An artist’s representation shows distribution of one dozen stellar streams that surround the Milky Way Galaxy. These streams were once small galaxies or star clusters that are now being torn apart by the Milky Way's gravity.Geraint F. Lewis/S5 Collaboration
To an imaginary observer far away in space, the Milky Way galaxy may appear to be an isolated island of stars adrift in the universe. Now, a new survey has provided one of the best looks yet at the multiple streams of stars that surround the galaxy in a tangle of long, diffuse strands – features that could help locate the unidentified dark matter that scientists believe makes up most of the Milky Way’s mass.
The stellar streams are the remains of smaller galaxies and star clusters that were drawn in by the Milky Way’s gravitational pull billions of years ago and then stretched apart like so much cosmic taffy. Ting Li, an astronomer at the University of Toronto, is leading an international team whose goal is to chart the streams to reveal details about the Milky Way’s past and current composition, a specialty known as galactic archeology.
So far Dr. Li and her colleagues have examined 20 such streams, 12 of which are outlined in a study that was presented on Tuesday during a virtual meeting of the American Astronomical Society. The study includes detailed measurements of the three-dimensional motions and properties of the stars that constitute the streams. By understanding the distribution of the streams well enough, Dr, Li said that she and her colleagues hope to spot kinks and other irregularities that may indicate past close encounters with clumps of dark matter.
“If we want to map the dark matter in our galaxy we really need dozens of streams, and we are starting to do that,” Dr. Li said.
While the existence of dark matter in our galaxy and others was first inferred decades ago through its gravitational effect on the motions of stars, its true nature remains a mystery. Julio Navarro, a cosmologist at the University of Victoria who works on the dark matter question, said that it will likely be some time yet before studies of stellar streams can provide definitive answers. But until dark matter is detected more directly, they remain one of the few options for testing different theories about it is made of.
Dr. Navarro added that stellar streams also reveal the diversity of galaxies and smaller clusters of stars that once populated the space near the Milky Way, but have now ended up splattered against it, like bugs on a windshield. In a separate study published last week in the journal Nature, a team led by the University of Victoria reported the discovery of a stellar stream whose constituent stars have the lowest concentration of heavy elements yet seen.
“There are no clusters like this that we know of,” Dr. Navarro said. “So this is the relic of an object that must have existed but now is nowhere to be found.”
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Ivan Semeniuk