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Newly Discovered Dipole Repeller is Pushing Our Local Group of Galaxies Through Space

02/02/2017 01:42PM

Newly Discovered Dipole Repeller is Pushing Our Local Group of Galaxies Through Space
For decades, astronomers have known that our Milky Way galaxy, together with our companion Andromeda galaxy, are moving through space at about 1.4 million miles per hour. Scientists have generally assumed that excessive gravity from two dense regions of the universe, known as the Great Attractor and the Shapley Concentration, were responsible for this motion.

But now, in a groundbreaking study, a team of researchers is reporting the discovery of a previously unknown and nearly empty region of space located in the opposite direction that is effectively exerting a repelling force and pushing our Local Group of galaxies through space. This void has been dubbed the "Dipole Repeller."

Astronomers initially attributed the Milky Way's motion to the Great Attractor, a region of a half dozen rich clusters of galaxies 150 million light-years away. Soon after, attention was drawn to a much larger structure called the Shapley Concentration, located 600 million light-years away, in the same direction as the Great Attractor. However, there has been ongoing debate about the relative importance of these two attractors and whether they suffice to explain our motion.

The newly discovered void was previously suggested by astronomers at the University of Hawaii, but obtaining observational confirmation of the absence of galaxies has proved challenging. Until now, studies mostly focused on the detailed distribution of galaxies -- where are the galaxies located and how much pull do they exert on the Milky Way?

In this new study, a team led by Yehuda Hoffman at Hebrew University's Racah Institutes of Physics, working with colleagues in the USA and France, tried a different approach. Instead of looking at the positions of galaxies, they used the galaxy motions instead. The team created a three dimensional map of the galaxy flow field and used this to infer the underlying mass distribution that consists of dark matter and luminous galaxies. This method revealed "overdense" regions that pull on the Milky Way as well as "underdense" ones that push it.

The region of space that is moving coherently away from the void and toward the attractors is huge, extending across more than a billion light years, or a tenth of the radius of the observable universe. This Laniakea Supercluster of galaxies identified by the team is embedded within this flow, like a cork in a stream.

"Through 3D mapping of the flow of galaxies through space, we found that our Milky Way galaxy is speeding away from a large, previously unidentified, region of low density that we call the Dipole Repeller, as well as towards the known Shapley Concentration," said Dr. Hoffman. "It has become apparent that push and pull are of comparable importance at our location."

"There was a hint of the void from studies of the distribution of rich clusters of galaxies that emit X-rays, discussed in articles over a decade ago by Dale Kocevski, Harald Ebeling, and myself at the University of Hawaii", said Br. Brent Tully from the University of Hawaii Institute for Astronomy, "but the statistics were not sufficient to be convincing."

By identifying the Dipole Repeller, the researchers were able to explain both the direction of the Milky Way's motion and its velocity relative to the rest of the Universe. They expect that future ultra-sensitive surveys at optical, near-infrared, and radio wavelengths will directly identify the few galaxies expected to lie in this void, and directly confirm the void associated with the Dipole Repeller.

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