Stars made in outflows may populate the stellar halo of the Milky Way

Sijie Yu*, James S. Bullock, Andrew Wetzel, Robyn E. Sanderson, Andrew S. Graus, Michael Boylan-Kolchin, Anna M. Nierenberg, Michael Y. Grudić, Philip F. Hopkins, Dušan Kereš, Claude André Faucher-Giguère

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review


We study stellar-halo formation using six Milky Way-mass galaxies in FIRE-2 cosmological zoom simulations. We find that 5 - 40% of the outer (50 - 300 kpc) stellar halo in each system consists of in-situ stars that were born in outflows from the main galaxy. Outflow stars originate from gas accelerated by super-bubble winds, which can be compressed, cool, and form co-moving stars. The majority of these stars remain bound to the halo and fall back with orbital properties similar to the rest of the stellar halo at z = 0. In the outer halo, outflow stars are more spatially homogeneous, metal rich, and alpha-element-enhanced than the accreted stellar halo. At the solar location, up to ∼ 10% of our kinematically-identified halo stars were born in outflows; the fraction rises to as high as ∼ 40% for the most metal-rich local halo stars ([Fe/H]>-0.5). We conclude that the Milky Way stellar halo could contain local counterparts to stars that are observed to form in molecular outflows in distant galaxies. Searches for such a population may provide a new, near-field approach to constraining feedback and outflow physics. A stellar halo contribution from outflows is a phase-reversal of the classic halo formation scenario of Eggen, Lynden-Bell & Sandange, who suggested that halo stars formed in rapidly infalling gas clouds. Stellar outflows may be observable in direct imaging of external galaxies and could provide a source for metal-rich, extreme velocity stars in the Milky Way.

Original languageEnglish (US)
JournalUnknown Journal
StatePublished - Dec 6 2019


  • Galaxies: evolution
  • Galaxies: formation
  • Galaxies: structure
  • Galaxies:halo
  • Methods: numerical

ASJC Scopus subject areas

  • General

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