We combine near-infrared photometry obtained with the VLT/ISAAC instrument and archival HST/WFPC2 optical images to determine VIK magnitudes and colours of globular clusters in two early-type galaxies , NGC 3115 and NGC 4365 .
The combination of near-IR and optical photometry provides a way to lift the age-metallicity degeneracy .
For NGC 3115 , the globular cluster colours reveal two major sub-populations , consistent with previous studies .
By comparing the V - I , V - K colours of the NGC 3115 globular clusters with stellar populations models , we find that the colour difference between the two \ga 10 Gyr old major sub-populations is primarily due to a difference in metallicity .
We find \Delta [ Fe/H ] = 1.0 \pm 0.3 dex and the blue ( metal-poor ) and red ( metal-rich ) globular cluster sub-populations being coeval within 3 Gyr .
In contrast to the NGC 3115 globular clusters , the globular cluster system in NGC 4365 exhibits a more complex age and metallicity structure . We find a significant population of intermediate-age very metal-rich globular clusters along with an old population of both metal-rich and metal-poor clusters .
Specifically , we observe a large population of globular clusters with red V - K colours but intermediate V - I colours , for which all current stellar population models give ages and metallicities in the range \sim 2 - 8 Gyr and \sim 0.5 Z _ { \odot } -3 Z _ { \odot } , respectively .
After 10 Gyr of passive evolution , the intermediate-age globular clusters in NGC 4365 will have colours which are consistent with the very metal-rich population of globular clusters in giant elliptical galaxies , such as M87 .
Our results for both globular cluster systems are consistent with previous age and metallicity studies of the diffuse galactic light .
In addition to the major globular cluster populations in NGC 3115 and NGC 4365 we report on the detection of objects with extremely red colours ( V - K \ga 3.8 mag ) , whose nature could not ultimately be revealed with the present data .