On the Size Difference between Red and Blue Globular Clusters

Several recent studies have reported a mean size difference of about 20% between the metal-rich and metal-poor subpopulations of globular clusters (GCs) in a variety of galaxies. In this paper, we investigate the possibility that the size difference might be a projection effect, resulting from a correlation between cluster size and galactocentric distance, combined with different radial distributions of the GC subpopulations. We find that projection effects may indeed account for a size difference similar to the observed one, provided that there is a steep relation between GC size and galactocentric distance in the central parts of the GC system and that the density of GCs flattens off near the center in a manner similar to a King profile. For more centrally peaked distributions such as a de Vaucouleurs R1/4 law, or for shallower size-radius relations, projection effects are unable to produce the observed differences in the size distributions.