Formation of massive disc galaxies in the IllustrisTNG simulation

We investigate the formation history of massive disc galaxies in hydrodynamical simulation - the IllustrisTNG, to study why massive disc galaxies survive through cosmic time. 83 galaxies in the simulation are selected with M-*,M-z = 0 > 8 x 10(10) M-circle dot and kinematic bulge-to-total ratio less than 0.3. We find that 8.4 per cent of these massive disc galaxies have quiet merger histories and preserve disc morphology since formed. 54.2 per cent have a significant increase in bulge components in history, then become discs again till present time. The rest 37.3 per cent experience prominent mergers but survive to remain discy. While mergers and even major mergers do not always turn disc galaxies into ellipticals, we study the relations between various properties of mergers and the morphology of merger remnants. We find a strong dependence of remnant morphology on the orbit type of major mergers. Specifically, major mergers with a spiral-in falling orbit mostly lead to disc-dominant remnants, and major mergers of head-on galaxy-galaxy collision mostly form ellipticals. This dependence of remnant morphology on orbit type is much stronger than the dependence on cold gas fraction or orbital configuration of merger system as previously studied.

Automated summary

The study investigates the formation history of massive disc galaxies in the IllustrisTNG hydrodynamical simulation to understand why some of these galaxies can maintain their disc morphology throughout cosmic time. The research shows that some massive disc galaxies have quiet merger histories, others experience an increase in bulge components before becoming discs again, and some survive prominent mergers yet remain disc-like. The morphology of merger remnants strongly depends on the orbit type of major mergers, with spiral-in falling orbits leading to disc-dominant remnants and head-on galaxy-galaxy collisions mostly forming ellipticals.