  The globular cluster (GC) systems of many galaxies reveal bimodal optical
color distributions. Based on stellar evolutionary models and the bimodal
colors and metallicities of Galactic GCs this is thought to reflect an
underlying bimodal metallicity distribution. However, stars at many different
phases of stellar evolution contribute to optical light. The I-H color is a
much cleaner tracer of metallicity because it primarily samples the metallicity
sensitive giant branch. Therefore, we use deep HST-NICMOS H, and WFPC2 optical
observations, of M87 GCs to study their metallicity distribution. The M87
clusters are bimodal in I-H, for which there is no known physical explanation
other than a bimodal metallicity distribution. Moreover, the two modes defined
by the B-I and I-H colors are comprised of roughly the same two sets of
objects, confirming that optical colors also primarily trace the metallicity.
This is inconsistent with a recent suggestion based on one model of metallicity
effects on the horizontal branch that bimodality arises from an underlying
unimodal metallicity distribution due to a specific color-metallicity relation.
We also find no discernable variation in the peak colors of the M87 GCs out to
roughly 75 kpc due to the declining ratio of red-to-blue GCs, as implied by
this model. Similarly, there is no evidence that the bimodal peaks are bluer
for systems with large blue-to-red GC ratio. Our observations confirm that the
primary cause of bimodality in cluster systems is an underlying bimodal
metallicity distribution, and not the specific color-metallicity relationship
defined by this horizontal branch model.
