Dr. Souradeep Bhattacharya

Andromeda (M31) is the nearest giant spiral galaxy to the Milky Way (MW) and the most massive member of the Local Group (LG). Given its proximity to the MW, it has long been the subject of many breakthrough discoveries. For a long time, it was considered a sister galaxy to the MW given its similar spiral morphology. However, the first deep images of its outskirts revealed a plethora of inner halo substructures that shed light on its tumultuous merger history in sharp contrast to the quiet merger history of the MW. I will present the observational evidences in the disc and inner halo of M31 that reveal it to be a spiral galaxy vastly different from the MW. I will discuss in detail the observational constraints on the merger history of M31 from our survey of Planetary Nebulae (emission-line nebulae in the late stages of stellar evolution). We revealed the chemodynamically distinct thin and thicker disc of M31 and studied the kinematics of the inner halo substructures. Our results strongly constrain the merger in M31 to a single 'wet' merger event ~2.5-4 Gyr ago with a gas-rich satellite galaxy that would then have been the third largest member of our LG, about twice as massive as M33.