Merger-induced galaxy transformations in the ARTEMIS simulations
  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 these 15 galaxies, 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. Focusing on the galaxies themselves, we find
multiple examples of stellar discs whose angular momentum (AM) axes change
orientation at rapid rates of $\sim60$ 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. Finally, we conduct a brief investigation into whether
bars form soon after the mergers. In a few galaxies we find a bar-like feature
whose emergence coincides with a significant merger.
