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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.

From giant clumps to clouds -- II. The emergence of thick disc kinematics from the conditions of star formation in high redshift gas rich galaxies