Milky way galaxy formation and age
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Milky Way Galaxy Formation: Early Assembly and Major Mergers
The Milky Way formed through a series of complex events, including the aggregation of gas and stars from smaller galaxies and several significant merger events. Evidence shows that the galaxy began forming more than 12 billion years ago, with material following two main evolutionary paths: one leading to the slowly rotating halo and central bulge, and the other to the rapidly rotating disk. The existence of distinct thick and thin disks suggests that ongoing mergers of satellite galaxies played a key role in shaping the Milky Way’s structure in its early history .
Recent studies using precise stellar ages and chemical abundances have revealed that the Milky Way’s formation can be divided into distinct phases. The oldest stars, which now reside mainly in the thick disk, began forming about 13 billion years ago, only 0.8 billion years after the Big Bang. This early phase was followed by the assembly of the inner halo, with most of these stars forming around 11 billion years ago, coinciding with the major Gaia-Sausage-Enceladus (GSE) merger event Xiang2022Montalbán2020Thulasidharan2024. This merger brought in a significant population of stars and contributed to the thickening of the disk, as indicated by an abrupt increase in disk thickness among stars of that age .
Age and Structural Evolution of the Milky Way
The Milky Way’s stellar populations show a clear age-metallicity split, with stars older than about 8 billion years reflecting the early, more chaotic phase of galaxy formation, and younger stars marking a later, more quiescent period of disk growth . The thick disk formed around 11–13 billion years ago, while the thin disk began to dominate after a transition period of about 2.6 billion years . The galaxy’s assembly was rapid for its mass, with half of its present-day halo mass in place by redshift z = 1.5 (about 9.5 billion years ago) and half of its stellar mass by z = 1.2 .
Globular clusters (GCs) provide further insight into the Milky Way’s assembly. About 40% of the Milky Way’s GCs formed outside the galaxy and were later accreted, mainly during early mergers with satellite galaxies. The most significant of these mergers occurred 8–11 billion years ago, contributing to the rapid early growth of the Milky Way Kruijssen2018Forbes2020.
Major Mergers and Their Impact
The Gaia-Sausage-Enceladus merger, which occurred around 10–11 billion years ago, was a decisive event in the Milky Way’s history. This merger not only brought in a large number of stars but also contributed to the formation of the thick disk and the kinematically hot old disk. After this event, the Milky Way experienced a period of relative calm, with no major mergers (mass ratios greater than 1:4) since about 12 billion years ago (z ≈ 4) Kruijssen2018Chandra2023Bonaca2020+1 MORE. The last significant disturbance to the disk was this merger, after which the thin disk formed from the remaining and newly accreted gas Chandra2023Bonaca2020.
Simulations and observational data suggest that the Milky Way’s disk grew inside-out, with the outer, metal-poor, low-alpha disk forming rapidly after the last major merger. Radial migration and secular heating have since shaped the disk’s structure, leading to the observed trends in metallicity and velocity dispersion with stellar age .
Conclusion
The Milky Way’s formation and age are marked by an early, rapid assembly phase, punctuated by major mergers—most notably the Gaia-Sausage-Enceladus event about 10–11 billion years ago. The thick disk and inner halo formed during this period, followed by a long, quiescent phase of disk growth and chemical enrichment. The galaxy’s current structure, including its thick and thin disks, is a direct result of these early formative events and subsequent evolutionary processes Kruijssen2018Chandra2023Montalbán2020+7 MORE.
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