The Great Attractor is an apparent gravitational anomaly in intergalactic space at the center of the local Laniakea Supercluster, in which the Milky Way is located, in the so-called Zone of Avoidance that is very difficult to observe in visible wavelengths due to the obscuring effects of our own galactic plane. This anomaly suggests a localized concentration of mass thousands of times more massive than the Milky Way.
The anomaly is observable by its effect on the motion of galaxies and their associated clusters over a region hundreds of millions of light-years across. These galaxies are all redshifted, in accordance with the Hubble Flow, indicating that they are receding relative to us and to each other, but the variations in their redshift are sufficient to reveal the existence of the anomaly. The variations in their redshifts are known as peculiar velocities, and cover a range from about +700 km/s to −700 km/s, depending on the angular deviation from the direction to the Great Attractor.
The Great Attractor is moving towards the Shapley Supercluster. Recent astronomical studies by a team of South African astrophysicists revealed a supercluster of galaxies, termed the Vela Supercluster, in the Great Attractor's theorized location.
The first indications of a deviation from uniform expansion of the universe were reported in 1973 and again in 1978. The location of the Great Attractor was finally determined in 1986: it is situated at a distance of somewhere between 150 and 250 Mly (million light years) (47–79 Mpc) (the latter being the most recent estimate) away from the Milky Way, in the direction of the constellations Triangulum Australe (The Southern Triangle) and Norma (The Carpenter’s Square). While objects in that direction lie in the Zone of Avoidance (the part of the night sky obscured by the Milky Way galaxy) and are thus difficult to study with visible wavelengths, X-ray observations have revealed that the region of space is dominated by the Norma cluster (ACO 3627), a massive cluster of galaxies containing a preponderance of large, old galaxies, many of which are colliding with their neighbours and radiating large amounts of radio waves.
Debate over apparent mass
In 1992, much of the apparent signal of the Great Attractor was attributed to the effect of Malmquist bias. In 2005, astronomers conducting an X-ray survey of part of the sky known as the Clusters in the Zone of Avoidance (CIZA) project reported that the Great Attractor was actually only one tenth the mass that scientists had originally estimated. The survey also confirmed earlier theories that the Milky Way galaxy is in fact being pulled towards a much more massive cluster of galaxies near the Shapley Supercluster, which lies beyond the Great Attractor, and which is called the Shapley attractor.
In astrophysics, "Dark flow" is a possible non-random component of the peculiar velocity of galaxy clusters. The measured velocity is the sum of that predicted by Hubble's Law added to a possible small, unexplained, "dark" velocity that flows in a direction common to the galaxy clusters.
The proposed Laniakea Supercluster is defined as the Great Attractor's basin, encompassing the former superclusters of Virgo and Hydra-Centaurus. Thus the Great Attractor would be the core of the new supercluster.
In 2016, a multinational team of South African, European and Australian researchers headed by South African astronomer Renée C. Kraan-Korteweg announced the discovery of a supercluster of galaxies that would largely explain the mysterious Great Attractor. Using data from the AAOmega spectrograph, the 3.9m Anglo-Australian Telescope, and the Southern African Large Telescope, astronomers detected a region of galactic overdensity consistent with the "supercluster" designation, which provides the requisite explanation for a gravitational anomaly in the Shapley supercluster neighborhood where the Great Attractor was theorized to be located.
- Dark flow – A possible non-random component of the peculiar velocity of galaxy clusters
- Dipole repeller – A center of effective repulsion in the large-scale flow of galaxies in the neighborhood of the Milky Way
- CfA2 Great Wall
- Dark matter – Hypothetical form of matter comprising most of the matter in the universe
- Hercules–Corona Borealis Great Wall
- Shapley Supercluster – Largest concentration of galaxies in our local universe
- "What Is The Great Attractor? - Universe Today". Universe Today. 2014-07-14. Retrieved 2018-06-24.
- "What Is The Great Attractor? - Universe Today". Universe Today. 2014-07-14.
- Kraan-Korteweg, Renée C. (2016-11-08). "Discovery of a supercluster in the ZOA in Vela". Monthly Notices of the Royal Astronomical Society. 466 (1): L29–L33. arXiv:1611.04615. Bibcode:2017MNRAS.466L..29K. doi:10.1093/mnrasl/slw229.
- "NASA - Hubble Focuses on "the Great Attractor"". Nasa.gov. 2013-01-18. Retrieved 2015-06-02.
- Kraan-Korteweg, Renée C. (2000). "Galaxies Behind the Milky Way and the Great Attractor". From the Sun to the Great Attractor. Lecture Notes in Physics. 556. pp. 301–344. CiteSeerX 10.1.1.338.3806. doi:10.1007/3-540-45371-7_8. ISBN 978-3-540-41064-5.
- Mukai, Koji; Mushotzky, Rich; Masetti, Maggie. "NASA's Ask an Astrophysicist: The Great Attractor". Archived from the original on February 18, 2003.
It is now thought that the Great Attractor is probably a supercluster, with Abell 3627 near its center.
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- New Map Locates Milky Way in Neighborhood of 100,000 Galaxies. Nadia Drake, for National Geographic, 3 September 2014
- Cosmography of the Local Universe at Vimeo showing the Great Attractor