TRACING THE PROPERTIES OF THE SAGITTARIUS STREAM ACROSS THE SKY WITH LAMOST SPECTRA

Abstract

The Sagittarius dwarf galaxy (Sgr) is a satellite that is currently being consumed by the Milky Way’s gravity. Its disruption has created the most prominent and widely studied tidal stream in our halo which wraps around our Galaxy with its leading arm in the northern Galactic hemisphere and its trailing arm in the southern hemisphere. By studying this stream, we can learn about how the dwarf galaxy formed as well as how its stripping has affected both its and the Milky Way’s evolution. Using optical spectra collected by the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) survey in combination with stellar proper motions measured by the GAIA satellite, we identify stars that belong to the Sagittarius tidal stream based on their positions, distances, velocities, and stellar parameters. We trace the velocity, distance, and metallicity of the Sagittarius stream asa function of its position over 200 degrees of its extent on the sky with a particular focus on the information we can obtain from the metallicity properties. Knowing how metallicity relates to other properties allows us to slightly unravel the star formation and orbital histories of Sgr. We find that for the parts of the stream we analyzed, the leading stream has a constant [Fe/H] of ≈-1.3 as a functionof position along it, while the [Fe/H] of the trailing stream increases as a function of position in the direction of the Sgr core. When analyzing the [α/Fe] ratio for Sgr, we found that the median ratios for the leading stream were close together while those for the trailing stream were spread out. Finally, we fit 3 gaussian components to the [Fe/H] distribution of our entire Sgr sample, with means of -0.79, -1.33, and -1.9. We believe that the constant metallicity we observe in the leading arm is due to those stars having been stripped at around the same time, and the gradient in the trailing stream is observed because it is comprised of stars that were stripped at different orbits of Sgr around the Milky Way.The observed metallicity gradient is also due to the star formation episodes of the pre-tidally stripped Sgr system itself, where the fitting of 3 different gaussians to the [Fe/H] distribution of the Sgr stream reveals the presence of 3 different stellar populations of the original Sgr system.