Anglo-Australian telescope uncovers dark matter mysteries in the Milky Way

A team from the University of Toronto has developed a map of 12 streams of stars orbiting within our galactic halo that may reveal the way in which dark matter holds the stars in their orbits, while also giving clues about the formation history of the Milky Way.

“We are seeing these streams being disrupted by the Milky Way’s gravitational pull and eventually becoming part of the Milky Way,” said Ting Li, an assistant professor and lead author on the new 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.”

A program called the Southern Stellar Stream Spectroscopic Survey (S5) was developed to measure the properties of stellar streams which are formed from the remains of neighbouring small galaxies and star clusters that are being torn apart by the Milky Way.

The speeds of the stars were measured using the four-metre Anglo-Australian Telescope, which is located in New South Wales, Australia. The Doppler shift of light was then used to determine how fast individual stars are moving.

S5 was used to measure as many streams as possible, the properties of which revealed the presence of the invisible dark matter of the Milky Way.

“Think of a Christmas tree,” said co-author professor Geraint 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. It is the same with stellar streams - their orbits reveal the dark matter.”

A crucial ingredient for the success of S5 were observations from the European Gaia space mission which provided precise measurements of positions and the motion of stars. As well as measuring their speeds, the astronomers used 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 assistant 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 new observations helped the astronomers to determine 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”.

In 2020, another team at the University of Technology in Melbourne discovered a new way to see the elusive dark matter haloes that surround galaxies.