The bursty origin of the Milky Way thick disc

TL;DR

This study uses cosmological simulations to link the formation of the Milky Way's thick disc to an early bursty star formation phase, showing that the transition to steady star formation correlates with thin disc development.

Contribution

It demonstrates that the thick disc's origin is tied to an early bursty star formation period, providing insights into the Milky Way's formation history through simulated galaxy evolution.

Findings

Thick disc stars formed during bursty phase have radial orbits.

Transition from bursty to steady star formation influences thin/thick disc properties.

Galaxies with earlier transition have higher thin-disc fractions.

Abstract

We investigate thin and thick stellar disc formation in Milky-Way-mass galaxies using twelve 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 $z=0$, whilst 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 LMC-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 $\sim$6.5 Gyr ago; prior to that time the Milky Way likely lacked a recognisable thin disc.