The Milky Way’s halo, a massive reservoir of hot gas, is spinning, according to astronomers at the University of Michigan’s College of Literature, Science and the Arts (LSA). The halo’s spinning direction is the same but at a slower speed as Milky Way’s disk, a portion of the galaxy that contains stars, planets, dust and gas.

According to Edmund Hodges-Kluck, assistant research scientist, this discovery is unexpected. In a press release, the assistant research scientist pointed out that astronomers previously assumed that the Milky Way’s halo does not move, unlike its disk. This is the first time that they found that the halo actually rotates in the same direction as the disk, although at a lower speed.

The data for the study, now published at the Astrophysical Journal, was acquired by a European Space Agency telescope named XMM-Newton. The research team found that the halo, which is composed of ionized plasma several times bigger than the galaxy’s disk, spins up to 400,000 miles per hour or more than 640,000 kilometers per hour. By comparison, Milky Way’s disk spins up to 540,000 miles per hour or more than 860,000 kilometers per hour.

Milky Way's halo

Milky Way. Credit: NASA/JPL-Caltech/S. Stolovy (SSC/Caltech)

Since the galaxy’s hot gaseous halo is the original source of many matter in the disk, Hodges Kluck says that it could shed light into how the Milky Way formed. Scientists estimate that 80 percent of the matter in the universe is dark matter while the 20 percent is normal matter, some of which can be found in the galaxy’s halo.

“Now that we know about the rotation, theorists will begin to use this to learn how our Milky Way galaxy formed — and its eventual destiny,” adds Joel Bregman, a professor of astronomy at the University of Michigan’s College of Literature, Science and the Arts. “We can use this discovery to learn so much more — the rotation of this hot halo will be a big topic of future X-ray spectrographs,” Bregman says.