Timing the Early Assembly of the Milky Way with the H3 Survey

Ana Bonaca, Charlie Conroy, Phillip A. Cargile, Rohan P. Naidu, Benjamin D. Johnson, Dennis Zaritsky, Yuan Sen Ting, Nelson Caldwell, Jiwon Jesse Han, Pieter Van Dokkum

Research output: Contribution to journalReview articlepeer-review

70 Scopus citations


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,, , and orbits from the H3 Survey and Gaia to time the major events in the early Galaxy. Located beyond the Galactic plane,, 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 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.

Original languageEnglish (US)
Article numberL18
JournalAstrophysical Journal Letters
Issue number1
StatePublished - Jul 1 2020

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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