| Authors : | Souradeep Bhattacharya (IUCAA, Pune, India), Magda Arnaboldi (ESO, Garching, Germany), Francois Hammer (GEPI, Observatoire de Paris, Paris, France), Yanbin Yang (GEPI, Observatoire de Paris, Paris, France), Ortwin Gerhard (MPE, Garching, Germany), Nelson Caldwell (CfA Harvard, MH, USA) and Kenneth C. Freeman (RSAA-ANU, Canberra, Australia) |
| Abstract : | At a distance of ~780 kpc, Andromeda (M31) is the nearest giant spiral galaxy to the Milky Way. Its proximity allows for a detailed study of its structure and stellar populations. M31 has experienced a tumultuous merger history as evidenced from the many substructures that are present in its inner halo, particularly the G1-Clump, NE- and W- shelves, and the Giant Stream. In a series of papers published over the recent years, we have explored the planetary nebulae (PNe) population of M31. Their line-of-sight velocity (LOSV) and chemical abundance measurements have revealed the distinct thin and thicker discs of M31. Now we present the LOSV measurements of PNe in the aforementioned four substructures. Including also measurements of red-giant-branch (RGB) LOSVs in these substructures, we compare with predictions of satellite and host material in a recent major merger (mass ratio = 1:4) simulation to help interpret the data. The observed projected radial distance vs. LOSV phase space for the two shelves and GS, links these substructures' formation in a single unique event, consistent with a major merger. We strikingly find the G1-clump to be dynamically cold, consistent with being composed primarily by pre-merger disc material. Such a structure cannot form in a minor merger (mass ratio ~1:20), and is therefore a smoking gun for the recent major merger event in M31. Juxtaposed with previous results for its disc, we now conclude that M31 has had a recent (2.5-4 Gyr ago) 'wet' major merger with the satellite falling along the giant stream disrupting the pre-merger disc to form the M31 thicker disc, while also forming the aforementioned inner-halo substructures and rebuilding the M31 thin disc from freshly infallen gas. M31 is now the galaxy with the most well constrained recent merger history. I will also address the possible scope for further constraints. |
