Abstract Details

Name: Souradeep Bhattacharya
Affiliation: IUCAA Pune
Conference ID: ASI2021_77
Title : The recent formation history of Andromeda (M31) revealed from Planetary Nebulae
Authors and Co-Authors : Souradeep Bhattacharya (IUCAA, Pune, India), Magda Arnaboldi (ESO, Garching, Germany), Nelson Caldwell (Harvard-Smithsonian CfA, Boston, USA), Ortwin Gerhard (MPE, Garching, Germany), Alan McConnachie (NRC Herzberg, Victoria, Canada), Ken Freeman (RSAA-ANU, Canberra, Australia), Johanna Hartke (ESO Chile, Santiago, Chile)
Abstract Type : Oral
Abstract Category : Extragalactic Astronomy
Abstract : Andromeda (M31) is the nearest giant spiral galaxy to our Milky Way (MW) and the most massive member of our Local Group. A plethora of substructures have been photometrically identified in its inner halo revealing its tumultuous recent formation history. M31 is thus a prime laboratory for the study of hierarchical formation of spiral galaxies and in sharp contrast to the MW which has had no major galaxy mergers over the past 10 Gyr. Uniform measurements of kinematics and chemical abundances over the entire disc and inner halo of M31 is essential to understand its structure and evolutionary history. Given its large angular size and contamination from MW halo stars, such measurements are only possible from discrete tracers firmly in the M31 system. Planetary Nebulae (PNe), bright emission-line nebulae in the late-phase of stellar evolution, are excellent discrete tracers of light, chemistry and motion in galaxies. We carry out a 54 sq. deg. uniform deep narrow-band OIII survey with MegaCam@CFHT to identify ~5000 PNe in M31. Spectroscopic follow-up of a subsample of the PNe in the M31 disc was carried out with Hectospec@MMT. The identified PNe were separated into high- and low-extinction samples having average ages of ~2.5 Gyr and ~4.5 Gyr, forming the dynamically colder thin and dynamically hotter thick discs of M31 respectively. The two discs are also found to be chemically distinct from PN argon abundance distributions and thus have distinct origins. This is the first identification of the kinematically and chemically distinct thin and thick discs in M31. We find that the age-velocity dispersion relation in M31 is consistent with a single major merger that occurred 2.5-4.5 Gyr ago with a merger mass ratio ~1:5. The cannibalised satellite, about twice as massive as M33, was then the third largest member of the Local Group.