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[*] Nuclear Star Clusters in Bulgeless Galaxies
Walcher C.J., Haering N., Boeker T., Rix H.-W., van der Marel R.P., Gerssen J., Ho L., Shields J.
in Carnegie Observatories Astrophysics Series, Vol. 1: Coevolution of Black Holes and Galaxies, ed. L. C. Ho (Pasadena: Carnegie Observatories, http://www.ociw.edu/ociw/symposia/series/symposium1/proceedings.html), in press, 2003


Centers of spiral galaxy bulges are very special in terms of their dynamics and star formation history. We investigate whether the centers of bulge-less galaxies are as unique in their structure and stellar populations. Our HST imaging survey of very late-type, bulge-less (Sd) galaxies has revealed unusually bright, nuclear star clusters at their photometric center in 75 percent of the cases. We have obtained high-resolution spectra with UVES at the VLT for 10 of those objects. We find that the majority of the clusters are younger than 1 Gyr. The masses of the clusters as determined from the velocity dispersion range between 10^6 and 10^8 solar masses and are typically one order of magnitude higher than would be expected from stellar synthesis models. We interpret this as an underlying population of older generations of stars that have already faded with time and thus do not show up in the spectrum. Consequently nuclear cluster formation is a repetitive event in very late type galaxies, which can contribute to the build-up of bulges.

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Last modified March 30, 2003.
Roeland van der Marel, marel@stsci.edu.
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