High redshift disc galaxies are more gas rich, clumpier, and more turbulentthan local Universe galaxies. This early era of galaxy formation imprints thedistribution and kinematics of the stars that we observe today, but it is notyet well established how. In this work, we use simulations of isolated MilkyWay-mass disc galaxies to study how kinematic properties of stars change whenvarying the gas fraction. This allows us to quantify the roles played byinternal processes, e.g. gas turbulence and gravitational scattering offmassive gas clumps, in establishing the observed stellar velocity dispersionsand orbital eccentricities. We find that models with gas fractions $>20$ percent feature a turbulent and clumpy interstellar medium (ISM), leading tozero-age stellar velocity dispersions $\sim 20-30~{\rm km\, s}^{-1}$ and highmean orbital eccentricities. Low eccentricities cannot arise from thesephysical conditions. For gas fractions below $20$ per cent, the ISM becomesless turbulent, with stellar velocity dispersions $<10~{\rm km\, s}^{-1}$, andnearly circular orbits for young stars. The turbulence present in gas-rich highredshift galaxies hence acts as a `barrier' against the formation of thindiscs. We compare our findings to the Milky Way's age-velocity dispersionrelation and argue that velocity dispersions imprinted already at starformation by the ISM contribute significantly at all times. Finally, we showthat observed orbital eccentricities in the Milky Way's thick and thin discscan be explained entirely as imprints by the star-forming ISM, rather than bymergers or secular processes.

Despite their ubiquity throughout the Universe, the formation of S0 galaxiesremains uncertain. Recent observations have revealed that S0 galaxies make up adiverse population which is difficult to explain with a single formationpathway, suggesting that the picture of how these galaxies form is morecomplicated than originally envisioned. Here we take advantage of the latesthydrodynamical cosmological simulations and follow up these studies with aninvestigation into the formation histories of S0s in IllustrisTNG. We firstclassify IllustrisTNG galaxies in a way which is fully consistent with theobservations, and reproduce the observed photometric and environmentaldistributions seen for the S0 population. We then trace the formation historiesof S0 galaxies back through time, identifying two main distinct pathways; thosewhich experienced gas stripping via group infalls (37 percent of S0s) orsignificant merger events (57 percent). We find that those forming via mergersfeature a transient star-forming ring, whose present-day occurrence ratematches observations. We find that these formation pathways together canreproduce the range in rotational support in observed S0s, concluding thatthere are two main formation pathways for S0 galaxies.