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H.L. Morrison (Case Western Reserve University)
The last 10-20 years has seen a profound shift in views of how the Galaxy's halo formed. The idea of a monolithic early collapse of a single system (Eggen, Lynden-Bell and Sandage 1962) has been challenged by observations at high redshift and by increasingly sophisticated cosmological models of the evolution of structure. These findings imply that we should see clear evidence of hierarchical formation processes in nearby galaxies. Recent studies of our Galaxy have begun to reveal tantalizing evidence of possible substructure in the halo. Most spectacularly, the discovery of the Sgr dwarf galaxy provides indisputable evidence that our Galaxy's halo is accreting stars and clusters today. The question has now shifted from `Did accretion play any role in the formation of the Galaxy?' to `How much of the Galaxy formed from accretion of hierarchical fragments?' I will review evidence for tidal streams associated with known Milky Way satellites and for star streams (some as close as the solar neighborhood) whose progenitors are still unknown. This will include results from recent surveys such as the Sloan Digital Sky Survey and our own ongoing pencil-beam halo survey. These data will be useful, in conjunction with theoretical models of the disruption of dwarf galaxies, in constraining models for the early formation of the Milky Way, a typical spiral galaxy.