The Nature of the Ubiquitous Nuclear Star Clusters in Spiral Galaxies from HST and VLT spectroscopyvan der Marel, Rossa, Walcher, Boeker, Charlot, Ho, Rix & Shields
It has been found from HST imaging studies that most spiral galaxies host a pronounced "nuclear" star cluster in their very center. To understand the nature of these clusters we obtained spectra with HST/STIS and VLT/UVES of the nuclear clusters in 40 spiral galaxies. To infer the star formation history, metallicity and dust extinction, we fitted weighted sums of single-age stellar population templates to the spectra. The luminosity-weighted age of the clusters ranges from 10 Myrs to 10 Gyrs, with clusters in late-type spirals having a younger luminosity-weighted mean age than those in early-type spirals. The stellar populations of the clusters are generally best fit as a mixture of populations of different ages, indicating that star formation is an ongoing process in these clusters. The average cluster mass is smaller in late-type spirals than in early-type spirals. The cluster mass correlates strongly with the luminosity of the host galaxy bulge, with the same slope as the well-known correlation between supermassive black hole mass and bulge luminosity. I discuss these results in the context of models for the formation and evolution of both nuclear star clusters and black holes in galaxy centers.