Are the Super–Star Clusters of NGC 1569 in a Poststarburst Phase?

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Abstract
We present spectroscopic observations in the near-ultraviolet, optical, and near-infrared of the super- star clusters (hereafter SSC A and SSC B) in NGC 1569. Previous studies have suggested that they are in a poststarburst phase. However, our data suggest a younger mean age and a significant age spread. The spectrum of SSC A shows the Wolf-Rayet (W-R) feature at 4686 Å, with a luminosity equivalent to 20-40 WNL stars. This qualifies NGC 1569 as a W-R galaxy. The small Balmer jump detected in both clusters also suggests strongly the presence of very massive (and therefore very young) stars. Evolutionary synthesis models are used to constrain the star formation scenario by explaining the spectral energy distribution, the Balmer jump, the equivalent width (EW) of the W-R bump, and the near-infrared Ca II triplet in absorption. No single-age stellar population fits all these constraints. We propose a two-burst model, with the younger burst having an age of 3 Myr (2 Myr for SSC B) and the older one having an age of 9 Myr (8 Myr for SSC B), to explain the simultaneous presence of hot massive stars and red supergiants. We speculate that sequential star formation is taking place in the SSCs of NGC 1569, with the younger burst probably located in the surroundings of and initiated as a consequence of the energetic stellar activity of the older central compact cluster. A deficit of ionized gas was found around the SSCs, which we ascribe to the strong stellar winds and supernova explosions of the older burst removing the gas from the vicinity of the clusters.