A new Canadian-led study has brought scientists one step closer to understanding dark matter, using a map of twelve “streams” of stars orbiting within our galaxy’s halo.
University of Toronto cosmologist Ting Li and collaborators are starting to map the Milky Way’s invisible mass in higher resolution than ever using the star streams — tiny, torn-apart galaxies and star clusters that drifted too close to the immense gravity of our galaxy, the Milky Way.
“Our Milky Way has a lot of these companions,” Li said, speaking of objects such as the Large Magellanic Cloud that are within our galaxy’s gravitational “well” or sphere of influence. While we can see many of these galactic companions in the sky, over our Universe’s long history, a few fell apart after drifting too close to the Milky Way, she explained. “So some of these companions, they get torn apart by our gravity.”
The remnants of dust and gas left behind by these star streams will likely point the way to better understanding the location of dark matter in the Milky Way, which is the term astronomers use to describe an unseen property of space that is responsible for most of the matter of the Universe.
Dark matter is not visible with conventional telescopes, as it doesn’t emit any radiation we can detect. But we can observe dark matter’s influence through gravitational effects such as “lensing.” Lensing temporarily occurs when the light of distant galaxies gets “pulled” into our line of sight as a massive and closer object, such as a big star, drifts in front of the galaxy from Earth’s point of view.
Now Li’s team suggests that for the Milky Way, we can use stellar streams to better understand where dark matter lurks in our neighbourhood. We don’t have the resolution to spot these streams in faraway galaxies, she warns. But perhaps we could make predictions about where dark matter resides in other galaxies, based on our up-close observations of the Milky Way.
“Dark matter is like a Christmas tree in a dark night,” Li explained. “We don’t see the tree, but we see the tree’s lights. It’s like how we see the [stellar] streams. The stream is wrapping around the galaxy, and that allows us to weigh how massive the galaxy is.”
To further the analogy, she said the more lights wrapping around a Christmas tree, the better a sense we get of the tree’s shape. This is why having 12 stellar streams is so valuable. Mapping so many streams simultaneously shows us the locations of dark matter in the Milky Way to higher resolution than ever before.
We also learned where the streams came from; their speeds and “metallicity” (proportion of elements other than hydrogen or helium) show that half of the streams came from dwarf galaxies, while half came from star groups known as globular clusters.
This study is more focused on understanding the properties of the streams, the authors said in their results, but with more work coming there will be information in future studies on what the study means for our understanding of dark matter.
That said, the authors are pointing to factors such as the velocity and dispersion of the streams as better constraining dark matter properties, after more modelling and study. Ti said one key debate in the community is how much of dark matter is “cold” (slow-moving) or “hot” (fast-moving), for example. The study further points to “gaps” and “kinks” in the streams as a key to showing the existence of dark matter structure in the zone surrounding the Milky Way.
Li is part of an international effort devoted to seeking out stellar streams, called the Southern Stellar Stream Spectroscopic Survey (S5). The survey mapped how fast these stars are moving using the four-metre Anglo-Australian Telescope in Australia, and captured the motion of 12 stellar streams in a single study. (By contrast, previous studies only focused on one at a time.)
Another key helper in the survey was the European Space Agency’s Gaia mission, which maps the motions and positions of stars to high resolution.
Research always takes place in a continuum, and as such Li said this study will eventually be part of a network of observations on stellar streams — with each study giving more resolution as to where dark matter is in our galaxy.
The team has in fact observed about 20 streams to date, with this study representing only 12 of the streams so far. Li pointed to other telescopic collaborations that are ongoing with S5, which will also produce more results through their various combinations of observations.
“I think the next step we are currently preparing is use all  streams to constrain the Milky Way’s dark matter distribution; hopefully that will come out in a year or two,” she said.
The study has been accepted for publication in the American Astronomical Society’s peer-reviewed Astrophysical Journal. A preprint version of the report is available on ArXiv.
This biweekly column by Canadian science and space journalist Elizabeth Howell focuses on a trending news topic in Canadian astronomy and space.