Follow the motion
As detection and analysis techniques improved, however, astronomers realized the Virgo Supercluster was not a gravitationally bound object. Its story is much more complicated, and a simple definition from the 1950s perhaps wouldn’t suffice.
De Vaucouleurs and other astronomers had originally looked at the rainbowlike spectrum of light from galaxies in the supercluster. Measuring how much that spectrum has shifted compared with a stationary source on Earth tells us how fast the galaxy is moving — and how far away it is. Astronomers combine that distance with the galaxy’s position on the sky, and do that for hundreds of galaxies, and then voilà! A 3D map of galaxy distribution within the supercluster.
By the 1980s, astronomers began to understand the detailed dynamics, or motions, of the structures apart from the universe’s background of cosmic expansion. No longer were they limited to mapping points of light on the sky. Now they could look at underlying structure by following galaxies’ movements.
That structure was surprising. Instead of all galaxies in the Local Supercluster moving toward the Virgo Cluster, and therefore the center of the supercluster, they seemed to be moving toward a spot that didn’t align with Virgo. Even the Virgo Cluster was moving toward that same area. Astronomers refer to that mysterious region as the Great Attractor.
But what lies beyond the Local Supercluster? Like a leaf carried along by a rushing river toward a lake, each galaxy follows the flow of gravity. Smaller lakes eventually feed into large basins of water. The Local Supercluster is one of those smaller lakes; what does it feed into? What enormous body of water contains the Great Attractor?
To solve that mystery, astronomers needed to unravel the many different movements of each galaxy. The largest comes from cosmic expansion, called the Hubble flow, which describes the expansion of the universe that carries things farther apart. But a smaller and more important motion to determine the structure in which a galaxy lies results from the gravitational pull between galaxies. This motion, called peculiar velocity, subtracts the Hubble flow. “The peculiar velocities are telling us where the mass is,” says Tully.
Over several years, Tully and his colleagues — including Hélène Courtois of the University of Lyon and Yehuda Hoffman from the Hebrew University of Jerusalem — have measured and mapped the motions of nearly 20,000 galaxies in the local universe. Their observations yield three numbers for a galaxy’s position, one number for its radial velocity (its velocity along our line of sight), and one number for the motion’s uncertainty. That’s five numbers each for 20,000 data points. But those data points aren’t isolated numbers; they all relate to one another because they’re all correlated through gravity. The goal of the team’s analysis was to figure out how.
Tully and his colleagues published their analysis December 1, 2017, in The Astrophysical Journal. Their paper shows how the Local Supercluster, defined 70 years ago, relates to an even larger volume of the local universe. The Great Attractor is the center of what is now called the Laniakea Supercluster, and the Local Supercluster is just an assemblage of that larger structure. They call Laniakea a true supercluster because anything within its boundaries will move gravitationally toward it, while whatever lies beyond those boundaries will move away.