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

We find that the globular cluster systems of the Milky Way and of our neighboring spiral galaxy, M31, comprise two distinct entities, differing in three respects. First, M31 has a set of young globular clusters (GCs), ranging in age from a few 100 Myr to 5 Gyr old, as well as old globular clusters. No such very young GCs are known in the Milky Way. Second, we confirm that the oldest M31 GCs have much higher nitrogen abundances than do Galactic GCs at equivalent metallicities. Third, Morrison et al. have shown that M31 has a subcomponent of GCs that follow closely the disk rotation curve of that galaxy. Such a GC system in our own Galaxy has yet to be found. The only plausible scenario for the existence of the young M31 GC comes from the hierarchical-clustering-merging (HCM) paradigm for galaxy formation. We infer that M31 has absorbed more of its contingent of dwarf systems in the recent past than has the Milky Way. This inference has three implications: First, that all spiral galaxies could differ in their globular cluster properties, depending on how many companions each galaxy has, and when the parent galaxy absorbs them. In this spectrum of possibilities, apparently the Milky Way ties down one end, in which almost all of its GCs were absorbed 10-12 Gyr ago. Second, it suggests that young globular Department of Physics and Astronomy, Box 871504, Arizona State University, Tempe, AZ 85287-1504 Mt. Stromlo and Siding Spring Observatories, ANU, Private Bag, Weston Creek, A.C.T. 2611, Australia Anglo-Australian Observatory, P.O. Box 296, Epping, New South Wales 2121, Australia Centre for Astrophysics & Supercomputing, Swinburne University, Mail #31, P.O. Box 218, Hawthorn, Victoria 3127, Australia Instituto de Astronomia, Geofisica e Ciências Atomsféricas, Universidade de São Paulo, Rua de Matão, 1226, Cidade Universitária, Sao Paulo, SP, CEP 05508-090, Brazil Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138 Lick Observatory, University of California at Santa Cruz, Santa Cruz, CA 95064