Selecting Sagittarius: a study in kinematics and metallicities

The Sagittarius dwarf galaxy obtained its name from the constellation of the Archer, in which it lies. The dwarf spheroidal itself is part of a much larger system, the parent to a stream of stars which gracefully arches around the entirety of the Milky Way. This system of the Sagittarius dwarf and stream can help us probe how galaxies like the Milky Way formed, and what the dark matter halo of our Galaxy looks like. The current hierarchical scenario for galaxy development holds that many smaller mergers eventually built up into larger galaxies, such as the one we reside in now. The left over building blocks from the process of galaxy assembly should remain identifiable today as distinct structures in the smooth Milky Way halo. The Sagittarius dwarf and stream provide us with the best studied, and most complete, example of such a structure. We investigate the Sagittarius system in detail, obtaining and analysing new spectroscopic data from the Anglo-Australian Telescope. This research project entailed the selection of candidate Sagittarius members from existing photometric data, followed by detailed spectroscopic observations of over 24,110 stars in regions of the Sagittarius dwarf and stream, a sample roughly an order of magnitude larger than previous studies. For each of these stars, we measure kinematics and metallicities. The project is primarily observational, examining the properties of the Sagittarius system, with a comparison to existing models in order to constrain the mass profile of the Milky Ways dark matter halo, through which Sagittarius falls. We find that the distribution of stellar radial velocities in the core corresponds to the predictions of a pressure-supported model for the progenitor to the Sagittarius dwarf galaxy and stream system. We also note that the average metallicity appears to rise in the innermost two degrees of the Sagittarius dwarf core, a property that has been observed in other Local Group dwarf galaxies. We develop a new selection technique to distinguish Sagittarius stream members from a model of the smooth Galactic halo, and find reasonable agreement between our data and the predictions of a simulation in which Sagittarius orbits a Milky Way with a triaxial dark matter halo. This selection technique also yields a surprise detection of the Sextans dwarf in the region of the Sagittarius stream. We additionally apply our methodology to observations of a single field in the Orphan Stream, with promising results. The results of this thesis, both methods and data, have a number of important applications for future research. The technique developed here for distinguishing likely members of stellar overdensities from the smooth Galactic halo can be applied to other datasets covering other areas of the sky. The stellar velocities and metallicities obtained for the Sagittarius core and stream can be used to refine new models for the interaction of Sagittarius and the Milky Way, thereby constraining the properties of the progenitor and of the Galaxy's dark matter halo. Finally, follow-up observations of our targets (such as with high resolution spectroscopy) will allow more detailed analysis of the properties of Sagittarius as well as the other stellar structures identified in this work.