As part of an ongoing effort to study the stellar nuclei of very late-type , bulge-less spirals , we present results from a high-resolution spectroscopic survey of nine such nuclear star clusters , undertaken with VLT/UVES .
We fit the spectra with population synthesis models and measure Lick-type indices to determine mean luminosity-weighted ages , which range from 4.1 \times 10 ^ { 7 } to 1.1 \times 10 ^ { 10 } years and are insensitive to assumed metallicity or internal extinction .
The average metallicity of nuclear clusters in late-type spirals is slightly sub-solar ( \langle Z \rangle = 0.015 ) but shows significant scatter .
Most of the clusters have moderate extinctions of 0.1 to 0.3 mags in the I -band .
The fits also show that the nuclear cluster spectra are best described by a mix of several generations of stars .
This is supported by the fact that only models with composite stellar populations yield mass-to-light ratios that match those obtained from dynamical measurements .
For our nine sample clusters , the last star formation episode was on average 34 Myr ago , while all clusters experienced some star formation in the last 100 Myr .
We thus conclude that the nuclear clusters undergo repeated episodes of star formation .
The robustness of our results with respect to possible contamination from the underlying galaxy disk is demonstrated by comparison to a similar analysis using smaller-aperture spectra obtained with HST/STIS .
Combining these results with those from Walcher et al .
( 2005 ) , we have thus shown that the stellar nuclei of these bulge-less galaxies are massive and dense star clusters that form stars recurrently until the present day .
This set of properties is unique among the various classes of star clusters .
It is almost inevitable to associate these unique properties with the location of the cluster in its host galaxy .
It remains a challenging question to elucidate exactly how very late-type spirals manage to create nuclei with such extreme characteristics .