3 Colour distribution

9 out of 17 GCs for which we have good quality spectra and are kinematically associated with NGC 3115 are within the HST/WFPC2 image. In order to assign colours to the remaining 8 GCs we derived an empirical relationship between a "pseudo-colour'' measured from our flux-calibrated spectra and the HST (V-I) colours. Of course our limited wavelength range does not allow us to measure a real (V-I) colour but one can get a rough idea on the overall spectral shape. On the spectra we measured the mean counts in two regions (3900-4600 Å and 4800-5500 Å) and plotted the difference versus the HST (V-I) colour (see Fig. 4). This procedure yields a linear relationship with a mean scatter of 0.037 mag in derived (V-I) colour. Note that here we have used all available spectra of GC candidates with measured HST colours (see Table 1), specifically including the ones with an  S/N < 12 per pixel. The derived (V-I) colours are listed in Table 1 in column seven where we have assigned a mean error of 0.037 mag.

Figure 4: The pseudo colour derived from the spectra versus $(V-I)_{\rm HST}$. The linear fit corresponds to $(V-I)_{\rm HST} = -1.08(\pm0.04) \times ({\rm spec\; colour}) + 0.95(\pm0.01)$. For details see Sect. 3.

The colour distribution of our sample of 17 GCs is shown in Fig. 5 together with the full HST sample by Kundu & Whitmore (1998). The HST sample shows a clear bimodal structure with peaks at (V-I) = 0.96 and (V-I) = 1.17. Taking the mean colour of the HST sample ( (V-I) = 1.06) as the dividing line between the red and blue clusters, our spectroscopic sample features eleven blue clusters and six red ones while spanning the range (V-I) = 0.87-1.18.

Figure 5: Histogram of (V-I) colours of the cluster candidates from Kundu & Whitmore (1998). Overplotted as a filled histogram is our spectroscopic sample of 17 confirmed globular clusters in NGC 3115. The vertical dashed line indicates the dividing line between red and blue clusters at (V-I) = 1.06.