Evolution of Disc Thickness in Simulated High-Redshift Galaxies

TL;DR

This study uses cosmological simulations to analyze how stellar disc thickness in high-redshift galaxies evolves, revealing that young stars form in thin discs and that disc thickening occurs over time due to shape changes and rapid orientation shifts.

Contribution

It introduces a new mechanism involving quick orientation changes that contribute to disc thickening, challenging the traditional 'upside-down' formation scenario.

Findings

Stars form in thin discs at high redshift.

Disc thickness increases with stellar age.

Rapid orientation changes contribute to disc inflation.

Abstract

We study the growth of stellar discs of Milky Way-sized galaxies using a suite of cosmological simulations. We calculate the half-mass axis lengths and axis ratios of stellar populations split by age in isolated galaxies with stellar mass $M_* = 10^7 - 10^{10} M_{\odot}$ at redshifts $z$ > 1.5. We find that in our simulations stars always form in relatively thin discs, and at ages below 100 Myr are contained within half-mass height $z_{1/2}$ ~ 0.1 kpc and short-to-long axis ratio $z_{1/2}/x_{1/2}$ ~ 0.15. Disc thickness increases with the age of stellar population, reaching median $z_{1/2}$ ~ 0.8 kpc and $z_{1/2}/x_{1/2}$ ~ 0.6 for stars older than 500 Myr. We trace the same group of stars over the simulation snapshots and show explicitly that their intrinsic shape grows more spheroidal over time. We identify a new mechanism that contributes to the observed disc thickness: rapid changes in the orientation of the galactic plane mix the configuration of young stars. The frequently mentioned "upside-down" formation scenario of galactic discs, which posits that young stars form in already thick discs at high redshift, may be missing this additional mechanism of quick disc inflation. The actual formation of stars within a fairly thin plane is consistent with the correspondingly flat configuration of dense molecular gas that fuels star formation.