The Pristine survey - X. A large population of low-metallicity stars permeates the Galactic disc

Federico Sestito*, Nicolas F. Martin, Else Starkenburg, Anke Arentsen, Rodrigo A. Ibata, Nicolas Longeard, Collin Kielty, Kristopher Youakim, Kim A. Venn, David S. Aguado, Raymond G. Carlberg, Jonay I. González Hernández, Vanessa Hill, Pascale Jablonka, Georges Kordopatis, Khyati Malhan, Julio F. Navarro, Rubén Sánchez-Janssen, Guillame Thomas, Eline TolstoyThomas G. Wilson, Pedro A. Palicio, Spencer Bialek, Rafael Garcia-Dias, Romain Lucchesi, Pierre North, Yeisson Osorio, Lee R. Patrick, Luis Peralta de Arriba

*Corresponding author for this work

Research output: Contribution to journalArticleAcademicpeer-review

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The orbits of the least chemically enriched stars open a window on the formation of our Galaxy when it was still in its infancy. The common picture is that these low-metallicity stars are distributed as an isotropic, pressure-supported component since these stars were either accreted from the early building blocks of the assembling Milky Way (MW), or were later brought by the accretion of faint dwarf galaxies. Combining the metallicities and radial velocities from the Pristine and LAMOST surveys and Gaia DR2 parallaxes and proper motions for an unprecedented large and unbiased sample of 1027 very metal poor stars at [Fe/H] ≤ -2.5 dex, we show that this picture is incomplete. We find that $31{{\ \rm per\ cent}}$ of the stars that currently reside spatially in the disc ( $|Z| \le 3{\rm \, kpc}$ ) do not venture outside of the disc plane throughout their orbit. Moreover, this sample shows strong statistical evidence (at the 5.0σ level) of asymmetry in their kinematics, favouring prograde motion. The discovery of this population implies that a significant fraction of stars with iron abundances [Fe/H] ≤ -2.5 dex merged into, formed within, or formed concurrently with the MW disc and that the history of the disc was quiet enough to allow them to retain their disc-like orbital properties, challenging theoretical and cosmological models.
Original languageEnglish
Pages (from-to)L7-L12
JournalMonthly Notices of the Royal Astronomical Society
Publication statusPublished - 1-Sept-2020


  • Galaxy: kinematics and dynamics
  • Galaxy: evolution
  • Galaxy: abundances
  • Galaxy: disc
  • Galaxy: halo
  • Galaxy: formation
  • Astrophysics - Astrophysics of Galaxies
  • Astrophysics - Solar and Stellar Astrophysics

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