The ACS Virgo Cluster Survey. XIV. Analysis of Color-Magnitude Relations in Globular Cluster Systems

We examine the correlation between globular cluster (GC) color and magnitude using HST/ACS imaging for a sample of 79 early-type galaxies (-21.7<M_B<-15.2 mag) with accurate SBF distances from the ACS Virgo Cluster Survey. Using the KMM mixture modeling algorithm, we find a highly significant correlation, d(g-z)/dz = -0.037 +- 0.004, between color and magnitude for the subpopulation of blue GCs in the co-added GC color-magnitude diagram of the three brightest Virgo galaxies (M49, M87 and M60): brighter GCs are redder than their fainter counterparts. For the single GC systems of M87 and M60, we find similar correlations; M49 does not appear to show a significant trend. There is no correlation between (g-z) and M_z for GCs of the red subpopulation. The correlation d(g-z)/dg for the blue subpopulation is much weaker than d(g-z)/dz. Using Monte Carlo simulations, we attribute this to the fact that the blue subpopulation in M_g extends to higher luminosities than the red subpopulation, which biases the KMM fits. The correlation between color and M_z thus is a real effect. This conclusion is supported by biweight fits to the same color distributions. We identify two environmental dependencies of the color-magnitude relation: (1) the slope decreases in significance with decreasing galaxy luminosity; and (2) the slope is stronger for GCs at smaller galactocentric distances. We examine several mechanisms that might give rise to the observed color-magnitude relation: (1) presence of contaminators; (2) accretion of GCs from low-mass galaxies; (3) stochastic effects; (4) capture of field stars by individual GCs; and (5) GC self-enrichment. We conclude that self-enrichment and field-star capture, or a combination of these processes, offer the most promising means of explaining our observations