Relics of the First Galactic Core: How the Milky Way’s Oldest Stars Reveal Its Fiery Beginnings
Shenglan Sun and collaborators explore how the Milky Way’s earliest stars, those with extremely low metal content, can uncover the Galaxy’s first billion years. Since metals (elements heavier than hydrogen and helium) are created in stars, a star’s metallicity indicates how many generations of stars formed before it. The team assembled the largest-ever 3D map of these “metal-poor giants” using data from several wide-field photometric surveys, revealing stars with metallicities as low as [Fe/H] = –3.5, about one-thousandth of the Sun’s metal content.
Building the Sample and Simulating a Proto-Milky Way
Sun and colleagues combined stars from multiple surveys (SMSS, SAGES, J-PLUS, and S-PLUS) and cross-matched them with Gaia data to estimate distances, metallicities, and motions. After careful filtering, their final dataset contained over five million giant stars, including more than 1.7 million with [Fe/H] < –1.0. To interpret these observations, they turned to Auriga 18 (Au18), a high-resolution cosmological simulation of a Milky Way–like galaxy that reproduces realistic disk and bulge structures.
A New Scenario: Proto-Galaxy Formation through Gas “Compaction”
The authors propose that the Milky Way’s earliest assembly was shaped by high-redshift gas compaction events, periods when gas rapidly collapsed into the galactic center, igniting intense bursts of star formation. These “blue nugget” (BN) phases occurred about 11–13 billion years ago (redshift z ≳ 3). In the Au18 simulation, three such compaction events created dense, star-forming cores that later expanded into the Galaxy’s bulge. This mechanism naturally explains why today’s oldest, most metal-poor stars are concentrated in the inner Galaxy: they are relics of these early, compact, and turbulent starbursts.
Observational Evidence: Spatial, Chemical, and Kinematic Clues
The researchers found three key lines of evidence linking their observations to the compaction scenario.
Spatial distribution: Metal-poor stars with [Fe/H] < –1.0 cluster tightly within 15,000 light-years of the Galactic center, forming a flattened, spheroidal structure similar to the simulated Au18 stars.
Metallicity distribution: A distinct population of very metal-poor (VMP) stars peaks around [Fe/H] ≈ –2.7, suggesting they were born during the first compaction episode.
Kinematics: These stars show little rotation and large random motions, forming a “kinematically hot” structure that contrasts with the younger, rotating disk. This matches the simulated transition from the chaotic proto-Galaxy to the emerging thick disk.
Were These Stars Born Here or Brought Here?
A major question is whether these ancient stars formed in situ, inside the Milky Way’s main progenitor, or were later accreted from smaller galaxies. Using orbital properties, Sun et al. estimate that roughly half of their sample likely formed in situ. However, distinguishing the two origins remains difficult because the early Galaxy was highly chaotic, mixing stars from multiple sources.
Implications for the Milky Way’s First Billion Years
The study connects the Milky Way’s earliest phase to the same physical processes seen in young, compact galaxies at high redshift. According to Sun and collaborators, the proto-Galaxy likely experienced several compaction-driven starburst episodes that established its dense central structure. These findings bridge “near-field” Galactic archaeology, using stars in our backyard, with the “far-field” view of galaxy formation in the early universe. By tracing metal-poor stars, the authors provide a unified picture of how the Milky Way’s heart was forged in the cosmic dawn.
Outlook
Future high-resolution spectroscopy and next-generation simulations will test this compaction-driven model further, helping astronomers pin down when and how the first stars and structures of our Galaxy formed. In doing so, they will refine our understanding of how galaxies like the Milky Way emerged from the turbulent, starbursting universe over 12 billion years ago.
Source: Sun
