The Next Generation Virgo cluster Survey (NGVS). XXVI. The issues of
  photometric age and metallicity estimates for globular clusters
  Large samples of globular clusters (GC) with precise multi-wavelength
photometry are becoming increasingly available and can be used to constrain the
formation history of galaxies. We present the results of an analysis of Milky
Way (MW) and Virgo core GCs based on five optical-near-infrared colors and ten
synthetic stellar population models. For the MW GCs, the models tend to agree
on photometric ages and metallicities, with values similar to those obtained
with previous studies. When used with Virgo core GCs, for which photometry is
provided by the Next Generation Virgo cluster Survey (NGVS), the same models
generically return younger ages. This is a consequence of the systematic
differences observed between the locus occupied by Virgo core GCs and models in
panchromatic color space. Only extreme fine-tuning of the adjustable parameters
available to us can make the majority of the best-fit ages old. Although we
cannot exclude that the formation history of the Virgo core may lead to more
conspicuous populations of relatively young GCs than in other environments, we
emphasize that the intrinsic properties of the Virgo GCs are likely to differ
systematically from those assumed in the models. Thus, the large wavelength
coverage and photometric quality of modern GC samples, such as used here, is
not by itself sufficient to better constrain the GC formation histories. Models
matching the environment-dependent characteristics of GCs in multi-dimensional
color space are needed to improve the situation.
