Bimodal Infrared Colors of the M87 Globular Cluster System: Peaks in the Metallicity Distribution

Abstract

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 discernible variation in the peak colors of the M87 GCs out to ~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 a 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.