Where are the most ancient stars in the Milky Way?

Kareem El-Badry, Joss Bland-Hawthorn, Andrew Wetzel, Eliot Quataert, Daniel R. Weisz, Michael Boylan-Kolchin, Philip F. Hopkins, Claude André Faucher-Giguère, Dušan Kereš, Shea Garrison-Kimmel

Research output: Contribution to journalArticlepeer-review

73 Scopus citations


The oldest stars in the MilkyWay (MW) bear imprints of the Galaxy's early assembly history. We use FIRE cosmological zoom-in simulations of three MW-mass disc galaxies to study the spatial distribution, chemistry, and kinematics of the oldest surviving stars (zform ≳ 5) in MW-like galaxies. We predict the oldest stars to be less centrally concentrated at z = 0 than stars formed at later times as a result of two processes. First, the majority of the oldest stars are not formed in situ but are accreted during hierarchical assembly. These ex situ stars are deposited on dispersion-supported, halo-like orbits but dominate over old stars formed in situ in the solar neighbourhood, and in some simulations, even in the galactic centre. Secondly, old stars formed in situ are driven outwards by bursty star formation and energetic feedback processes that create a time-varying gravitational potential at z ≳ 2, similar to the process that creates dark matter cores and expands stellar orbits in bursty dwarf galaxies. The total fraction of stars that are ancient is more than an order of magnitude higher for sight lines away from the bulge and inner halo than for inward-looking sight lines. Although the task of identifying specific stars as ancient remains challenging, we anticipate that million-star spectral surveys and photometric surveys targeting metal-poor stars already include hundreds of stars formed before z = 5.We predict most of these targets to have higher metallicity (-3 < [Fe/H] < -2) than the most extreme metal-poor stars.

Original languageEnglish (US)
Pages (from-to)652-668
Number of pages17
JournalMonthly Notices of the Royal Astronomical Society
Issue number1
StatePublished - Oct 11 2018
Externally publishedYes

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


  • Galaxy: evolution
  • Galaxy: formation
  • Galaxy: stellar content


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