Great Attractor

The Great Attractor is a gravitational anomaly in intergalactic space and the apparent central gravitational point of the Laniakea Supercluster. The observed anomalies suggest a localized concentration of mass millions of times more massive than the Milky Way. However, it is inconveniently obscured by our own Milky Way's galactic plane, lying behind the Zone of Avoidance (ZOA), so that, in visible light wavelengths, the Great Attractor is difficult to observe directly.[1]

Panoramic view of the entire near-infrared sky. The location of the Great Attractor is shown following the long blue arrow at bottom right.
Hubble Telescope image of the region of the sky where the Great Attractor is located

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 the universe. These galaxies are observable above and below the ZOA; all are redshifted in accordance with the Hubble Flow, indicating that they are receding relative to us and to each other, but the variations in their redshifts are large enough and regular enough to reveal that they are slightly drawn towards 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 itself is moving towards the Shapley Supercluster.[1] 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.[2]

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 larger 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).[3] 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),[4][5] 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 a statistical effect called Malmquist bias.[6] 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.[7]

Dark flow
Main article: Dark flow

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.

Laniakea Supercluster

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.[8]

Vela 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 3.9 m 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.[2]

See also

References
  1. "What is the Great Attractor?". Universe Today. 14 July 2014. Retrieved 24 June 2018.
  2. Kraan-Korteweg, Renée C. (8 November 2016). "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.
  3. "Hubble focuses on "the Great Attractor"". NASA. 18 January 2013. Retrieved 24 October 2020.
  4. 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.
  5. Mukai, Koji; Mushotzky, Rich; Masetti, Maggie. "The Great Attractor". NASA's Ask an Astrophysicist. Archived from the original on 18 February 2003. It is now thought that the Great Attractor is probably a supercluster, with Abell 3627 near its center.
  6. Landy, Stephen D.; Szalay, Alexander S. (1992). "A general analytical solution to the problem of Malmquist bias due to lognormal distance errors". The Astrophysical Journal. 391: 494. Bibcode:1992ApJ...391..494L. doi:10.1086/171365.
  7. "X-rays reveal what makes the Milky Way move" (Press release). Ifa.hawaii.edu. 11 January 2006. Retrieved 24 October 2020.
  8. Tully, R. Brent; Courtois, Hélène; Hoffman, Yehuda; Pomarède, Daniel (2014). "The Laniakea supercluster of galaxies". Nature. 513 (7516): 71–73. arXiv:1409.0880. Bibcode:2014Natur.513...71T. doi:10.1038/nature13674. PMID 25186900.

Further reading
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