New Constraints on a complex relation between globular cluster colors
  and environment
  We present an analysis of high-quality photometry for globular clusters (GCs)
in the Virgo cluster core region, based on data from the Next Generation Virgo
Cluster Survey (NGVS) pilot field, and in the Milky Way (MW) based on
VLT/X-Shooter spectrophotometry. We find significant discrepancies in
color-color diagrams between sub-samples from different environments,
confirming that the environment has a strong influence on the integrated colors
of GCs. GC color distributions along a single color are not sufficient to
capture the differences we observe in color-color space. While the average
photometric colors become bluer with increasing radial distance to the cD
galaxy M87, we also find a relation between the environment and the slope and
intercept of the color-color relations. A denser environment seems to produce a
larger dynamic range in certain color indices. We argue that these results are
not due solely to differential extinction, IMF variations, calibration
uncertainties, or overall age/metallicity variations. We therefore suggest that
the relation between the environment and GC colors is, at least in part, due to
chemical abundance variations, which affect stellar spectra and stellar
evolution tracks. Our results demonstrate that stellar population diagnostics
derived from model predictions which are calibrated on one particular sample of
GCs may not be appropriate for all extragalactic GCs. These results advocate a
more complex model of the assembly history of GC systems in massive galaxies
that goes beyond the simple bimodality found in previous decades.
