Timing the Early Assembly of the Milky Way with the H3 Survey
Abstract
The archeological record of stars in the Milky Way opens a uniquely detailed window into the early formation and assembly of galaxies. Here we use 11,000 main-sequence turn-off stars with well-measured ages, $[\mathrm{Fe}/{\rm{H}}]$ , $[\alpha /\mathrm{Fe}]$ , and orbits from the H3 Survey and Gaia to time the major events in the early Galaxy. Located beyond the Galactic plane, $1\lesssim | Z| /\mathrm{kpc}\lesssim 4$ , this sample contains three chemically distinct groups: a low-metallicity population, and low-α and high-α groups at higher metallicity. The age and orbit distributions of these populations show that (1) the high-α group, which includes both disk stars and the in situ halo, has a star formation history independent of eccentricity that abruptly truncated 8.3 ± 0.1 Gyr ago (z ≃ 1); (2) the low-metallicity population, which we identify as the accreted stellar halo, is on eccentric orbits and its star formation truncated $10.2{.}_{-0.1}^{+0.2}$ Gyr ago (z ≃ 2); (3) the low-α population is primarily on low-eccentricity orbits and the bulk of its stars formed less than 8 Gyr ago. These results suggest a scenario in which the Milky Way accreted a satellite galaxy at z ≈ 2 that merged with the early disk by z ≈ 1. This merger truncated star formation in the early high-α disk and perturbed a fraction of that disk onto halo-like orbits. The merger enabled the formation of a chemically distinct, low-α disk at z ≲ 1. The lack of any stars on halo-like orbits at younger ages indicates that this event was the last significant disturbance to the Milky Way disk.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- July 2020
- DOI:
- 10.3847/2041-8213/ab9caa
- arXiv:
- arXiv:2004.11384
- Bibcode:
- 2020ApJ...897L..18B
- Keywords:
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- Milky Way stellar halo;
- Milky Way dynamics;
- Galaxy evolution;
- Astrometry;
- Galaxy chemical evolution;
- Galaxy formation;
- Spectroscopy;
- Stellar ages;
- Milky Way Galaxy;
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- 1051;
- 594;
- 80;
- 580;
- 595;
- 1558;
- 1581;
- 1054;
- Astrophysics - Astrophysics of Galaxies
- E-Print:
- submitted to AAS Journals