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The bursty origin of the Milky Way thick disc

  • Author(s): Yu, S;
  • Bullock, JS;
  • Klein, C;
  • Stern, J;
  • Wetzel, A;
  • Ma, X;
  • Moreno, J;
  • Hafen, Z;
  • Gurvich, AB;
  • Hopkins, PF;
  • Kereš, D;
  • Faucher-Giguère, CA;
  • Feldmann, R;
  • Quataert, E
  • et al.
Abstract

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 ~6.5 Gyr ago; prior to that time the Milky Way likely lacked a recognizable thin disc.

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