HST/STIS 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 various Hubble types using spectra obtained with STIS on-board HST. We observed the nuclear clusters in 40 galaxies, selected from two previous HST/WFPC2 imaging surveys. The spectra provide a better separation of cluster light from underlying galaxy light than is possible with ground-based spectra. To infer the star formation history, metallicity and dust extinction, we fit weighted superpositions of single-age stellar population templates to the spectra. The luminosity-weighted age ranges from 10 Myrs to 10 Gyrs. The stellar populations of 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 had additional star formation long after the oldest stars formed. On average, the sample clusters in late-type spirals have a younger luminosity-weighted mean age 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 than in early-type spirals (log(M/Msun) = 6.25+/-0.21 vs. 7.63+/-0.24), and exceed the masses typical of globular clusters. The cluster mass correlates strongly with 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 between supermassive black hole mass and bulge luminosity. The properties of both nuclear clusters and black holes are therefore intimately connected to the properties of the host galaxy. 
Joern Rossa (STScI), Roeland P. van der Marel (STScI), Torsten Boeker   (ESTEC), Joris Gerssen (U of Durham), Luis C. Ho (OC...