Galactic halo substructure in the Sloan Digital Sky Survey: The ancient tidal stream from the Sagittarius dwarf galaxy

Two studies have recently reported the discovery of pronounced halo substructure in the Sloan Digital Sky Survey (SDSS) commissioning data. Here we show that this halo substructure is, almost in its entirety, due to the expected tidal stream torn off the Sagittarius dwarf galaxy during the course of its many close encounters with the Milky Way. This interpretation makes strong predictions on the kinematics and distances of these stream stars. Comparison of the structure in old horizontal branch stars, detected by the SDSS team, with the carbon star structure discovered in our own survey, indicates that this halo stream is of comparable age to the Milky Way. It would appear that the Milky Way and the Sagittarius dwarf galaxy have been a strongly interacting system for most of their existence. Once complete, the SDSS will provide a unique data set with which to constrain the dynamical evolution of the Sagittarius dwarf galaxy; it will also strongly constrain the mass distribution of the outer Milky Way.