The Milky Way's Halo Spins With The Galaxy

Our galaxy is surrounded by a gaseous halo that extends for many hundreds of thousands of light-years from the center. It has a mass comparable to the Milky Way itself and it was believed to be still, compared to our quickly rotating galaxy.

"This flies in the face of expectations," lead author Edmund Hodges-Kluck said in a statement. "People just assumed that the disk of the Milky Way spins while this enormous reservoir of hot gas is stationary – but that is wrong. This hot gas reservoir is rotating as well, just not quite as fast as the disk."

The gas in the halo is incredibly hot, millions of degrees, but very spread out so it's difficult to estimate how quickly it's moving. The researchers had to carefully detect movement as the gas moved in front of very bright extragalactic sources, like active supermassive black holes and quasars.

In a paper published in the Astrophysical Journal, they reported that the gas is moving at about 180 kilometers per second (400,000 mph), which is only slightly slower than the rotational velocity at the rim of the Milky Way (240 km/s, 540,000 mph).

"The rotation of the hot halo is an incredible clue to how the Milky Way formed," continued Hodges-Kluck. "It tells us that this hot atmosphere is the original source of a lot of the matter in the disk."

Galaxies are believed to have formed when intergalactic materials began to fall into the large gravitational wells formed by dark matter. The rotation of the halo tells us how quickly the material must fall towards the center, and also how quickly the Milky Way came together.

"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," added Professor Joel Bregman, co-author of the study.