Title: Chemical Evolution in the Sagittarius Stream
Authors: Stefan Keller, Gary Da Costa, David Yong, Ken Freeman, Michael Bessell

Abstract: We have determined the metallicity and [alpha/Fe] ratio for M-giants in the trailing arm of the Sagittarius dwarf debris stream. Our targets sample the chemistry of the trailing arm in two regions and, therefore, at two epochs (~1Gyr and ~2Gyrs since tidal disruption). Through comparison to each other, and extant samples of the main body of the dSph, we have developed a unique view of the evolution of the metallicity distribution function and [alpha/Fe] in a galaxy accretion event. Our work addresses an outstanding difficulty for Lambda CDM hierarchical structure formation: if the halo is composed of accreted dSph why is [alpha/Fe] high in the halo and low in dSphs?