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 integral-field-unit instrument on the VLT as part of the Fornax3D project. By fitting recently developed population-orbital superposition models to the observed surface brightness, 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 that is identified as the relic of past massive mergers. By comparing to analogs 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 × 10¹⁰ M_⊙ (about one-fifth of its current stellar mass) in the case of NGC 1380 and of 1.5_-0.7^+1.6 ×10¹⁰ M_⊙ (about one-fourth of its current stellar mass) in the case of NGC 1427. Moreover, we infer that the last massive merger in NGC 1380 happened ∼10 Gyr ago based on the stellar age distribution of the regrown dynamically cold disk, whereas the merger in NGC 1427 ended t ≲ 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.