The bursty origin of the Milky Way thick disc

We investigate thin and thick stellar disc formation in Milky Way-mass galaxies using 12 FIRE-2 cosmological zoom-in simulations. All simulated galaxies experience an early period of bursty star formation that transitions to a late-time steady phase of near-constant star formation. Stars formed during the late-time steady phase have more circular orbits and thin-disc-like morphology at z = 0, while stars born during the bursty phase have more radial orbits and thick-disc structure. The median age of thick-disc stars at z = 0 correlates strongly with this transition time. We also find that galaxies with an earlier transition from bursty to steady star formation have a higher thin-disc fractions at z = 0. Three of our systems have minor mergers with Large Magellanic Cloud-size satellites during the thin-disc phase. These mergers trigger short starbursts but do not destroy the thin disc nor alter broad trends between the star formation transition time and thin/thick-disc properties. If our simulations are representative of the Universe, then stellar archaeological studies of the Milky Way (or M31) provide a window into past star formation modes in the Galaxy. Current age estimates of the Galactic thick disc would suggest that the Milky Way transitioned from bursty to steady phase similar to 6.5 Gyr ago; prior to that time the Milky Way likely lacked a recognizable thin disc.

Automated summary

This study investigates thin and thick stellar disc formation in Milky Way-mass galaxies through 12 FIRE-2 cosmological zoom-in simulations. The research shows that simulated galaxies experience an early bursty star formation period followed by a late-time steady phase of near-constant star formation. There is a correlation between the transition time from bursty to steady star formation and the thin/thick-disc properties.