Great Attractor

The Great Attractor is a gravitational anomaly in intergalactic space at the center of the Laniakea Supercluster that reveals the existence of a localised concentration of mass tens of thousands of times more massive than the Milky Way. This mass 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 Great Attractor is moving towards the Shapley Supercluster.

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 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) 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/or radiating large amounts of radio waves.

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