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J. Rossa, R. P. van der Marel (STScI, Baltimore, MD), T. Boeker (ESTEC, Noordwijk, The Netherlands), J. Gerssen (University of Durham, U.K.), L. C. Ho (OCIW, Pasadena, CA), H.-W. Rix (MPIA, Heidelberg, Germany), J. C. Shields (Ohio University, Athens, OH), C.-J. Walcher (MPIA, Heidelberg, Germany)
We present results of a spectroscopic HST/STIS investigation of nuclear star clusters (NCs) in early- and late-type spiral galaxies. Our spectroscopic sample comprises 40 galaxies, which were selected from two previous HST/WFPC2 imaging surveys. We derive NC ages for 19 galaxies, which had sufficiently high signal-to-noise to perform stellar population synthesis by fitting Bruzual-Charlot stellar templates to our spectra. We derive various quantities, including the best-fit luminosity-weighted age, metallicity and extinction, using a sum of stellar templates. The derived ages of the NCs in our sample range from 10 Myrs to 10 Gyrs. Approximately 53% of the NCs for which we performed stellar population synthesis have populations that are younger than 1 Gyr. This indicates that at least for the late-type sample star formation is an ongoing process, as the ages are younger than a Hubble time. The NCs in late-type spirals have mean ages that are younger by 0.9 dex than the NCs in early-type spirals. We also have derived masses of the NCs, which were calculated from the M/L values and the inferred luminosity. NCs in early-type spirals clearly have larger masses, which on average are larger by 1.4 dex. Overall, we find that there is no evidence that the average NC masses for early-type and late-type spirals are systematically biased. However, the inferred average ages are probably biased in the sense that they are younger than the true average for all NCs in spiral galaxies. We find a correlation of the NC mass with the B-band bulge luminosity of the host galaxy, which is remarkably similar to the well established relation of bulge luminosity with black hole mass for galaxies.
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Bulletin of the American Astronomical Society, 37 #4
© 2005. The American Astronomical Soceity.