Twelve for dinner: How the Milky Way ‘ate’ smaller star clusters and galaxies

Study including UChicago scientists creates map of nearby stellar streams and dark matter

Astronomers are one step closer to revealing 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, the streams also tell us about the formation and history of the Milky Way. In particular, the analysis revealed how the Milky Way has steadily grown 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,” said University of Toronto Professor Ting Li, the lead author of the paper. “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.”

Prof. Li and a team of collaborators from institutions around the world, including the University of Chicago, initiated a dedicated program called the Southern Stellar Stream Spectroscopic Survey (S5) to measure the properties of many stellar streams. They measured the speeds of stars using the Anglo-Australian Telescope, a 4-meter optical telescope in Australia. Li and her team used the Doppler shift of light – the same property used by radar guns to capture speeding drivers – to find out how fast individual stars are moving.

The properties of stellar streams reveal the presence of the invisible dark matter of the Milky Way. “Think of a Christmas tree,” said 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," said Asst. Prof. Alex Ji at the University of Chicago, a co-author on the study. "Stars stripped from a cluster all have the same chemical composition because they are born at the same time, while stars stripped from a galaxy have differing compositions. These two types of streams provide different insights into the nature of dark matter."

According to 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.”

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 Anglo-Australian Telescope,” said co-author Professor Daniel Zucker of Macquarie University.

A crucial ingredient for the success of S5 was also 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,” said Sergey Koposov, reader in observational astronomy in the University of Edinburgh and a co-author of the study.

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 said. “We are trail-blazers and pathfinders on this journey. It is going to be very exciting!”

Other UChicago co-authors on the paper were Nora Shipp, PhD’21, now at MIT; undergraduate student Kiyan Tavangar, BS’21; and Asst. Prof. Alex Drlica-Wagner.

The results have been accepted for publication in the American Astronomical Society’s Astrophysical Journal. A preprint of the accepted version can be found here.

Citation: “S5: The Orbital and Chemical Properties of One Dozen Stellar Streams.” Li et al, Astrophysical Journal, 2022.

Funding: NASA, National Science Foundation, Australian Research Council, UNSW Scientia Fellowship Program, Netherlands Organization for Scientific Research, Thacher Research Award, Carnegie Fellowship.