12 Stellar Streams Within Our Galactic Halo: The Milky Way’s Feeding Habits Shine a Light on Dark Matter

Milky Way Galaxy Surrounded by Dozens of Stellar Streams

Artist’s representation of our Milky Way Galaxy surrounded by dozens of stellar streams. These streams were the companion satellite galaxies or globular clusters that are now being torn apart by our Galaxy’s gravity. Credit: James Josephides and S5 Collaboration

Astronomers are one step closer to revealing the properties of dark matter enveloping our Milky Way galaxy, thanks to a new map of twelve streams of stars orbiting within our galactic halo.

Understanding these star streams is very important for astronomers. As well as revealing the dark matter that holds the stars in their orbits, they also tell us about the formation history of the Milky Way, revealing that the Milky Way has steadily grown over billions of years by shredding and consuming smaller stellar systems.

A movie showing the 3-D location of individual stars in the dozen streams observed by S5. The colors of individual points are according to a star’s 3-D velocity. Credit: Sergey Koposov, S5 Collaboration

“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,” said University of Toronto Professor Ting Li, the lead author of the paper.

Prof. Li and her international team of collaborators initiated a dedicated program – the Southern Stellar Stream Spectroscopic Survey (S5) – to measure the properties of stellar streams: the shredded remains of neighboring small galaxies and star clusters that are being torn apart by our own Milky Way.

Twelve Stellar Streams Observed by S5

Artist’s impression of twelve stellar streams observed by S5, seen from the Galactic South Pole. Credit: Geraint F. Lewis, S5 Collaboration

Li and her team are the first group of scientists to study such a rich collection of stellar streams, measuring the speeds of stars using the Anglo-Australian Telescope (AAT), a 4-meter (13-foot) optical telescope in Australia. Li and her team used the Doppler shift of light – the same property used by radar guns to catch speeding drivers – to find out how fast individual stars are moving.

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

Dozen Streams As Seen Across the Sky by Gaia

Location of the stars in the dozen streams as seen across the sky. The background shows the stars in our Milky Way from the European Space Agency’s Gaia mission. The AAT is a Southern Hemisphere telescope so only streams in the Southern sky are observed by S5. Credit: Ting Li, S5 Collaboration and European Space Agency

The properties of stellar streams reveal the presence of the invisible dark matter of the Milky Way. “Think of a Christmas tree,” says 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 said. “It is the same with stellar streams – their orbits reveal the dark matter.”

As well as measuring their speeds, the astronomers can use these observations to work out the chemical compositions of the stars, telling us where they were born. “Stellar streams can come either from disrupting galaxies or star clusters,” says 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.”

The tidal disruption of ten globular clusters in the Milky Way for 8 billion years. The red particles show the dark matter of the simulated Milky Way and the green particles show the disrupting globular clusters. The stars from the disrupting globular cluster form long stellar streams which follow the orbit. Astronomers use these streams to measure the mass distribution and clumpiness of dark matter in the Milky Way, as well as the accretion history of our Galaxy. Credit: Denis Erkal, S5 Collaboration

According to Prof. Li, these new observations are essential for determining how our Milky Way arose from the featureless universe after the Big Bang. “For me, this is one of the most intriguing questions, a question about our ultimate origins,” Li said. “It is the reason why we founded S5 and built an international collaboration to address this.”

A crucial ingredient for the success of S5 were observations from the European Gaia space mission. “Gaia provided us with exquisite measurements of positions and motions of stars, essential for identifying members of the stellar streams,” says Dr. Sergey Koposov, reader in observational astronomy in the University of Edinburgh and a co-author of the study.

This follows one globular cluster being torn into a tidal stream over 8 billion years. The red particles show the dark matter of a large galaxy and the green particles show a disrupting globular cluster. The stars near the progenitor form a characteristic “S”-shape due to the gravitational influence of the globular cluster. Credit: Denis Erkal, S5 Collaboration

Li’s team plans to produce more measurements on stellar streams in the Milky Way. In the meantime, she is pleased with these results as a starting point. “Over the next decade, there will be a lot of dedicated studies looking at stellar streams,” Li says. “We are trail-blazers and pathfinders on this journey. It is going to be very exciting!”

The results have been accepted for publication in the American Astronomical Society’s Astrophysical Journal.

Reference: “S5: The Orbital and Chemical Properties of One Dozen Stellar Streams” by Ting S. Li, Alexander P. Ji, Andrew B. Pace, Denis Erkal, Sergey E. Koposov, Nora Shipp, Gary S. Da Costa, Lara R. Cullinane, Kyler Kuehn, Geraint F. Lewis, Dougal Mackey, Jeffrey D. Simpson, Daniel B. Zucker, Peter S. Ferguson, Sarah L. Martell, Joss Bland-Hawthorn, Eduardo Balbinot, Kiyan Tavangar, Alex Drlica-Wagner, Gayandhi M. De Silva, Joshua D. Simon, S5 Collaboration, 24 March 2022, The Astrophysical Journal.
DOI: 10.3847/1538-4357/ac46d3

3 Comments on "12 Stellar Streams Within Our Galactic Halo: The Milky Way’s Feeding Habits Shine a Light on Dark Matter"

  1. Please cease sending.

  2. Howard Jeffrey Bender, Ph.D. | January 12, 2022 at 6:06 am | Reply

    This headline is misleading. “12 Stellar Streams Within Our Galactic Halo: The Milky Way’s Feeding Habits Shine a Light on Dark Matter” sounds like matter streaming into the galactic center while the article is really talking about orbiting star streams. Oh, well.

    Let’s stick to Dark Matter. Another possibility, from a view of String Theory, is that Dark Matter appears to us as an effect of string/anti-string annihilations. As you may know, quantum mechanics requires that strings must be formed as pairs in the quantum foam – a string and an anti-string – that immediately annihilate each other. Quantum mechanics also requires both the string and anti-string to be surrounded by “jitters” that reduce their monstrous vibrating energies. What if this jitter remains for a fraction of an instant after their string/anti-string annihilations? This temporary jitter would be seen by us as matter, via E=mc2, for that instant before it too returns to the foam. That’s why we never see it – the “mass” lasts only for that instant but is repeated over and over and over, all over. Specifics on this can be found in my YouTube, Dark Matter – A String Theory Way at https://www.youtube.com/watch?v=N84yISQvGCk

  3. Cynthia Binder | March 30, 2022 at 4:49 am | Reply

    Globals cluster…on Gaia. Nothing more nothing less. Rain down on me cool heavy. 🧊 ❄cubes,liquid 💧🌊🔥🔥🔥hothothot. Quantim,, mechanics string theory.. go Gaia 🍕🌭🍕👍new data🥤

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