovide a significant fraction (20 - 25%) of the total optical andnear-infrared light in the central region of the galaxy. The photometricproperties of the super star cluster population in NGC 1569 resemble those ofthe populous clusters in the Magellanic Clouds