Dr. Joern Rossa
University of Florida

University of Florida Astronomy Colloquium - Sept. 13th, 2006

The Physical Properties of Nuclear Star Clusters in Spiral Galaxies as Revealed with HST/STIS: Dependence of Age and Mass on Hubble Type

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. At a spatial resolution of 0.2" the spectra provide a better separation of cluster light from underlying galaxy light than is possible with ground-based spectra. Approximately half of the spectra have a sufficiently high signal-to-noise ratio for detailed stellar population analysis. To infer the star formation history, metallicity and dust extinction, we fit weighted superpositions of single-age stellar population templates to the high signal-to-noise spectra. We use the results to determine the luminosity-weighted age, mass-to-light ratio, and masses of the clusters. Approximately half of the sample clusters contain a population younger than 1 Gyr. 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 ( = 8.37+-0.25 vs. 9.23+-0.21). The average mass-weighted ages are older by ~0.7 dex, indicating that there often is an underlying older population that does not contribute much light, but does contain most of the mass. The average cluster masses are smaller in late-type spirals than in early-type spirals ( = 6.25+-0.21 vs. 7.63+-0.24), and exceed the masses typical of globular clusters. The cluster mass correlates loosely with total galaxy luminosity. It correlates more 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 in the centers of spiral galaxies are therefore intimately connected to the properties of the host galaxy, and in particular its bulge component. Plausible formation scenarios will have to account for this. We discuss various possible selection biases in our results, but conclude that none of them can explain the differences seen between clusters in early- and late-type spirals. The inability to infer spectroscopically the populations of faint clusters does introduce a bias towards younger ages, but not necessarily towards higher masses.