Evidence from APOGEE for the presence of a major building block of the halo buried in the inner Galaxy

TL;DR

This study uses APOGEE data to identify a new metal-poor stellar structure near the Galactic center, likely resulting from an early accretion event, and compares it with cosmological simulations to estimate its progenitor's mass.

Contribution

It provides the first evidence of a distinct metal-poor structure in the inner Galaxy linked to an ancient accretion event, with detailed chemical and dynamical analysis.

Findings

Discovery of a new metal-poor stellar structure within 4 kpc of the Galactic center.

Estimated progenitor galaxy mass of approximately 5×10^8 solar masses.

The accreted to in situ ratio in the metal-poor bulge supports cosmological simulation predictions.

Abstract

We report evidence from APOGEE for the presence of a new metal-poor stellar structure located within $\sim$4~kpc of the Galactic centre. Characterised by a chemical composition resembling those of low mass satellites of the Milky Way, this new inner Galaxy structure (IGS) seems to be chemically and dynamically detached from more metal-rich populations in the inner Galaxy. We conjecture that this structure is associated with an accretion event that likely occurred in the early life of the Milky Way. Comparing the mean elemental abundances of this structure with predictions from cosmological numerical simulations, we estimate that the progenitor system had a stellar mass of $\sim5\times10^8M_\odot$, or approximately twice the mass of the recently discovered Gaia-Enceladus/Sausage system. We find that the accreted:{\it in situ} ratio within our metal-poor ([Fe/H]$<$--0.8) bulge sample is somewhere between 1:3 and 1:2, confirming predictions of cosmological numerical simulations by various groups.