Stellar Populations in the Nuclei of Late‐Type Spiral Galaxies

(Abridged) As part of an ongoing effort to study the stellar nuclei of verylate-type, bulge-less spirals, we present results from a high-resolutionspectroscopic survey of nine such nuclear star clusters, undertaken withVLT/UVES. We fit the spectra with population synthesis models and measureLick-type indices to determine mean luminosity-weighted ages, which range from4.1*10^7 to 1.1*10^10 years and are insensitive to assumed metallicity orinternal extinction. The average metallicity of nuclear clusters in late-typespirals is slightly sub-solar ( = 0.015) but shows significant scatter. Thefits also show that the nuclear cluster spectra are best described by a mix ofseveral generations of stars. This is supported by the fact that only modelswith composite stellar populations yield mass-to-light ratios that match thoseobtained from dynamical measurements. The last star formation episode was onaverage 34 Myr ago, while all clusters experienced some star formation in thelast 100 Myr. We thus conclude that the nuclear clusters undergo repeatedepisodes of star formation. The robustness with respect to possiblecontamination from the underlying galaxy disk is demonstrated by comparison tospectra obtained with HST/STIS. Combining these results with those from Walcheret al. (2005), we have thus shown that the stellar nuclei of these bulge-lessgalaxies are massive and dense star clusters that form stars recurrently untilthe present day. This unique set of properties is likely due to the centrallocation of these clusters in their host galaxies.Comment: ApJ submitted (original submission Oct 19, 2005, present version includes changes based on referee recommendations). 53 pages, 12 figures, 8 table