Milky Way's thin disk is billions of years older than we thought

An international team of astronomers has uncovered the veil shrouding the formation and evolution of our Milky Way galaxy. The study shows that our galaxy’s thin disk started forming much earlier than what was previously thought.

The research was led by Samir Nepal, a researcher from the Leibniz Institute for Astrophysics Potsdam (AIP).

Understanding Milky Way formation

To grasp the importance of this discovery, it is crucial to understand some key aspects of the Milky Way galaxy. The Milky Way is structured into several distinct regions.

At its core lies the central bulge, which is a densely packed area of stars. Surrounding this central bulge is the thick disk, characterized by a higher concentration of stars and gas compared to other regions.

Extending outward from these regions is the thin disk, which contains the majority of the galaxy’s stars, including our Sun.

The thin disk region is significant as it is thought to have commenced its formation around 8 to 10 billion years ago, indicating a lengthy process of star formation and evolution.

Understanding these structural components and their histories provides valuable context for interpreting the implications of the recent discovery.

Galactic map to Milky Way formation

Galactic archaeology involves mapping our galaxy’s past through the ages, compositions, and movements of stars. These maps, known as chrono-chemo-kinematical maps, help us to piece together the history of our galaxy.

But, how do scientists spot an ancient star? Stars are like time capsules. An easy way to step back into the early days of our galaxy is to study very metal-poor stars.

Old, metal-poor stars are the senior citizens of the universe. They formed when everything was mostly hydrogen and helium, before heavier elements were created by new generations of stars.

Sifting through data from the European Space Agency‘s Gaia Mission, the researchers analyzed the stellar neighborhood, about 3,200 light years around our Sun.

The experts found an unexpected number of ancient stars, older than 10 billion years – some even pushing the 13 billion year mark – all in the thin disk orbits.

Behind the spectral signatures

Even though these ancient stars are old, they are not all metal-poor. Some have twice the metal content of our Sun, implying that a rapid phase of metal enrichment took place early in our Milky Way’s life.

This surprising discovery suggests that the thin disk of the Milky Way may have started forming billions of years earlier than we previously thought.

“These ancient stars in the disk suggest that the formation of the Milky Way’s thin disk began much earlier than previously believed, by about 4–5 billion years,” explained Nepal.

Revamping the galactic timeline

“Our study suggests that the thin disk 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,” noted study co-author 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.”

This discovery puts our galaxy’s disk formation timeline on par with those of high-redshift galaxies being observed by the James Webb Space Telescope (JWST) and Atacama Large Millimeter Array (ALMA) Radio Telescope. It signifies that cold disks could form and stabilize very early in the universe’s history, shedding new light on the evolution of galaxies.

The future of galactic exploration

The study was made possible by the third data release of the Gaia mission. With the help of innovative machine learning methods, the team analyzed stellar parameters of over 800,000 stars. They compiled an extensive dataset which includes gravity, temperature, metal content, distances, kinematics and age of the stars.

The future looks bright, with similar techniques aiming to analyze millions of spectra, collected by the upcoming 4MIDABLE-LR survey, which is set to kick-start operations in 2025.

So, what does this all mean for us? It reframes our understanding of our home galaxy. The research not only pushes the boundary of our knowledge of the Milky Way’s past, but it also propels the future of galactic exploration in unprecedented ways.

As we uncover more of these cosmic treasures, who knows what other fascinating secrets about our galactic home are waiting to be unearthed?

The study is published in arXiv.

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