A 2–3 billion year old major merger paradigm for the Andromeda galaxy and its outskirts

Abstract

Recent observations of our neighbouring galaxy M31 have revealed that its disc was shaped by widespread events. The evidence for this includes the high dispersion (V/σ ≤ 3) of stars older than 2 Gyr, and a global star formation episode, 2–4 Gyr ago. Using the modern hydrodynamical code, gizmo, we have performed 300 high-resolution simulations to explore the extent to which these observed properties can be explained by a single merger. We find that the observed M31 disc resembles models having experienced a 4:1 merger, in which the nuclei coalesced 1.8–3 Gyr ago, and where the first passage took place 7–10 Gyr ago at a large pericentre distance (32 kpc). We also show that within a family of orbital parameters, the Giant Stream (GS) can be formed with various merger mass ratios, from 2:1 to 300:1. A recent major merger may be the only way to create the very unusual age–dispersion relation in the disc. It reproduces and explains the long-lived 10 kpc ring, the widespread and recent star formation event, the absence of a remnant of the GS progenitor, the apparent complexity of the 3D spatial distribution of the GS, the NE and G Clumps and their formation process, and the observed slope of the halo profile. These modelling successes lead us to propose that the bulk of the substructure in the M31 halo, as well as the complexity of the inner galaxy, may be attributable to a single major interaction with a galaxy that has now fully coalesced with Andromeda.

The M31/M33 tidal interaction: a hydrodynamic simulation of the extended gas distribution

“…In contrast to these observations, we have shown that M31's Giant Stellar Stream cannot be reproduced with a hydrodynamical model for the infall of M33 onto M31 following an orbit with a close encounter in the remote past. This is consistent with the idea that the GSS is a structure created some ∼ 1 − 3 Gyr ago by the direct collision of M31 with a companion featuring a range of possible masses (∼ 10 9 − 10 11 M ; Fardal et al 2006;Hammer et al 2018), and which we have not included in our model. Therefore, the existence of the GSS and the plausibility of a close encounter between M31 and M33 in the distant past are not mutually exclusive.…”

“…Others (e.g. Hammer et al 2018) argue for a major merger event as the source of the GSS. In view of the new orbital history presented here, it is worth exploring whether the origin of this structure could be attributed to a close encounter with M33 in the distant past Similarly, the observed distribution of gas around M31 strongly suggests that this galaxy has had a number of interactions with its companions.…”

“…Earlier simulations of the tidal interaction between M31 and M33 (e.g Bekki 2008;McConnachie et al 2009) were constrained by (now outdated) M33's PM alone -since M31's PM were not available at the time, or targeted specific aspects of the rich structure observed around in and around either M31 (e.g. Hammer et al 2018) or M33 (e.g. Semczuk et al 2018).…”

Comparing the Quenching Times of Faint M31 and Milky Way Satellite Galaxies

“…The M31 halo hosts rich stellar substructures (e.g., streams, over-densities) that suggest an active history of mergers in M31 (see Mc-Connachie et al 2018 and references therein) and its stellar halo and outer disk have large populations of intermediate age stars, as revealed by oldest MSTO-depth CMD analysis (e.g., Brown et al 2006;Bernard et al 2015). Several models have posited a major merger between M31 and (what would have been) the third largest member of the LG ∼2-4 Gyr ago (e.g., Hammer et al 2018;D'Souza & Bell 2018). These models can qualitatively explain some observed features of M31, such as a global burst of star formation 2-4 Gyr ago (e.g., Bernard et al 2015;Williams et al 2017) and the metal-rich inner halo.…”

The Large-scale Structure of the Halo of the Andromeda Galaxy. II. Hierarchical Structure in the Pan-Andromeda Archaeological Survey

“…If this was the case then, with reference to Figure 12, it would imply the Giant Stream, the Western Shelf, the Eastern Shelf, Andromeda North-East and the G1 clump (5 of the 13 highlighted features) are all causally connected via a single accretion event (the first three as remnants of the merged satellite, and the latter 2 as debris from the disk of M31 as it responded to the induced gravitational perturbation). It is worth noting that recent work by Hammer et al (2018) consider a relatively major (4:1) merger in Andromeda about 2 -3 Gyrs ago, and they conclude that it is possible that the majority of inner halo substructures may have been formed in a single event. Huxor et al (2011) have already noted the excellent agreement between the shape of the radial profile of the globular clusters and that of the stellar halo that is visible in Figure 13.…”