Exploring the origin of thick disks using the NewHorizon and Galactica
  simulations
  Ever since a thick disk was proposed to explain the vertical distribution of
the Milky Way disk stars, its origin has been a recurrent question. We aim to
answer this question by inspecting 19 disk galaxies with stellar mass greater
than $10^{10}\,\rm M_\odot$ in recent cosmological high-resolution zoom-in
simulations: Galactica and NewHorizon. The thin and thick disks are reasonably
reproduced by the simulations with scale heights and luminosity ratios as
observed. We then spatially classify the thin and thick disks and find that the
thick disk stars are older, metal-poorer, kinematically-hotter, and higher in
accreted star fraction, while both disks are dominated by the stars formed in
situ. Half of the in-situ stars in the thick disks are formed before the
galaxies develop their disks, and the rest are formed in spatially and
kinematically thinner disks and then thickened with time by heating. However,
the 19 galaxies have various properties and evolutionary routes, highlighting
the need for statistically-large samples to draw general conclusions. We
conclude from our simulations that the thin and thick disk components are not
entirely distinct in terms of formation processes, but rather markers of the
evolution of galactic disks. Moreover, as the combined result of the thickening
of the existing disk stars and the continued formation of young thin-disk
stars, the vertical distribution of stars does not change much after the disks
settle, pointing to the modulation of both orbital diffusion and star formation
by the same confounding factor: the proximity of galaxies to marginal
stability.
