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

Abstract

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 (≤5 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.