We study the globular cluster (GC) system of the Virgo giant elliptical
galaxy NGC 4365, using new wide-field VIK imaging. The GC colour distribution
has (at least) two peaks, but the colours of the red GCs appear more strongly
weighted towards intermediate colours compared to most other large ellipticals
and the integrated galaxy light. The intermediate-color/red peak may itself be
composed of two sub-populations, with clusters of intermediate colours more
concentrated towards the centre of the galaxy than both the blue and red GCs.
Nearly all intermediate-colour and red GCs in our sample show an offset towards
red V-K and/or blue V-I colours compared to SSP models for old ages in a
(V-K,V-I) diagram. This has in the past been interpreted as evidence for
intermediate ages. We also combine our VIK data with previously published
spectroscopy. The differences between observed and model colour-metallicity
relations are consistent with the offsets observed in the two-colour diagram,
with the metal-rich GCs being too red (by about 0.2 mag) in V-K and too blue
(by about 0.05 mag) in V-I compared to the models at a given metallicity. These
offsets cannot easily be explained as an effect of younger ages. We conclude
that, while intermediate GC ages cannot be definitively ruled out, an
alternative scenario is more likely whereby all the GCs are old but the
relative number of intermediate-metallicity GCs is greater than typical for
giant ellipticals. The main obstacle to reaching a definitive conclusion is the
lack of robust calibrations of integrated spectral and photometric properties
for stellar populations with near-solar metallicity. In any case, it is
puzzling that the intermediate-colour GCs in NGC 4365 are not accompanied by a
corresponding shift of the integrated galaxy light towards bluer colours.Comment: 23 pages, including 20 figures and 5 tables. Accepted for publication
in A&
