The survey of planetary nebulae in Andromeda (M31)

VII. Predictions of a major merger simulation model compared with chemodynamical data of the disc and inner halo substructures

¹ Department of Astrophysics, Astronomy & Mechanics, Faculty of Physics, National and Kapodistrian University of Athens, Panepistimiopolis, Zografou 15784, Greece
² European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany
³ Inter University Centre for Astronomy and Astrophysics, Ganeshkhind, Post Bag 4, Pune 411007, India
⁴ Centre for Astrophysics Research, Department of Physics, Astronomy and Mathematics, University of Hertfordshire, Hatfield AL10 9AB, UK
⁵ GEPI, Observatoire de Paris, PSL Research University, CNRS, Place Jules Janssen, F-92190 Meudon, France
⁶ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstraße, 85748 Garching, Germany
⁷ Department of Physics & Astronomy, The Johns Hopkins University, Baltimore, MD 21218, USA

^⋆ Corresponding author: haristsak@phys.uoa.gr

Received: 26 November 2024
Accepted: 16 April 2025

Abstract

Context. The nearest giant spiral, the Andromeda galaxy (M31), exhibits a kinematically hot stellar disc, a global star formation episode ∼2–4 Gyr ago, and conspicuous substructures in its stellar halo that are suggestive of a recent accretion event.

Aims. Recent chemodynamical measurements in the M31 disc and inner halo can be used as additional constraints for N-body hydrodynamical simulations that successfully reproduce the disc age-velocity dispersion relation and star formation history as well as the morphology of the inner halo substructures.

Methods. We combined an available N-body hydrodynamical simulation of a major merger (mass ratio 1:4) with a well-motivated chemical model to predict abundance distributions and gradients in the merger remnant at z = 0. We computed the projected phase space and the [M/H] distributions for the substructures in the M31 inner halo, namely, the Giant Stellar Stream (GSS) and the North-East (NE) and Western (W) shelves. We compared the chemodynamical properties of the simulated M31 remnant with recent measurements for the M31 stars in the inner halo substructures.

Results. This major merger model predicts (i) multiple distinct components within each of the substructures; (ii) a high mean metallicity and large spread in the GSS and NE and W shelves, which explain various photometric and spectroscopic metallicity measurements; (iii) simulated phase space diagrams that qualitatively reproduce various features identified in the projected phase space of the substructures in published data from the Dark Energy Spectroscopic Instrument (DESI); (iv) a large distance spread in the GSS, as suggested by previous tip of the red giant branch measurements; and (v) phase space ridges caused by several wraps of the secondary as well as up-scattered main M31 disc stars that also have plausible counterparts in the observed phase spaces.

Conclusions. These results provide further strong and independent arguments for a major satellite merger in M31 ∼3 Gyr ago and a coherent explanation for many of the observational results that make M31 appear so different from the Milky Way.