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

We report evidence from APOGEE for the presence of a new metal-poor stellar structure located within similar to 4 kpc of the Galactic Centre. Characterized 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 similar to 5 x 10(8) M-circle dot, or approximately twice the mass of the recently discovered Gaia-Enceladus/Sausage system. We find that the accreted: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.

Automated summary

Evidence from APOGEE suggests the presence of a new metal-poor stellar structure near the Galactic Center, possibly associated with an accretion event in the early Milky Way. This structure, termed the Inner Galaxy Structure (IGS), displays chemical composition similarities to low-mass Milky Way satellites and is dynamically detached from more metal-rich populations in the inner Galaxy. Comparisons with cosmological simulations indicate a progenitor system for the IGS with a stellar mass approximately double that of the Gaia-Enceladus/Sausage system. The ratio of accreted to in situ stars within the metal-poor bulge sample is estimated to be between 1:3 and 1:2, in line with predictions from various cosmological numerical simulations.