Evolution of Disc Thickness in High-Redshift Galaxies
  We study the growth of stellar discs of Milky Way-sized galaxies using a
suite of cosmological simulations. We calculate the half-mass axis lengths and
axis ratios of stellar populations split by age in isolated galaxies with
stellar mass $M_* = 10^7 - 10^{10} M_{\odot}$ at redshifts $z$ > 1.5. We find
that in our simulations stars always form in relatively thin discs, and at ages
below 100 Myr are contained within half-mass height $z_{1/2}$ ~ 0.1 kpc and
short-to-long axis ratio $z_{1/2}/x_{1/2}$ ~ 0.15. The disc thickness increases
with the age of stellar population, reaching median $z_{1/2}$ ~ 0.8 kpc and
$z_{1/2}/x_{1/2}$ ~ 0.6 for stars older than 500 Myr. We trace the same group
of stars over the simulation snapshots and show explicitly that their intrinsic
shape grows more spheroidal over time. The observed increase in disc thickness
is due to the gradual vertical expansion of the originally thin stellar
population, combined with the rapidly changing orientation of the galactic
plane which mixes the configuration of young stars. The frequently mentioned
"upside-down" formation scenario of galactic discs, which posits that young
stars form in already thick discs at high redshift, is describing only the
final product of this quick disc inflation while missing the actual formation
of stars within a fairly thin plane. The location of very young stars in thin
discs is due to the correspondingly flat configuration of dense molecular gas
that participates in star formation.
