The assembly of the most rotationally supported disc galaxies in the TNG100 simulations

Context. Disc-dominated galaxies can be difficult to accommodate in a hierarchical formation scenario such as Λ cold dark matter (ΛCDM), where mergers are an important growth mechanism. However, observational evidence indicates that these galaxies are common in the Universe. Aims. We seek to characterise the conditions that lead to the formation of disc-dominated galaxies within ΛCDM. Methods. We used dynamical decomposition of the stellar particles in all galaxies with stellar mass M ∗ = [10 10 − 10 11 ] M ⊙ within the cosmological hydrodynamical simulation Illustris TNG100. We selected a sample of 43 mostly-disc galaxies that have less than ∼10% of their mass in a bulge component. For comparison, we also studied two additional stellar-mass matched samples: 43 intermediate galaxies having ∼30% of their stellar mass in the bulge and 43 with a purely spheroidal-like morphology. Results. We find that the selection purely based on stellar dynamics is able to reproduce the expected stellar population trends of different morphological types, with higher star-formation rates and younger stars in disc-dominated galaxies. Halo spin seems to play no role in the morphology of the galaxies, in agreement with previous works. At a fixed M * , our mostly-disc and intermediate samples form in dark matter haloes that are two to ten times less massive than the spheroidal sample, highlighting a higher efficiency in disc galaxies to retain and condensate their baryons. On average, mergers are less prevalent in the buildup of discs than in spheroidal galaxies, but there is a large scatter, including the existence of mostly-disc galaxies, with 15%–30% of their stars coming from accreted origin. Discs start to form early on, settling their low vertical velocity dispersion as early as 9–10 Gyr ago, although the dominance of the disc over the spheroid was established more recently (3–4 Gyr lookback time). The most rotationally supported discs form in haloes with the lowest virial mass in the sample and the best aligned distribution of angular momentum in the gas.