The super-star cluster NGC 1569-A resolved on subparsec scales with Hubble Space Telescope spectroscopy

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⊙ 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.