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Abstract Our Milky Way should host an ancient, metal-poor, and centrally concentrated stellar population, which reflects the star formation and enrichment in the few most massive progenitors that coalesced at high redshift to form the proto-Galaxy. While metal-poor stars are known to reside in the inner few kiloparsecs of our Galaxy, current data do not yet provide a comprehensive picture of such a metal-poor “heart” of the Milky Way. We use information from Gaia Data Release 3, especially the XP spectra, to construct a sample of 2 million bright ( G BP σ R GC ∼ 2.7 kpc around the GC, with most orbits confined to the inner Galaxy. At high orbital eccentricities, there is clear evidence for accreted halo stars in their pericentral orbit phase. Most stars show α /Fe enhancement and Al/Fe–Mn/Fe abundances expected for an origin in the more massive portions of the proto-Galaxy. Stars with M/H < −2 show no net rotation, whereas those with M/H ∼ −1 are rotation dominated. These central, metal-poor stars most likely predate the oldest disk population ( τ age ≈ 12.5 Gyr), which implies that they formed at z ≳ 5, forging the proto-Milky Way.
Rix et al. (Thu,) studied this question.