2006ApJ...649..692W

As part of an ongoing effort to study the stellar nuclei of very late type, bulge-less spirals, we present results from a high-resolution spectroscopic survey of nine such nuclear star clusters, undertaken with the VLT UVES. We fit the spectra with population synthesis models and measure Lick-type indices to determine mean luminosity-weighted ages, which range from 4.1x10⁷ to 1.1x10¹⁰ yr and are insensitive to assumed metallicity or internal extinction. The average metallicity of nuclear clusters in late-type spirals is slightly subsolar (<Z≥0.015) but shows significant scatter. Most of the clusters have moderate extinctions of 0.1-0.3 mag in the I band. The nuclear cluster spectra are best described by age-composite stellar populations, as only such models yield mass-to-light ratios that match those obtained from dynamical measurements. For our nine sample clusters, the last star formation episode was on average 34 Myr ago, while all clusters experienced some star formation in the last 100 Myr. We thus conclude that the nuclear clusters undergo repeated episodes of star formation. Our results are robust to possible contamination from the underlying galaxy disk, as demonstrated by comparison to a similar analysis using smaller aperture spectra obtained with the HST STIS. Also considering the 2005 results from Walcher et al., we have thus shown that the stellar nuclei of bulge-less galaxies are massive and dense star clusters that form stars recurrently until the present day. This set of properties is unique among the various classes of star clusters. To elucidate whether and how these unique properties are related to the nuclear location of the cluster in its host galaxy remains a challenging question.