ar X iv : a st ro - p h / 02 10 36 2 v 1 1 6 O ct 2 00 2 VLT spectroscopy of NGC 3115 globular clusters

We present results derived from VLT–FORS2 spectra of 17 globular clusters associated with the nearby lenticular galaxy NGC3115. Comparing line-strength indices to new stellar population models by Thomas et al. we determine ages, metallicities and element abundance ratios. Our data are also compared with the Lick/IDS observations of Milky Way and M31 globular clusters. Our best age estimates show that the observed clusters which sample the bimodal colour distribution of NGC3115 globular clusters 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 M 31 while the Milky Way seems to harbour clusters which are mainly consistent with [α/Fe] ≃ 0.3. 1 The Sample and observations The candidate GCs were selected from the HST/WFPC2 investigation of Kundu & Whitmore (1998) who detected 144 globular cluster (herafter GC) candidates in the central region of NGC 3115. In order to keep integration times reasonably short only clusters with V < 22 (the peak of the GC luminosity function is at V = 22.37± 0.05) were selected while keeping a balance between red (V − I < 1.06) and blue (V − I ≥ 1.06) clusters. The overall GC population of NGC3115 shows a clear bimodal colour distribution with mean metallicities at [Fe/H] ≃ −0.37 and [Fe/H] ≃ −1.36 as estimated from the V − I colours. In order to utilize the full field-of-view of FORS2 we supplemented this list with GC candidates from a low-resolution spectroscopy survey reported in Kavelaars (1998) and also placed some slits on promising objects without prior information. In total 29 spectra were obtained. Applying a cut in recession velocity and S/N (S/N ≥ 12 per pixel) yielded a final sample of 17 GCs. 2 Harald Kuntschner et al. The observations were carried out with FORS2 on the VLT with the blue 600 l/mm grism and 1 wide MXU slitlets giving a resolution of ∼5 Å (FWHM). The total exposure time was 12 440 s.