As the Milky Way hurtles through space, it doesn’t travel alone. It is part of a galactic supercluster which a team of astronomers has just succeeded in mapping. Their map, which graces the cover of September’s Nature, is named Laniakea, Hawaiian for “immeasurable heaven”.
Yet measure it is precisely what the astronomers have done. They say it contains about 100,000 galaxies clustered into a roughly heart-shaped structure, all engaged in a common galactic waltz.
We’ve known for decades that the Milky Way must reside in a supercluster because of the sheer number of galaxies that surround us. But figuring out the supercluster’s size, shape and movement has been impossible, until now.
The problem was a matter of perspective, explains Matthew Colless, an astronomer at the Australian National University. We could see the boundaries of distant superclusters but couldn’t easily see our own for the simple reason that we are inside it. “It’s much easier to see the shape of an island if you can fly over it than if you’re standing in the middle of it,” he says.
The team of astronomers led by Brent Tully at the University of Hawaii and Helene Courtois at the University of Lyon didn’t simply map the positions of nearby galaxies. They took a new approach, also factoring in each galaxy’s motion through space.
All galaxies are flying apart from one another due to the expansion of the universe. But on top of this expansion there is a local movement of galaxies driven by gravity – like little eddies on the surface of a rushing tide. It was these eddies, known as the “peculiar velocities”, that the group measured and mapped.
The resulting data has allowed the creation of a virtual intergalactic time-lapse photograph. Resembling a vast cosmic anemone, each curving tendril is a light trail tracing the motion of a single galaxy. Seeing the galaxies this way – caught in their cosmic dance rather than their static positions – reveals where they are going.
It also allows astronomers to define the structure of the galactic cluster in a new way. By tracing the eddy patterns of Laniakea the boundaries have at last become clear: the team could establish where Laniakea ends and its neighbours begin – a first for astronomers.
The finding solves the mystery of the “Great Attractor”.
The galaxies whorling within Laniakea are all converging on the same spot. This is the “gravity minimum” for the cluster – the place where the pull of gravity from each galaxy cancels out so the galaxies are all drawn there, like water down a waterfall. Galaxies at Laniakea’s fringe may be nearby but are parts of other dances, drawn to the gravity minimums of other clusters.
The finding solves the mystery of the “Great Attractor”. Three decades ago astronomers found that the Milky Way and nearby galaxies appeared to be hurtling at hundreds of kilometres per second towards a mysterious area of the universe, now identified as Laniakea’s gravity minimum.
“Now we’ve understood that this Great Attractor thing is not a mysterious spherical object,” says Courtois, “it’s more like the central attraction of a big ballet, a gravitational ballet.”
Courtois, who has been working on the problem for 20 years, says their discovery of the inner workings of superclusters could only be made using recent improvements in technology. “We had to wait for the telescopes,” she says.
To survey Laniakea the team compiled data from telescopes all around the world, including the refurbished Parkes radio telescope in New South Wales, the Arecibo radio telescope in Puerto Rico, and optical telescopes in Chile and Hawaii. Advances in computational power and new techniques of visualising data in 3D helped them to pull it all together.
Meanwhile, Colless and his team have just released an even larger dataset covering 10 times the volume of Laniakea. With this they hope to map other nearby superclusters to try to understand how Laniakea itself may be pulled around by them.
These studies of large-scale structures could also help us try to solve some of the great mysteries of astronomy, such as dark matter and dark energy, Courtois explains.
Tully also sees a philosophical aspect to this work, which comes from further defining our place in the universe. “There was a huge psychological impact in seeing the images of the Earth from space. Now we know that our address includes the Laniakea supercluster of 100,000 galaxies,” he says.
Cathal O'Connell is a science writer based in Melbourne.
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