Hubble Space TelescopeSTIS Spectra of Nuclear Star Clusters in Spiral Galaxies: Dependence of Age and Mass on Hubble Type

(Abridged) We study the nuclear star clusters in spiral galaxies of variousHubble types using spectra obtained with STIS on-board HST. We observed thenuclear clusters in 40 galaxies, selected from two previous HST/WFPC2 imagingsurveys. The spectra provide a better separation of cluster light fromunderlying galaxy light than is possible with ground-based spectra. To inferthe star formation history, metallicity and dust extinction, we fit weightedsuperpositions of single-age stellar population templates to the spectra. Theluminosity-weighted age ranges from 10 Myrs to 10 Gyrs. The stellar populationsof NCs are generally best fit as a mixture of populations of different ages.This indicates that NCs did not form in a single event, but instead they hadadditional star formation long after the oldest stars formed. On average, thesample clusters in late-type spirals have a younger luminosity-weighted meanage than those in early-type spirals (log(age/yr) = 8.37+/-0.25 vs.9.23+/-0.21). The average cluster masses are smaller in late-type spirals thanin early-type spirals (log(M/Msun) = 6.25+/-0.21 vs. 7.63+/-0.24), and exceedthe masses typical of globular clusters. The cluster mass correlates stronglywith both the Hubble type of the host galaxy and the luminosity of its bulge.The latter correlation has the same slope as the well-known correlation betweensupermassive black hole mass and bulge luminosity. The properties of bothnuclear clusters and black holes are therefore intimately connected to theproperties of the host galaxy.Comment: AJ submitted (original submission Nov 30, 2005, present version includes changes based on referee recommendations). 69 pages, 16 figures, 7 table