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

TL;DR

This study uses high-resolution cosmological simulations to investigate the origins of thick disks in galaxies, finding they are formed through a combination of early in-situ star formation and subsequent heating, with properties varying across galaxies.

Contribution

It provides new insights into thick disk formation, showing they are not entirely distinct from thin disks but are markers of disk evolution, based on analysis of 19 simulated galaxies.

Findings

Thick disks are older, metal-poorer, and kinematically hotter than thin disks.

Both disks are mainly formed in situ, with some stars accreted.

Vertical distribution remains stable after disk settling.

Abstract

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.