Ancient Stars Found in Milky Way's Thin Disk
The Milky Way consists of a halo, a central bulge and bar, a thick disk, and a thin disk. The majority of stars, including our 4.6 billion-year-old Sun, reside in the thin disk and follow a consistent rotation around the galactic center. Previously, it was believed that the thin disk started forming around 8 to 10 billion years ago.
Galactic archaeology aims to understand the Milky Way's formation by creating detailed maps that illustrate the ages, chemical compositions, and movements of stars. These maps, known as chrono-chemo-kinematical maps, require large datasets of stars with precisely known ages.
To tackle this challenge, astronomers often study very metal-poor stars, which are old and provide insights into the early Milky Way. These stars formed when the universe was mostly hydrogen and helium, before heavier elements were created and spread by successive generations of stars.
Using data from the European Space Agency (ESA) Gaia Mission, an international team led by astronomers from the Leibniz Institute for Astrophysics Potsdam (AIP) examined stars in the solar neighborhood, approximately 3200 light-years around the Sun. They found many very old stars in thin disk orbits; most of these stars are over 10 billion years old, with some even surpassing 13 billion years. These ancient stars exhibit a range of metal compositions, from very metal-poor to double the metal content of our younger Sun, indicating rapid metal enrichment during the early Milky Way's evolution.
"These ancient stars in the disc suggest that the formation of the Milky Way's thin disc began much earlier than previously believed, by about 4-5 billion years," explained Samir Nepal from AIP and the study's first author. "This study also highlights that our galaxy had intense star formation at early epochs, leading to very fast metal enrichment in the inner regions and the formation of the disc. This discovery aligns the Milky Way's disc formation timeline with those of high-redshift galaxies observed by the James Webb Space Telescope (JWST) and Atacama Large Millimeter Array (ALMA) Radio Telescope. It indicates that cold discs can form and stabilize very early in the universe's history, providing new insights into the evolution of galaxies."
"Our study suggests that the thin disc of the Milky Way may have formed much earlier than we had thought, and that its formation is strongly related to the early chemical enrichment of the innermost regions of our Galaxy," added Cristina Chiappini. "The combination of data from different sources and the application of advanced machine learning techniques have enabled us to increase the number of stars with high-quality stellar parameters, a key step to lead our team to these new insights."
The third data release of the Gaia mission made these findings possible. The team analyzed the stellar parameters of more than 800,000 stars using a novel machine learning method that combines various data types to provide enhanced stellar parameters with high precision. These precise measurements include gravity, temperature, metal content, distances, kinematics, and the age of the stars. In the future, a similar machine learning technique will be used to analyze millions of spectra collected by the 4MIDABLE-LR survey with the 4-metre Multi-Object Spectroscopic Telescope (4MOST), starting operations in 2025.
Research Report:Discovery of the local counterpart of disc galaxies at z > 4: The oldest thin disc of the Milky Way using Gaia-RVS