Home, supercluster home. Astronomers studying the skies have found that our galaxy, the Milky Way, is part of an enormous supercluster of galaxies that has been named Laniakea, which comes from the Hawaiian words for "immeasurable heaven."
Laniakea, described in the journal Nature, stretches some 500 million light years across and holds 100,000 galaxies with the mass of a hundred million billion suns. This supercluster, scientists said, provides a whole new lens on the mysterious internal dynamics of such giant clusters, the biggest structures in the universe.
The universe has a certain order, defined by gravity: Planets circle stars, which spin around in galaxies, which hang out together in large clusters, which are nestled in gargantuan superclusters. These superclusters give the cosmos its large-scale structure — matter concentrates in massive, dense nodes and thin tendrils connect those nodes across the empty spaces in between.
Even though superclusters define the nodes of this cosmic web, scientists know very little about how they work. Until now, they didn't even know where exactly our own Milky Way lives — just that we're in a local group of at least 54 galaxies, which is itself part of a larger conglomeration long called the Virgo (or Local) Supercluster.
It's a little like knowing your street address without knowing which city you live in — which sounds like a pretty basic problem, but has proved deeply challenging to solve.
"This is what I've been doing for a long time, trying to understand where we live," said lead author R. Brent Tully, an astronomer at the University of Hawaii in Honolulu.
The giveaway is gravity. Gravity holds superclusters together, so watching the movements of galaxies speeding around their neighborhoods could help define a supercluster's borders.
Here's the problem. The universe is expanding faster and faster, due to a mysterious force called dark energy. This mucks up the speed readings: Scientists don't know how much of a particular galaxy's movement is due to the inward pull from the gravity of a cluster, and how much is due to the outward pull of the universe expanding faster and faster.
So a team of cosmic cartographers used the Superflows-2 database, a catalog of the motions of galaxies, to tease apart these movements. For each of more than 8,000 galaxies, they subtracted the expansion-related movement from the overall motions — leaving only the local, gravitationally driven movement.
The researchers found that the galaxies were flowing along these long, beautiful lines, many of them toward a gravitational dense basin of galaxies known as the Great Attractor. The boundary between those flowing toward this spot and those flowing away marked the edges of the galaxy supercluster.
It's like watching a watershed where the flows divide, Tully said.
"If you're standing at a certain place at the divide, water might be going in two different directions — one going into the Mississippi basin, one going off to the Great Lakes," Tully said.
The Milky Way sits in a backwater among these flowing galaxies, near a far edge of the supercluster. In fact, the whole Virgo Supercluster is just a small fraction of the Laniakea Supercluster.
"The Local Supercluster, we now see quite clearly, is just an appendange on Laniakea," Tully said. "It's like a suburb in a city of a metropolitan area."
Elmo Tempel, an astronomer at Tartu Observatory in Estonia who was not involved in the paper, praised the findings.
"This is the first clear definition of a supercluster," Tempel wrote in a commentary. "The downside of it is that it requires dynamical information that is available only for the nearby universe."
Understanding the dynamics of our own supercluster could help shed light on the large-scale forces at work in the universe, Tully said, particularly dark matter, the mysterious unseen mass whose gravity holds such enormous structures together, and dark energy, the strange force that is tearing them inevitably apart.