Beijing, China — An international team of astronomers led by researchers from the Department of Astronomy (DoA), School of Physics, Peking University (PKU), the Kavli Institute for Astronomy and Astrophysics (KIAA) at PKU and the University of Chinese Academy of Sciences (UCAS) has constructed the largest three-dimensional chemodynamical map to date of metal-poor giant stars in the inner Milky Way. By resolving the positions, chemical compositions and motions of ancient stars in our own Galaxy, this work provides a new way to connect near-field cosmology with theories of galaxy formation in the early Universe, where young galaxies underwent rapid growth and structural transformation.
The study was recently published in The Astrophysical Journal Letters, with Shenglan Sun, a PhD student in the DoA at PKU, as the first author. The corresponding authors are Yang Huang, an associate professor at UCAS, Fangzhou Jiang, an assistant professor at KIAA and Huawei Zhang, a professor in the DoA at PKU.
A Large Fossil Census of the Inner Galaxy Reveals a Very Metal-Poor Component Near the Galactic Center
In Galactic astronomy, [Fe/H] measures a star’s iron abundance relative to the Sun on a logarithmic scale, so [Fe/H] = −1 corresponds to one-tenth of the solar iron abundance. Stars with [Fe/H] < −1 are generally called metal-poor stars. Because such stars are among the oldest surviving stellar populations, their metallicities, positions and motions provide fossil records of the Milky Way’s earliest assembly history. In this study, the team assembled a catalog of more than five million giant stars, including more than 1.7 million metal-poor giants, enabling a statistical census of these ancient tracers across the inner Galaxy.
The new census reveals that stars spanning the metallicity range −4 ≲ [Fe/H] < −1 form a centrally concentrated, flattened spheroidal structure extending to Galactocentric radii of approximately 15 kpc. This finding provides compelling evidence that the inner Galaxy preserves a relic population from the Milky Way’s ancient proto-Galactic phase.
One of the clearest chemical signatures of this ancient inner-Galaxy population is a distinct very metal-poor peak in the metallicity distribution. In the selection-function-corrected analysis relative to Gaia DR3, this component is centered at [Fe/H] ≈ −2.7. It is present throughout the inner 15 kpc from the Galactic center but becomes most prominent within the central 1–3 kpc, where it accounts for up to approximately 6% of the stellar population.
Figure 1: Edge-on distribution of metal-poor giant stars in the inner Milky Way, revealing a centrally concentrated, flattened spheroidal structure across a wide range of stellar metallicities.
Figure 2: Metallicity distribution function of inner-Galaxy stars. The leftmost Gaussian peak marks the very metal-poor peak near [Fe/H] ∼ −2.7, which is strongest 1-3 kpc from the Galactic center.
The team then investigated the kinematics of these stars and uncovered a clear dependence of stellar motions on metallicity. Within the inner 15 kpc, stars with metallicities of −3.5 ≲ [Fe/H] ≲ −1.4 exhibit only weak rotational support, meaning that their ordered rotation is small compared with their velocity dispersion. In other words, these stars form a dynamically hot population: their motions are dominated by random, disordered orbits rather than coherent rotation, consistent with assembly during the Galaxy’s earliest growth phases. A pronounced transition toward a rotationally supported disk emerges at [Fe/H] ≈ −1.4, marking the onset of efficient disk formation.
A possible link to high-redshift “blue nugget” phases
Motivated by the sharp transition to significant rotational support at a characteristic metallicity (which corresponds to a galaxy mass scale), the team considers a possible picture in which the young Milky Way once experienced a rapid central build-up phase. In this scenario, large amounts of gas were driven into the center of the Galaxy’s progenitor, possibly by gas-rich mergers, cold gas streams, or instabilities in the young disk. As the gas piled up, it formed stars rapidly and produced a compact, dense stellar core. Such a compact star-forming stage is known in studies of high-redshift galaxy formation as a “blue nugget” phase. Modern simulations further suggest that stable galactic disks generally form only after this phase, with the compact central “nugget” providing the gravitational anchor for subsequent disk growth.
Figure 3. Schematic illustration of the high-redshift “blue nugget” phase of galaxy evolution (adapted from Lapiner et al. 2023). In modern cosmological simulations, stable, extended galactic disks such as that of the Milky Way typically form only after the compaction-driven nugget phase.
“The most striking result is the discovery of a distinct very metal-poor component centered at [Fe/H] ≈ −2.7 within the central 1–3 kpc of the Galaxy,” said Shenglan Sun, a graduate student at Peking University and first author of the study. “This population provides a concrete observational target for investigating how the earliest Milky Way was assembled.”
“This work demonstrates the power of modern narrow- and medium-band photometric surveys for Galactic archaeology,” said Prof. Yang Huang, one of the corresponding authors of the study from UCAS. “With millions of metal-poor giants, we can now trace the chemical structure of the inner Galaxy down to metallicities that were previously difficult to explore statistically.”“The combination of metallicity, distance, and kinematic information allows us to move beyond a purely spatial census,” said Prof. Huawei Zhang, one of the corresponding authors of the study from the Department of Astronomy of PKU. “It shows that the inner Galaxy preserves the chemical and dynamical memory the proto-Galaxy.”
“The blue-nugget scenario offers a physical bridge between near-field Galactic archaeology and high-redshift galaxy formation,” said Prof. Fangzhou Jiang, one of the corresponding authors of the study from KIAA. “Multiple lines of evidence suggest that the Milky Way assembled earlier than a typical galaxy of similar mass. If so, our Galaxy may have reached the characteristic mass scale for the blue-nugget phase during the cosmic dawn, implying that many of the accretion events uncovered by Gaia and Galactic archaeology occurred after the compaction phase of the Milky Way’s progenitor. If this picture is correct, it could sharpen our understanding of the Milky Way’s formation history by placing many of its best-known assembly events within a post-compaction evolutionary stage. We are now searching for clues to this possibility in the metal-poor stellar relics preserved near the Galactic center.”
By linking the extremely metal-poor component in the inner Galaxy to a possible high-redshift compaction event, the KIAA- and UCAS-led team explores a new avenue for testing theories of early galaxy formation using resolved stellar populations in our own cosmic backyard.
Publication
Shenglan Sun, Yang Huang, Fangzhou Jiang, Huawei Zhang, et al.,
A 3D Chemodynamical Census of Inner-Galaxy Metal-poor Giants to [Fe/H] ∼ −3.5
The Astrophysical Journal Letters, 1000, L27 (2026)
https://iopscience.iop.org/article/10.3847/2041-8213/ae4aab
