Near-infrared spectroscopy of a young super-star cluster in NGC 6946: chemical abundances and abundance patterns

Using the NIRSPEC spectrograph at Keck II, we have obtained H- and K-band echelle spectra for a young (∼10–15 Myr), luminous (MV∼−13.2) super-star cluster in the nearby spiral galaxy NGC 6946. From spectral synthesis and equivalent width measurements we obtain for the first time accurate abundances and abundance patterns in an extragalactic super-star cluster. We find [Fe/H]=−0.45 ± 0.08 dex, an average α-enhancement of ≈+0.22 ± 0.1 dex, and a relatively low 12 C/13 C ≈ 8 ± 2 isotopic ratio. We also measure a velocity dispersion of ≈9.1 km s−1, in agreement with previous estimates. We conclude that integrated high-dispersion spectroscopy of massive star clusters is a promising alternative to other methods for abundance analysis in extragalactic young stellar populations.