Something hidden behind the Milky Way is tugging on our galaxy
Our galaxy and thousands of its neighbours are drifting towards a patch of sky we can barely see, blocked by the dust and stars of the Milky Way. Astronomers named the target the Great Attractor in the 1980s. It turned out to be neither single nor final: it too is being pulled towards something farther away.
Galaxies generally fly apart as the universe expands, but each also has its own extra motion, called a peculiar velocity, caused by the gravity of nearby clumps and voids. To measure it, astronomers compare a galaxy's distance, found by independent methods, with the speed implied by its redshift. Hints that the local universe was not expanding smoothly emerged in the 1970s, and the cosmic microwave background showed that our Local Group of galaxies is moving relative to the wider cosmos.
In the 1980s a team nicknamed the Seven Samurai, among them Sandra Faber, Alan Dressler and Donald Lynden-Bell, measured about 400 elliptical galaxies and mapped a coherent stream heading towards an unrecognised mass concentration. Dressler coined the name Great Attractor. Over a vast swath of space, galaxies move up to roughly 700 kilometres per second faster or slower than pure expansion predicts, in a broad flow rather than a plunge towards one point.
Seeing the culprit is difficult because it lies in the Zone of Avoidance, the band of sky hidden by our own galaxy, towards the constellations Norma and Triangulum Australe. X-ray, radio and infrared surveys eventually found the Norma Cluster, a massive galaxy cluster about 220 million light-years away, sitting within a long filament called the Norma Wall. Early estimates had put the region's mass near ten thousand trillion Suns, but later X-ray work showed it was lighter than first thought.
Much of the pull actually comes from the far richer Shapley Supercluster, lying in roughly the same direction but much farther off, towards which the Great Attractor itself is moving. In 2014 R. Brent Tully's team defined our home supercluster, Laniakea, like a watershed of converging galaxy flows spanning about 500 million light-years, with the Great Attractor near its heart. In 2017 researchers added a push from behind: an empty region dubbed the Dipole Repeller.
Source: Great Attractor
