The Pristine survey - X. A large population of low-metallicity stars permeates the Galactic disc
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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 per cent31 per cent of the stars that currently reside spatially in the disc (|Z|≤3kpc|Z|≤3kpc) 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.
Sestito , F , Martin , N F , Starkenburg , E , Arentsen , A , Ibata , R A , Longeard , N , Kielty , C , Youakim , K , Venn , K A , Aguado , D S , Carlberg , R G , Gonzalez Hernandez , J , Hill , V , Jablonka , P , Kordopatis , G , Malhan , K , Navarro , J F , Sanchez-Janssen , R , Thomas , G , Tolstoy , E , Wilson , T G , Palicio , P A , Bialek , S , Garcia-Dias , R , Lucchesi , R , North , P , Osorio , Y , Patrick , L R & de Arriba , L P 2020 , ' The Pristine survey - X. A large population of low-metallicity stars permeates the Galactic disc ' , Monthly Notices of the Royal Astronomical Society: Letters , vol. 497 , no. 1 , pp. L7-L12 . https://doi.org/10.1093/mnrasl/slaa022
Monthly Notices of the Royal Astronomical Society: Letters
Copyright © 2020 the Author(s). Published by Oxford University Press on behalf of the Royal Astronomical Society. This work has been made available online in accordance with publisher policies or with permission. Permission for further reuse of this content should be sought from the publisher or the rights holder. This is the final published version of the work, which was originally published at https://doi.org/10.1093/mnrasl/slaa022.
DescriptionFunding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement. FS thanks the Initiative dExcellence IdEx from the University of Strasbourg and the Programme Doctoral International PDI for funding his Ph.D. This work has been published under the framework of the IdEx Unistra and benefits from a funding from the state managed by the French National Research Agency as part of the investments for the future program. FS, NFM, and RAI gratefully acknowledge support from the French National Research Agency (ANR) funded project ‘Pristine’ (ANR-18-CE31-0017) along with funding from CNRS/INSU through the Programme National Galaxies et Cosmologie and through the CNRS grant PICS07708. ES, AA, and KY gratefully acknowledge funding by the Emmy Noether program from the Deutsche Forschungsgemeinschaft (DFG). The authors acknowledge the support and funding of the International Space Science Institute (ISSI) for the international team ‘Pristine’. FS acknowledges the support and funding of the Erasmus + programme of the European Union.
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