A Young Stellar Cluster in the Nucleus of NGC 4449

摘要

We have obtained 1–2 ? resolution optical echellette spectra of the nuclear star cluster in the nearby starburst galaxy NGC 4449. The light is clearly dominated by a very young (6–10 Myr) population of stars. For our age dating, we have used recent population synthesis models to interpret the observed equivalent width of stellar absorption features such as the H I Balmer series and the Ca II triplet around 8500 ?. We also compare the observed spectrum of the nuclear cluster with synthesized spectra for stellar populations of varying ages. All these approaches yield a consistent cluster age. Metallicity estimates based on the relative intensities of various ionization lines yield no evidence for significant enrichment in the center of this low-mass galaxy: the metallicity of the nuclear cluster is about one-fourth of the solar value, in agreement with independent estimates for the disk material of NGC 4449.