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Ever since a thick disk was proposed to explain the vertical distribution ofthe Milky Way disk stars, its origin has been a recurrent question. We aim toanswer this question by inspecting 19 disk galaxies with stellar mass greaterthan $10^{10}\,\rm M_\odot$ in recent cosmological high-resolution zoom-insimulations: Galactica and NewHorizon. The thin and thick disks are reasonablyreproduced by the simulations with scale heights and luminosity ratios asobserved. We then spatially classify the thin and thick disks and find that thethick disk stars are older, metal-poorer, kinematically-hotter, and higher inaccreted star fraction, while both disks are dominated by the stars formed insitu. Half of the in-situ stars in the thick disks are formed before thegalaxies develop their disks, and the rest are formed in spatially andkinematically thinner disks and then thickened with time by heating. However,the 19 galaxies have various properties and evolutionary routes, highlightingthe need for statistically-large samples to draw general conclusions. Weconclude from our simulations that the thin and thick disk components are notentirely distinct in terms of formation processes, but rather markers of theevolution of galactic disks. Moreover, as the combined result of the thickeningof the existing disk stars and the continued formation of young thin-diskstars, the vertical distribution of stars does not change much after the diskssettle, pointing to the modulation of both orbital diffusion and star formationby the same confounding factor: the proximity of galaxies to marginalstability.

Exploring the origin of thick disks using the NewHorizon and Galactica simulations