The Poor Old Heart of the Milky Way

Rix, Hans-Walter; Chandra, Vedant; Andrae, René; Price-Whelan, Adrian M.; Weinberg, David H.; Conroy, Charlie; Fouesneau, Morgan; Hogg, David W.; De Angeli, Francesca; Naidu, Rohan P.; Xiang, Maosheng; Ruz-Mieres, Daniela

doi:10.3847/1538-4357/ac9e01

The Poor Old Heart of the Milky Way

Journal Article · Mon Dec 12 00:00:00 EST 2022 · The Astrophysical Journal

DOI:https://doi.org/10.3847/1538-4357/ac9e01· OSTI ID:2425357

^[1]; ; ; ; Weinberg, David H.; ; Fouesneau, Morgan; ; ; Naidu, Rohan P.; ;

Max Planck Inst. fuer Astronomie, Heidelberg (Germany); SDSS Collaboration. et al.

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° of the Galactic center (GC) with robust [M/H] estimates, δ[M/H] ≲ 0.1. For ~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 ~18,000 stars with -2.7 < [M/H] < -1.5, representing a stellar mass of ≳5 × 10⁷ M_⊙. The spatial distribution of these [M/H] < -1.5 stars has a Gaussian extent of only σ_{R$$_{GC}$$} 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.

View Accepted Manuscript (DOE)

Research Organization:: US Department of Energy (USDOE), Washington, DC (United States). Office of Science, Sloan Digital Sky Survey (SDSS)

Sponsoring Organization:: USDOE Office of Science (SC)

OSTI ID:: 2425357

Journal Information:: The Astrophysical Journal, Journal Name: The Astrophysical Journal Journal Issue: 1 Vol. 941; ISSN 0004-637X

Publisher:: IOP PublishingCopyright Statement

Country of Publication:: United States

Language:: English

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The Poor Old Heart of the Milky Way Andrae, Rene; Rix, Hans-Walter; Chandra, Vedant Zenodo https://doi.org/10.5281/zenodo.7035809	dataset	January 2022

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