The Super-Star Cluster NGC 1569-A Resolved on Subparsec Scales with Hubble Space Telescope Spectroscopy

Abstract

We present 3000-10000 Å Hubble Space Telescope (HST)/Space Telescope Imaging Spectrograph long-slit spectroscopy of the bright super-star cluster A (SSC-A) in the dwarf starburst galaxy NGC 1569. The 0.05" HST angular resolution allows us, for the first time, to probe for spatial variations in the stellar population of an ~10⁶ M_solar SSC. Integrated ground-based spectra of SSC-A have previously revealed young Wolf-Rayet (W-R) signatures that coexist with features from supposedly older, red supergiant (RSG) populations. We find that the W-R emission complexes come solely from the subcluster A2, identified in previous HST imaging, and are absent from the main cluster A1, thus resolving the question of whether the W-R and RSG features arise in a single or distinct clusters. The equivalent widths of the W-R features in A2-including the C IV λ5808 complex that we detect in this object for the first time-are larger than previously observed in other W-R galaxies. Models with subsolar metallicity, as inferred from the nebular emission lines of this galaxy, predict much lower equivalent widths. On the ``clean'' side of A1, opposite to A2, we find no evidence for radial gradients in the observed stellar population at 0.05"<R<0.40" (~0.5-5 pc) in broadband, low-resolution spectra or in medium-resolution spectra of the infrared Ca II triplet. Based on observations made with the Hubble Space Telescope, which is operated by AURA, Inc., under NASA contract NAS5-26555.