2023MNRAS.522.6010B

M 31 has experienced a recent tumultuous merger history, as evidenced from the many substructures that are still present in its inner halo, particularly the G1-Clump, NE-, and W-shelves and the Giant Stream (GS). We present planetary nebulae (PNe) line-of-sight velocity (LOSV) measurements covering the entire spatial extent of these four substructures. We further use predictions for the satellite and host stellar particle phase space distributions for a major merger (mass ratio = 1:4) simulation to help interpret the data. The measured PN LOSVs for the two shelves and GS are consistent with those from red giant branch stars. Their projected radius versus LOSV phase space, links the formation of these substructures in a single unique event, consistent with a major merger. We find the G1-clump to be dynamically cold compared to the M 31 disc ($\rm \sigma _{LOS, PN}=27$ km s^–1), consistent with pre-merger disc material. Such a structure can not form in a minor merger (mass ratio ∼1:20) and is therefore a smoking gun for the recent major merger event in M 31. The simulation also predicts the formation of a predominantly in situ halo from splashed-out pre-merger disc material, in qualitative agreement with observations of a metal-rich inner halo in M 31. Juxtaposed with previous results for its discs, we conclude that M 31 has had a recent (2.5-4 Gyr ago) 'wet' major merger with the satellite falling along the GS, heating the pre-merger disc to form the M 31 thicker disc, rebuilding the M 31 thin disc, and creating the aforementioned inner-halo substructures.