Globular Cluster and Galaxy Formation: M31, the Milky Way, and Implications for Globular Cluster Systems of Spiral Galaxies

The globular cluster (GC) systems of the Milky Way and of our neighboringspiral galaxy, M31, comprise 2 distinct entities, differing in 3 respects. 1.M31 has young GCs, ages from ~100 Myr to 5 Gyr old, as well as old globularclusters. No such young GCs are known in the Milky Way. 2. We confirm that theoldest M31 GCs have much higher nitrogen abundances than do Galactic GCs atequivalent metallicities. 3. Morrison et al. found M31 has a subcomponent ofGCs that follow closely the disk rotation curve of M31. Such a GC system in ourown Galaxy has yet to be found. These data are interpreted in terms of thehierarchical-clustering-merging (HCM) paradigm for galaxy formation. We inferthat M31 has absorbed more of its dwarf systems than has the Milky Way. Thisinference has 3 implications: 1. All spiral galaxies likely differ in their GCproperties, depending on how many companions each galaxy has, and when theparent galaxy absorbs them. The the Milky Way ties down one end of thisspectrum, as almost all of its GCs were absorbed 10-12 Gyr ago. 2. It suggeststhat young GCs are preferentially formed in the dwarf companions of parentgalaxies, and then absorbed by the parent galaxy during mergers. 3. Young GCsseen in tidally-interacting galaxies might come from dwarf companions of thesegalaxies, rather than be made a-new in the tidal interaction. There is no readyexplanation for the marked difference in nitrogen abundance for old M31 GCsrelative to the oldest Galactic GCs. The predictions made by Li & Bursteinregarding the origin of nitrogen abundance in globular clusters are consistentwith what is found for the old M31 GCs compared to that for the two 5 Gyr-oldM31 GCs.Comment: to be published in ApJ, Oct 2004; 13 pages of text, 2 tables, 7 postscript figure