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Milky Way’s eating habits may reveal dark matter’s violent history

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A new map of twelve streams of stars circling within our galactic halo sheds more light on the features of dark matter enveloping our Milky Way galaxy.

Thanks to a new map of twelve streams of stars circling within our galactic halo, astronomers are one step closer to understanding the features of dark matter engulfing our Milky Way galaxy.

Studying these star streams is critical for astronomers. They not only reveal the dark matter that keeps the stars in their orbits, but they also explain the Milky Way’s development history, demonstrating that the Milky Way has grown steadily over billions of years by shredding and consuming smaller stellar systems.

“We are seeing these streams being disrupted by the Milky Way’s gravitational pull, and eventually becoming part of the Milky Way. This study gives us a snapshot of the Milky Way’s feeding habits, such as what kinds of smaller stellar systems it ‘eats’. As our galaxy is getting older, it is getting fatter,” says professor Ting Li, the lead author of the paper.

Prof. Li and her international team of partners launched the Southern Stellar Stream Spectroscopic Survey (S5) to study the properties of stellar streams, which are the shredded remains of nearby tiny galaxies and star clusters that are being ripped apart by our own Milky Way.

The Anglo-Australian Telescope (AAT), a 4-meter optical telescope in Australia, was used by Li and her colleagues to measure the velocity of stars, making them the first group of scientists to analyze such a diverse array of stellar streams. To determine how quickly particular stars are moving, Li and her team employed the Doppler shift of light, which is the same phenomenon used by radar guns to catch speeding drivers.

Unlike prior studies that rely on one stream at a time, “S5 is dedicated to measuring as many streams as possible,” says co-author Professor Daniel Zucker of Macquarie University, “which we can do very efficiently with the unique capabilities of the AAT.”

The features of star streams suggest the existence of the Milky Way’s invisible dark matter.

“Think of a Christmas tree,” adds co-author Professor Geraint F. Lewis of the University of Sydney.

“On a dark night, we see the Christmas lights, but not the tree they are wrapped around. But the shape of the lights reveals the shape of the tree,” he adds. “It is the same with stellar streams – their orbits reveal the dark matter.”

Astronomers may use these data to work out the chemical compositions of the stars, which can tell us where they were born, in addition to determining their speeds.

“Stellar streams can come either from disrupting galaxies or star clusters,” according to Professor Alex Ji at the University of Chicago, a co-author on the study. “These two types of streams provide different insights into the nature of dark matter.”

These new observations, according to Prof. Li, are critical for identifying how our Milky Way emerged from the featureless universe following the Big Bang.

“For me, this is one of the most intriguing questions, a question about our ultimate origins,” Li says. “It is the reason why we founded S5 and built an international collaboration to address this.”

Li’s group intends to collect more data on star streams in the Milky Way.

Meanwhile, she is satisfied with the outcomes as a starting point.

“Over the next decade, there will be a lot of dedicated studies looking at stellar streams,” Li adds. “We are trail-blazers and pathfinders on this journey. It is going to be very exciting!”

Source: arXiv:2110.06950

Image Credit: Geraint F. Lewis, S5 Collaboration

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