Two ancient stellar streams that formed the early Milky Way galaxy have been discovered

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Astronomers have used the Gaia Space Telescope to spy some of the Milky Way's first building blocks: streams of two ancient stars, Shakti and Shiva, that helped our home galaxy grow and evolve 12 billion years ago.

Named after Hindu deities, the stellar streams appear to be the remnants of two galaxies that merged into an early version of the Milky Way between 12 billion and 13 billion years ago when the first galaxies formed across the universe. The structures are so ancient that they formed before the Milky Way's iconic spiral arms and the most ancient parts of the central disk.

A study detailing the observations came out Thursday The Astrophysical Journal.

“What's really surprising is that we can detect these ancient structures,” said lead study author Dr. Kyati Malhan, a postdoctoral scholar and Humboldt Research Associate at Stockholm University in Sweden. But the unprecedented data we're getting from Gaia made it possible.”

Observing the stellar wonders of Shakti and Shiva could help astronomers unlock the secrets of the Milky Way's early days and the evolution of similarly massive galaxies throughout the universe, researchers say.

The Gaia space telescope, launched in 2013 by the European Space Agency, began observing the universe the following year. Astronomers are using observations of Gaia to discover previously unknown structures in the Milky Way, helping to piece together the galaxy's history. The telescope's dataset provided astronomers with positions, distances and motions Almost 2 billion stars in the galaxy.

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In 2022, study co-author Hans-Walter Riggs and his colleagues used Gaia to peer into the heart of the Milky Way and discovered the oldest stars ever discovered in the galaxy.Stellar Archaeology” Observations. Analysis of data from nearly 6 million stars observed by Gaia and the Sloan Digital Sky Survey revealed two streams that stand out from the rest.

The survey data included details about the stars' chemical composition.

“For metal-poor stars of a certain size, we observed that stars tend to cluster around two specific combinations of energy and angular momentum,” Malhan said.

Shakti and Shiva are near the heart of the Milky Way, and each stream is estimated to encompass about 10 million suns. Here, the ancient stars were all similar in age, orbital path, and composition, which helped astronomers determine that the two streams were threads from an external source that wove together and became part of the Milky Way.

The researchers compared the discovery of Shiva and Shakti to the discovery of the earliest traces of an ancient settlement that eventually grew into a large, modern city.

“The stars there are so old that they don't contain many of the heavy metal elements that have formed over the lifetime of the universe. These heavy metals are forged into stars and scattered into space as they die. The stars at the heart of our galaxy are metal-free, so we called this region the 'poor old heart' of the Milky Way,” he said. said Riggs, director of the Max Planck Institute for Astronomy and Astronomy in Germany.

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“Until now, we've only recognized these early fragments that coalesced to form the ancient heart of the Milky Way. We now see the first fragments that appear older than Shakti and Shiva but are still out there. These represent the first steps in the evolution of our galaxy toward its current size.

The Milky Way began small and grew in size as it merged with other galaxies and received hydrogen to form stars. Every galaxy contains hydrogen gas, which helps in the birth of stars. When galaxies merge and collide, their clouds of hydrogen gas are disrupted, creating a frenzy of star birth.

Over time, the Milky Way's long filaments of gas and dust coalesced to form the modern spiral structure of the galaxy today.

Gaia has already helped astronomers determine when the Milky Way experienced different merger events in the past, and future observations may unlock further insights.

“One of Gaia's goals is to reveal more about our galaxy's infancy, and it is certainly achieving that,” said Timo Brusti, Gaia's project scientist at the European Space Agency, who was not involved in the study.

“We need to pinpoint the subtle and important differences between the stars in the Milky Way to understand how our galaxy formed and evolved. This requires incredibly precise data – and now, thanks to Gaia, we have that data. We're looking at surprising parts of our galaxy like the Shiva and Shakti Streams.” As we discover, we fill in the gaps and paint a complete picture of not just our current home, but our early cosmic history.

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