Merger-induced galaxy transformations in the ARTEMIS simulations

TL;DR

This study uses the ARTEMIS simulations to analyze how galaxy mergers influence the dynamical and structural evolution of Milky Way-like galaxies, revealing significant changes in stellar and dark matter components.

Contribution

It provides new insights into merger-induced galaxy transformations, including stellar debris features, halo shape changes, and disc angular momentum reorientations, based on high-resolution cosmological simulations.

Findings

Approximately one-third of simulated galaxies show Gaia Sausage-like features.

Dark matter haloes often become more spherical after mergers.

Stellar discs can experience large angular momentum reorientations.

Abstract

Using the ARTEMIS set of 45 high-resolution cosmological simulations, we investigate a range of merger-induced dynamical transformations of Milky Way-like galaxies. We first identify populations of accreted stars on highly radial orbits, similar to the 'Gaia Sausage' in the Milky Way. We show that $\approx1/3$ of the ARTEMIS galaxies contain a similar feature, and confirm that they usually comprise stellar debris from the most massive accreted satellite. Selecting 15 galaxies with discs at the present-day, we study their changes around the times of the GS-like mergers. Dark matter haloes of many of these exhibit global changes in shape and orientation, with almost half becoming significantly more spherical when the mergers occur. Focusing on the galaxies themselves, we find that 4/15 have stellar discs which experience large changes in the orientation of their angular momentum (AM) axes, at rates of up to $\sim60$ degrees Gyr$^{-1}$. By calculating the orbital angular momentum axes of the satellites before they are accreted, we show that there is a tendency for the disc's AM to become more aligned with this axis after the merger. We also investigate the origin of in situ retrograde stars, analogous to the 'Splash' in the Milky Way. Tracing them back to earlier snapshots, we demonstrate that they were often disrupted onto their extreme orbits by multiple early mergers. We also find that the total mass of these stars outside the central regions positively correlates with the total accreted stellar mass.