Great Attractor



The Great Attractor is a region of gravitational attraction in intergalactic space and the apparent central gravitational point of the Laniakea Supercluster of galaxies that includes the Milky Way galaxy, as well as about 100,000 other galaxies.

The observed attraction suggests a localized concentration of mass having the order of 1016 solar masses. However, it is obscured by the 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.

The attraction is observable by its effect on the motion of galaxies and their associated clusters over a region of hundreds of millions of light-years across the universe. These galaxies are observable above and below the Zone of Avoidance; all are redshifted in accordance with the Hubble flow, indicating that they are receding relative to the Milky Way 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 attraction. 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.

History
The Great Attractor was named by Alan Dressler in 1987, following decades of redshift surveys that built up a large dataset of redshift values. The redshift values and distance measurements independent of redshift measurements were then combined to create maps of peculiar velocity.

Through a series of peculiar velocity tests, astrophysicists found that the Milky Way was moving in the direction of the constellation of Centaurus at about 600km/s. Then, the discovery of cosmic microwave background (CMB) dipoles was used to reflect the motion of the Local Group of galaxies towards the Great Attractor. The 1980s brought many discoveries about the Great Attractor, such as the fact that the Milky Way is not the only galaxy impacted. Approximately 400 elliptical galaxies are moving toward the Great Attractor beyond the Zone of Avoidance caused by the Milky Way galaxy light.

Intense efforts to work through the difficulties caused by the occlusion by the Milky Way during the late 1990's identified the Norma Cluster at the center of the Great Attractor region.

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). 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 region of space to be 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 a statistical effect called 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 toward a much more massive cluster of galaxies near the Shapley Supercluster, which lies beyond the Great Attractor, and which is called the Shapley Attractor.

Laniakea Supercluster
The proposed Laniakea Supercluster is defined as the Great Attractor's basin. It covers approximately four main galaxy superclusters, including superclusters of Virgo and Hydra–Centaurus, and spans across 500 million light years. Because it is not dense enough to be gravitationally bound, it should be dispersing as the universe expands, but it is instead anchored by a gravitational focal point. Thus the Great Attractor would be the core of the new supercluster.