The gaseous halo of our Milky Way Galaxy is spinning in the same direction and at a similar speed as the Galaxy’s disk, which contains stars, planets, gas, and dust, according to a team of astronomers using data from ESA’s XMM-Newton X-ray Observatory.
Our Milky Way Galaxy is surrounded by an enormous halo of plasma (seen in blue in this artists’ rendition). Image credit: NASA / CXC / M.Weiss / Ohio State / A. Gupta et al.
“This flies in the face of expectations. People just assumed that the disk of the Milky Way spins while this enormous reservoir of hot gas is stationary – but that is wrong,” said lead author Dr. Edmund Hodges-Kluck, from the University of Michigan.
“This hot gas reservoir is rotating as well, just not quite as fast as the disk.”
Milky Way’s halo is several times larger than the Galaxy’s disk and composed of ionized plasma. Because motion produces a shift in the wavelength of light, Dr. Hodges-Kluck and co-authors measured such shifts around the sky using lines of very hot oxygen.
What they found was groundbreaking – the line shifts measured by the researchers show that Milky Way’s halo spins in the same direction as the disk of the Milky Way and at a similar speed: about 400,000 mph for the halo vs. 540,000 mph for the disk.
“The rotation of the hot halo is an incredible clue to how the Milky Way formed. It tells us that this hot atmosphere is the original source of a lot of the matter in the disk,” Dr. Hodges Kluck said.
Astronomers have long puzzled over why almost all galaxies, including the Milky Way, seem to lack most of the matter that they otherwise would expect to find.
They believe that about 80% of the matter in the Universe is the so-called ‘dark matter’ that, so far, can only be detected by its gravitational pull.
But even most of the remaining 20% of ‘normal’ matter is missing from galaxy disks.
More recently, some of the ‘missing’ matter has been discovered in the halo.
“Learning about the direction and speed of the spinning halo can help us learn both how the material got there in the first place, and the rate at which we expect the matter to settle into the Galaxy,” Dr. Hodges Kluck and his colleagues said.
The team’s findings were published in the April 27 issue of the Astrophysical Journal (arXiv.org preprint).
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Edmund J. Hodges-Kluck et al. 2016. The Rotation of the Hot Gas around the Milky Way. ApJ 822, 21; doi: 10.3847/0004-637X/822/1/21
