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

10 pages, 5 figures, accepted for publication in ApJL