The Poor Old Heart of the Milky Way

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) < 15.5 mag) giant stars within 30 degrees of the Galactic center (GC) with robust [M/H] estimates, delta[M/H] ? 0.1. For similar to 1.25 million stars we calculate orbits from Gaia Radial Velocity Spectrometer velocities and astrometry. This sample reveals an extensive, ancient, and metal-poor population that includes similar to 18,000 stars with -2.7 < [M/H] < -1.5, representing a stellar mass of ?5 x 10(7) M-?. The spatial distribution of these [M/H] < -1.5 stars has a Gaussian extent of only 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 [alpha/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] similar to -1 are rotation dominated. These central, metal-poor stars most likely predate the oldest disk population (tau(age) asymptotic to 12.5 Gyr), which implies that they formed at z ? 5, forging the proto-Milky Way.

Automated summary

Using data from the Gaia Data Release 3, we have constructed a sample of 2 million bright giant stars, revealing an ancient and metal-poor population in the Milky Way. These stars have a spatial distribution concentrated around the Galactic center, with most orbits confined to the inner Galaxy. They likely predate the oldest disk population, implying they formed in the early stages of the Milky Way.