A 3D Chemodynamical Census of Inner-Galaxy Metal-poor Giants to [Fe/H] ∼ −3.5

Abstract The earliest assembly of the Milky Way remains poorly understood, yet the spatial, chemical, and kinematic properties of its most metal-poor stars provide a unique fossil record of its proto-Galaxy phase. Understanding how this ancient component formed is essential for linking near-field Galactic archaeology to high-redshift galaxy evolution. We construct the currently largest 3D map of inner-Galaxy metal-poor giants by combining several narrow/medium-band photometric surveys, reaching metallicities down to [Fe/H] ∼ −3.5. Our final sample contains 5,095,676 giants, including 1,717,610 stars with [Fe/H] < −1. Across −4 ≤ [Fe/H] < −1, the density distribution reveals a centrally concentrated, flattened spheroidal component extending to r gc ∼ 15 kpc, together with a prominent overdensity near X ∼ −5 kpc that is dominated by metal-poor stars on disklike orbits, with a kinematically hot background also present. The selection-function-corrected metallicity distribution function shows a distinct, very metal-poor component around [Fe/H] ∼ −2.7 that becomes most prominent at 1 < r gc < 3 kpc. Stars with −3.5 ≲ [Fe/H] ≲ −1.4 exhibit weak net rotation and low rotational support within r gc < 15 kpc. Finally, we briefly note that the centrally enhanced very metal-poor component could be qualitatively consistent with one or more early dissipative build-up episodes (e.g., high- z compaction/“blue-nugget” phases) as one possible interpretation.