On the origin of the thick discs of spiral galaxies from high-resolution cosmological simulations

  • Published : 2020.10.13

Abstract

Ever since 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 solar mass in recent cosmological high-resolution (>34 pc) zoom-in simulations: Galactica and New Horizon. The thin and thick disks are reproduced by the simulations with scale heights and luminosity ratios that are in reasonable agreement with observations. When we spatially classify the disk stars into thin and thick disks by their heights from the galactic plane, the "thick" disk stars are older, less metal-rich, kinematically hotter, and higher in accreted star fraction than the "thin" disk counterparts. However, we found that the the thick disk stars were spatially and kinematically thinner when they were born. Indeed, a large fraction of thick disk stars was born near the galactic plane at earlier times and get heated with time, eventually occupying high altitudes and exhibiting different population properties compared to the thin-disk stars. In conclusion, from our simulations, the thin and thick disk components are not entirely distinct at birth, but rather a result of the time evolution of the stars born in the main disk of the galaxy. (excerpted from the abstract of the upcoming paper submitted to Astrophysical Journal: Park, M.-J., Yi, S.K. et al. 2020)

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