VLT spectroscopy of NGC 3115 globular clusters ⋆

We present results derived from VLT–FORS2 spectra of 24 different globular clusters associated with the lenticular galaxy NGC3115. A subsample of 17 globular clusters have sufficiently high signal–to–noise to allow precision measurements of absorption line-strengths. Comparing these indices to new stellar population models by Thomas et al. we determine ages, metallicities and element abundance ratios. For the first time these stellar population models explicitly take abundance ratio biases in the Lick/IDS stellar library into account. Our data are also compared with the Lick/IDS observations of Milky Way and M31 globular clusters. Unpublished higher order Balmer lines (HγA,F and HδA,F ) from the Lick/IDS observations are given in the Appendix. Our best age estimates show that the observed clusters which sample the bimodal colour distribution of NGC3115 are coeval within our observational errors (2–3 Gyr). Our best calibrated age/metallicity diagnostic diagram (Hβ vs [MgFe]) indicates an absolute age of 11–12 Gyr consistent with the luminosity weighted age for the central part of NGC3115. We confirm with our accurate line-strength measurements that the (V − I) colour is a good metallicity indicator within the probed metallicity range (−1.5 < [Fe/H] < 0.0). The abundance ratios for globular clusters in NGC3115 give an inhomogeneous picture. We find a range from solar to super-solar ratios for both blue and red clusters. This is similar to the data for M31 while the Milky Way seems to harbour clusters which are mainly consistent with [α/Fe] ≃ 0.3. From our accurate recession velocities we detect, independent of metallicity, clear rotation in the sample of globular clusters. In order to explain the metallicity and abundance ratio pattern, particularly the range in abundance ratios for the metal rich globular clusters in NGC3115, we favour a formation picture with more than two distinct formation episodes.