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 $\mathrm{[Fe/H]}\sim-3.5$. Our final sample contains 5,095,676 giants, including 1,717,610 stars with $\mathrm{[Fe/H]}<-1$. Across $-4\le \mathrm{[Fe/H]}<-1$, the density distribution reveals a centrally concentrated, flattened spheroidal component extending to $r_{\rm gc}\sim15$ kpc, together with a prominent overdensity near $X\sim-5$ 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 $\mathrm{[Fe/H]}\sim-2.7$ that becomes most prominent at 1$<r_{\rm gc}<$3 kpc. Stars with $-3.5\lesssim\mathrm{[Fe/H]}\lesssim-1.4$ exhibit weak net rotation and low rotational support within $r_{\rm gc}<15$ 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.