Evolution of disc thickness in simulated high-redshift galaxies

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 galaxies with stellar mass M-* = 10(7)-10(10) M-circle dot 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) similar to 0.1kpc and short-to-long axial ratio z(1/2)/x(1/2) similar to 0.15. Disc thickness increases with the age of stellar population, reaching median z(1/2) similar to 0.8 kpc and z(1/2)/x(1/2) similar to 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.

Automated summary

The study shows that stars always form in relatively thin discs in cosmological simulations. The thickness of the discs increases with the age of stellar population, and a new mechanism contributes to the observed disc thickness.