Andromeda (M31) has a giant, faint stellar disc that reaches tens of kiloparsecs from its center. Spectroscopic surveys find stars out to projected radii of 40–70 kpc that rotate nearly as fast as the galaxy’s neutral hydrogen gas. New work uses a high-resolution N-body hydrodynamical simulation to show that a recent, gas-rich major merger can naturally produce that vast, rotating and warped outer disc.

The authors start from a published major-merger model (mass ratio about 1:4, the highest-resolution run called model336) that was already shown to match many inner-halo features of M31 and the timing of a broad star-formation episode about 2.5 billion years ago. They compare the positions and motions of simulated stars and globular clusters to the resolved tracers observed in Andromeda’s outskirts. The simulation follows both collisionless stars and gas, so it captures how gas can settle and form a new inner disc while older stars are tidally disturbed.

In the model the merger strongly distorts the original disc. Pre-merger stars are pulled out, heated (meaning their motions become less ordered) and rearranged. The post-merger disc becomes asymmetric and is stretched by about a factor of two. Its stellar extent in the simulation reaches beyond 40 kpc. Older stars show a clear warp: the inclination of the stellar layer steadily decreases with radius so that the outer parts appear less edge-on than the inner parts. The simulation also reproduces the prograde rotation of compact groups of outer globular clusters found at projected radii >25 kpc.

This matters because it gives a single physical picture that links several independent observations: the inner-halo debris, the thickened, kinematically hot stellar disc, the disc-wide starburst 2–3 billion years ago, and the fast-rotating extended disc and outer globular clusters. Earlier work had estimated that roughly 10% of M31’s luminous disc mass and perhaps 30% of its angular momentum could lie in this extended component. The merger scenario explains how a young, gas-rich inner disc can reform while older stars settle into a warped, extended structure.