From giant clumps to clouds -- II. The emergence of thick disc
  kinematics from the conditions of star formation in high redshift gas rich
  galaxies
  High redshift disc galaxies are more gas rich, clumpier, and more turbulent
than local Universe galaxies. This early era of galaxy formation imprints the
distribution and kinematics of the stars that we observe today, but it is not
yet well established how. In this work, we use simulations of isolated Milky
Way-mass disc galaxies to study how kinematic properties of stars change when
varying the gas fraction. This allows us to quantify the roles played by
internal processes, e.g. gas turbulence and gravitational scattering off
massive gas clumps, in establishing the observed stellar velocity dispersions
and orbital eccentricities. We find that models with gas fractions $>20$ per
cent feature a turbulent and clumpy interstellar medium (ISM), leading to
zero-age stellar velocity dispersions $\sim 20-30~{\rm km\, s}^{-1}$ and high
mean orbital eccentricities. Low eccentricities cannot arise from these
physical conditions. For gas fractions below $20$ per cent, the ISM becomes
less turbulent, with stellar velocity dispersions $<10~{\rm km\, s}^{-1}$, and
nearly circular orbits for young stars. The turbulence present in gas-rich high
redshift galaxies hence acts as a `barrier' against the formation of thin
discs. We compare our findings to the Milky Way's age-velocity dispersion
relation and argue that velocity dispersions imprinted already at star
formation by the ISM contribute significantly at all times. Finally, we show
that observed orbital eccentricities in the Milky Way's thick and thin discs
can be explained entirely as imprints by the star-forming ISM, rather than by
mergers or secular processes.
