Rutgers University Astrophysics Seminar

Modern spectroscopic studies of resolved stellar populations in Andromeda (M31) will connect the Milky Way (MW) to future observations of disk galaxies in the Local Volume. Thus, our understanding of M31's merger history will be a foundation upon which we interpret this next observational frontier. Outstanding questions remain concerning M31's assembly history: has its disk survived a major merger within the last few billion years, and does this merger coincide with the formation of M31's Giant Stellar Stream (GSS) and its various tidal shells? In this talk, I will present novel results from the Spectroscopic and Photometric Landscape of Andromeda's Stellar Halo (SPLASH) survey on M31's Northeast (NE) Shelf, in addition to M31's inner stellar halo along the disk major axis. I will show that the shelf agrees with expectations of a tidal shell formed in a radial merger and provides strong evidence in favor of GSS formation models where it originates from the second orbital wrap of the tidal debris. Although remnants from this merger are predicted to pollute the major-axis halo, I will demonstrate that this stellar component is distinct from the NE Shelf, as well as the stellar halo along the minor axis that is far from the disk. These data represent the first complete kinematical detection of a resolved galactic shell system in the local universe and the first large-scale chemodynamical analysis of M31's northeastern disk region. I will conclude by discussing the implications of these results for M31's formation history in the context of the MW.