Globular Clusters as Fossils of Galaxy Formation

The globular clusters in the halos of large galaxies like our own are almost certainly fossil remnants of the early star-forming subsystems from which these galaxies were built. The ages of the halo clusters in our Galaxy indicate a prolonged period of galaxy building lasting at least several Gyr, and their masses indicate that they were formed in very massive star-forming complexes in protogalactic subsystems that may have resembled the present ‘blue compact dwarf’ galaxies. The surviving descendants of these subsystems are probably among the present dwarf spheroidal or nucleated dwarf galaxies, and the recently discovered Sagittarius dwarf is probably an example of such an object just now being accreted by our Galaxy and depositing into its halo four globular clusters including the second most luminous one in our Galaxy, M54.