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

**Authors:** Minjung J. Park, Sukyoung K. Yi, Sebastien Peirani, Christophe Pichon, Yohan Dubois, Hoseung Choi, Julien Devriendt, Sugata Kaviraj, Taysun Kimm, Katarina Kraljic, Marta Volonteri  
**arXiv:** [2009.12373](https://arxiv.org/abs/2009.12373)  
**Date:** 2020-09-25  
**Summary page:** https://www.summarizepaper.com/en/paper/2009.12373v1/

## Key points

- Study by Minjung J. Park and colleagues on thick disks in 19 disk galaxies
- Aim to understand vertical distribution of Milky Way disk stars through simulations
- Thin and thick disks accurately reproduced in simulations, aligning with observational data
- Distinct characteristics between thin and thick disk stars based on distance from galactic plane
- Thick disk stars older, less metal-rich, kinematically hotter, with higher fraction of accreted stars
- Both thin and thick disks mainly consist of in-situ stellar particles
- In-situ stars in thick disks originate before galaxy's disk development or form initially in thinner disks before heating process leads to thickening over time
- Thin and thick disk components not entirely distinct formation processes but markers for galactic disk evolution trajectory
- Vertical distribution of stars remains stable due to existing disk star thickening and ongoing formation of young thin-disk stars
- Stability influenced by proximity to marginal stability affecting orbital diffusion and star formation processes

## Summary

In their study, Minjung J. Park and colleagues delve into the origin of thick disks using high-resolution zoom-in simulations of 19 disk galaxies with stellar masses exceeding $10^{10}\,\rm M_\odot$, namely Galactica and NewHorizon. The researchers aim to shed light on the recurrent question surrounding the vertical distribution of Milky Way disk stars by examining the thin and thick disks within these simulated galaxies. Their findings reveal that both thin and thick disks are accurately reproduced by the simulations, displaying scale heights and luminosity ratios that align reasonably well with observational data. By spatially classifying disk stars based on their distances from the galactic plane, distinct characteristics emerge between the "thin" and "thick" components. Thick disk stars are identified as older, less metal-rich, kinematically hotter, and possessing a higher fraction of accreted stars compared to their thin disk counterparts. Interestingly, despite these differences, both thin and thick disks predominantly consist of stellar particles formed in situ. The researchers observe that approximately half of the in-situ stars in thick disks originate even before galaxies develop their disks, while the other half form initially in thinner disks before undergoing a process of heating that leads to their thickening over time. From these simulations, it is concluded that thin and thick disk components do not represent entirely distinct formation processes but rather serve as markers for the evolutionary trajectory of galactic disks. Furthermore, as a combined result of existing disk star thickening and ongoing formation of young thin-disk stars, the vertical distribution of stars remains relatively stable once the disks settle. This stability hints at a modulation effect on both orbital diffusion and star formation processes driven by a common factor: proximity to marginal stability. Overall, this comprehensive analysis provides valuable insights into the complex interplay between various factors shaping galactic disk evolution and offers a deeper understanding of the origins and dynamics underlying thick disk formations in galaxies.