The Milky Way's thick disc dates back to the first billion years of the universe

Some of the Milky Way is far older than we thought.
Derya Ozdemir
The Milky Wayalex-mit/iStock

A remarkable discovery has just added another layer to our quest to piece together our galaxy's history in unprecedented detail.

A team of scientists from the Max-Planck Institute for Astronomy in Germany has discovered that the "thick disc" of the Milky Way formed 13 billion years ago, some two billion years earlier than previously thought, according to a press release.

In case you missed it, this means it began to form only 0.8 billion years after the Big Bang.

Creating a timeline of the Milky Way's formation

The Milky Way galaxy is made up of two parts: the halo and the disc. The disc is divided into two parts: the thin disc, which includes the majority of the stars, and the thick disc, which is more than twice as tall as the thin disc but contains just a small percentage of the Milky Way's stars. The team was able to construct a history of the Milky Way's formation by detecting subgiant stars in these various areas. 

To make this discovery, astronomers Maosheng Xiang and Hans-Walter Rix from the Max-Planck Institute for Astronomy combined data from the European Space Agency's (ESA) Gaia mission, which is an ambitious endeavor that aims to create a three-dimensional map of our Galaxy, with measurements of the stars’ chemical compositions compiled by China’s Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST). 

The researchers analyzed a survey of roughly 250,000 stars ranging in age from 13.8 billion to 15 billion years to trace the expansion of our galaxy by cross-referencing their life cycles with the movements of the Milky Way.

According to the study published in the journal Nature, the stellar ages reflect two periods in our galaxy's history: The thick disc began generating stars in the first phase, which began only 0.8 billion years after the Big Bang, and the thin disc of stars that holds the Sun originated during the galaxy's subsequent second phase of development.

On a quest to discover the Milky Way

The analysis also shows that after the star-forming boom caused by our Milky Way galaxy's merger with a dwarf galaxy known as Gaia-Sausage-Enceladus, the thick disc continued to generate stars until the gas was depleted roughly 6 billion years after the Big Bang.

"Since the discovery of the ancient merger with Gaia-Sausage-Enceladus, in 2018, astronomers have suspected that the Milky Way was already there before the halo formed, but we didn't have a clear picture of what that Milky Way looked like," said Maosheng, who is currently a postdoc at the Max-Planck Institute for Astronomy. "Our results provide exquisite details about that part of the Milky Way, such as its birthday, its star-formation rate and metal enrichment history. Putting together these discoveries using Gaia data is revolutionizing our picture of when and how our galaxy was formed." 

This is an intriguing find that foreshadows what's to come in the future. The innovative James Webb Space Telescope, for example, has been designed to examine the universe's oldest Milky Way-like galaxies, and we may expect to witness many more new observations in the near future once it is fully operational.

Gaia, on the other hand, will also deliver its full third data release on June 13 of this year, which will include spectra and derived information like ages and metallicity. Thanks to this, astronomers will be able to enrich the glorious story of our galaxy with even more exciting details.

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