The Pristine Inner Galaxy Survey (PIGS) – V. A chemo-dynamical investigation of the early assembly of the Milky Way with the most metal-poor stars in the bulge

• Published in: Monthly notices of the Royal Astronomical Society Vol. 518; no. 3; pp. 4557 - 4578
• Main Authors: Sestito, Federico, Venn, Kim A, Arentsen, Anke, Aguado, David, Kielty, Collin L, Lardo, Carmela, Martin, Nicolas F, Navarro, Julio F, Starkenburg, Else, Waller, Fletcher, Carlberg, Raymond G, François, Patrick, González Hernández, Jonay I, Kordopatis, Georges, Vitali, Sara, Yuan, Zhen
• Format: Journal Article
• Language: English
• Published: Oxford University Press (OUP): Policy P - Oxford Open Option A 01.01.2023
• Subjects: Sciences of the Universe; Galaxy: evolution; Galaxy: bulge; stars: kinematics and dynamics; stars: Population II; Galaxy: formation; Galaxy: abundances
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1093/mnras/stac3332

ABSTRACT The investigation of the metal-poor tail in the Galactic bulge provides unique information on the early Milky Way assembly and evolution. A chemo-dynamical analysis of 17 very metal-poor stars (VMP, [Fe/H]<−2.0) selected from the Pristine Inner Galaxy Survey was carried out based on Gemini/GRACES spectra. The chemistry suggests that the majority of our stars are very similar to metal-poor stars in the Galactic halo. Orbits calculated from Gaia EDR3 imply these stars are brought into the bulge during the earliest Galactic assembly. Most of our stars have large [Na,Ca/Mg] abundances, and thus show little evidence of enrichment by pair-instability supernovae. Two of our stars (P171457 and P184700) have chemical abundances compatible with second-generation globular cluster stars, suggestive of the presence of ancient and now dissolved globular clusters in the inner Galaxy. One of them (P171457) is extremely metal-poor ([Fe/H]<−3.0) and well below the metallicity floor of globular clusters, which supports the growing evidence for the existence of lower-metallicity globular clusters in the early Universe. A third star (P180956, [Fe/H]∼−2) has low [Na,Ca/Mg] and very low [Ba/Fe] for its metallicity, which are consistent with formation in a system polluted by only one or a few low-mass supernovae. Interestingly, its orbit is confined to the Galactic plane, like other very metal-poor stars found in the literature, which have been associated with the earliest building blocks of the Milky Way.