The Milky Way’s feeding habits shine a light on dark matter

Astronomers are closer to revealing the 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.

An international team of collaborators, which includes Dr Sergey Koposov, reader in observational astronomy in the School of Physics and Astronomy, initiated a dedicated program - the Southern Stellar Stream Spectroscopic Survey (S5) - to measure the properties of stellar streams: the shredded remains of neighbouring small galaxies and star clusters that are being torn apart by our own Milky Way.

The team, led by Prof Ting Li from the University of Toronto, 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 optical telescope in Australia. They used the Doppler shift of light, used by the police radar guns to capture speeding drivers, to find out how fast individual stars are moving.

Observations from the European Gaia space mission were also used. These enabled astronomers to efficiently identify individual stars belonging to streams among much more populous stellar populations of the Milky Way.

The properties of stellar streams reveal the presence of the invisible dark matter of the Milky Way. Such observations are essential for determining how our Milky Way arose from the featureless universe after the Big Bang.

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