The Fornax3D project: discovery of ancient massive merger events in the
  Fornax cluster galaxies NGC 1380 and NGC 1427
  We report the discovery of ancient massive merger events in the early-type
galaxies NGC 1380 and NGC 1427, members of the Fornax galaxy cluster. Both
galaxies have been observed by the MUSE IFU instrument on the VLT, as part of
the Fornax3D project. By fitting recently-developed population-orbital
superposition models to the observed surface brightness as well as stellar
kinematic, age, and metallicity maps, we obtain the stellar orbits, age and
metallicity distributions of each galaxy. We then decompose each galaxy into
multiple orbital-based components, including a dynamically hot inner stellar
halo component which is identified as the relic of past massive mergers. By
comparing to analogues from cosmological galaxy simulations, chiefly TNG50, we
find that the formation of such a hot inner stellar halo requires the merger
with a now-destroyed massive satellite galaxy of $3.7_{-1.5}^{+2.7} \times
10^{10}$ Msun (about $1/5$ of its current stellar mass) in the case of NGC 1380
and of $1.5_{-0.7}^{+1.6} \times10^{10}$ Msun (about $1/4$ of its current
stellar mass) in the case of NGC 1427. Moreover, we infer that the last massive
merger in NGC 1380 happened $\sim10$ Gyr ago based on the stellar age
distribution of the re-grown dynamically cold disk, whereas the merger in NGC
1427 ended $t\lesssim 8$ Gyr ago based on the stellar populations in its hot
inner stellar halo. The major merger event in NGC 1380 is the first one with
both merger mass and merger time quantitatively inferred in a galaxy beyond the
Local Volume. Moreover, it is the oldest and most massive merger uncovered in
nearby galaxies so far.
