A Comparison Between the Half-Light Radii, Luminosities, and UBV Colors of Globular Clusters in M31 and the Galaxy

The Milky Way System and the Andromeda galaxy experienced radically different evolutionary histories. Nevertheless, it is found that these two galaxies ended up with globular cluster systems in which individual clusters have indistinguishable distributions of half-light radii. Furthermore globulars in both M31 and the Galaxy are found to have radii that are independent of their luminosities. In this respect globular clusters differ drastically from early-type galaxies in which half-light radius and luminosity are tightly correlated. Metal-rich globular clusters in M31 occupy a slightly larger volume than do those in the Galaxy. The specific globular cluster frequency in the Andromeda galaxy is found to he significantly higher than it is in the Milky Way System. The present discussion is based on the 107 Galactic globular clusters, and 200 putative globulars in M31, for which UBV photometry was available.