Resolving the Metallicity Distribution of the Stellar Halo with the H3 Survey

Charlie Conroy, Rohan P. Naidu, Dennis Zaritsky, Ana Bonaca, Phillip Cargile, Benjamin D. Johnson, Nelson Caldwell

Research output: Contribution to journalArticlepeer-review

58 Scopus citations


The Galactic stellar halo is predicted to have formed at least partially from the tidal disruption of accreted dwarf galaxies. This assembly history should be detectable in the orbital and chemical properties of stars. The H3 Survey is obtaining spectra for 200,000 stars and, when combined with Gaia data, is providing detailed orbital and chemical properties of Galactic halo stars. Unlike previous surveys of the halo, the H3 target selection is based solely on magnitude and Gaia parallax; the survey therefore provides a nearly unbiased view of the entire stellar halo at high latitudes. In this paper we present the distribution of stellar metallicities as a function of Galactocentric distance and orbital properties for a sample of 4232 kinematically selected halo giants to 100 kpc. The stellar halo is relatively metal-rich,, and there is no discernible metallicity gradient over the range 6 < R gal < 100 kpc. However, the halo metallicity distribution is highly structured, including distinct metal-rich and metal-poor components at R gal < 10 kpc and R gal > 30 kpc, respectively. The Sagittarius stream dominates the metallicity distribution at 20-40 kpc for stars on prograde orbits. The Gaia-Enceladus merger remnant dominates the metallicity distribution for radial orbits to ≈30 kpc. Metal-poor stars with [Fe/H] < -2 are a small population of the halo at all distances and orbital categories. We associate the "in situ" stellar halo with stars displaying thick disk chemistry on halo-like orbits; such stars are confined to |z| < 10 kpc. The majority of the stellar halo is resolved into discrete features in chemical-orbital space, suggesting that the bulk of the stellar halo formed from the accretion and tidal disruption of dwarf galaxies. The relatively high metallicity of the halo derived in this work is a consequence of the unbiased selection function of halo stars and, in combination with the recent upward revision of the total stellar halo mass, implies a Galactic halo metallicity that is typical for its mass.

Original languageEnglish (US)
Article number237
JournalAstrophysical Journal
Issue number2
StatePublished - Dec 20 2019

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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