The stellar content of the super-star clusters in NGC 1569

We discuss HST Faint Object Spectrograph (FOS) ultraviolet spectroscopy and Near-Infrared Camera and Multiobject Spectrometer (NICMOS) photometry of four young super-star clusters in the central region of the irregular starburst galaxy NGC 1569. The new observations, coupled with previous HST WFPC2 photometry and ground-based optical spectroscopy, allow us to isolate and age-date the hot and cool stellar components of these massive clusters. We analyze the two components, A1 and A2, of the brightest super-star cluster NGC 1569-A. This cluster received previous attention owing to the simultaneous presence of Wolf-Rayet stars and red supergiants. The FOS spectra provide the first evidence for O stars in NGC 1569-A, indicating a young (less than or equal to5 Myr) stellar component in A1 and/or A2. Comparison with other high-mass star-forming regions suggests 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 infrared photometric bands and compared with evolutionary synthesis models. No indications for an anomalous initial mass function are found, consistent with a scenario where this cluster is the progenitor of present-day globular clusters. The clusters A1 and A2 are compared with clusters B and No. 30. The latter two clusters are older and fully dominated by red supergiants. All four super-star clusters provide a significant fraction (20%-25%) of the total optical and near-infrared light in the central region of the galaxy. The photometric properties of the super-star cluster population in NGC 1569 resemble those of the populous clusters in the Magellanic Clouds.