Beyond the Milky Way, a galactic wall
Astronomers have discovered that there is a vast wall across the southern border of the local cosmos.
The South Pole Wall, as it is known, consists of thousands of galaxies — beehives of trillions of stars and dark worlds, as well as dust and gas — aligned in a curtain arcing across at least 700 million light-years of space. It winds behind the dust, gas and stars of our own galaxy, the Milky Way, from the constellation Perseus in the Northern Hemisphere to the constellation Apus in the far south. It is so massive that it perturbs the local expansion of the universe.
But don’t bother trying to see it. The entire conglomeration is behind the Milky Way, in what astronomers quaintly call the zone of avoidance.
An international team of astronomers led by Daniel Pomarède of Paris-saclay University and R. Brent Tully of the University of Hawaii announced this new addition to the local universe on Friday in a paper in Astrophysical Journal. The paper is festooned with maps and diagrams of blobby and stringy features of our local universe as well as a video tour of the South Pole Wall.
It is the latest installment of an ongoing mission to determine where we are in the universe — to fix our neighborhood among the galaxies and the endless voids — and where we are going.
“The surprise for us is that this structure is as big as the Sloan Great Wall and twice as close, and remained unnoticed, being hidden in an obscured sector of the southern sky,” Pomarède said in an email.
“The discovery is a wonderful poster child for the power of visualizations in research,” Tully said.
The new wall joins a host of other cosmographic features: arrangements of galaxies, or a lack of them, that astronomers have come to know and love over the past few decades, with names like the Great Wall, the Sloan Great Wall, the Hercules-corona Borealis Great Wall and the Bootes Void.
The new paper was based on measurements, performed by Tully and his colleagues, of the distances of 18,000 galaxies as far away as 600 million light-years. By comparison, the most distant objects we can see — quasars and galaxies that formed shortly after the Big Bang — are about 13 billion light years away.
The galaxies in the wall cannot be seen, but Pomarède and his colleagues were able to observe their gravitational effects by assembling data from telescopes around the world.
In the expanding universe, as described in 1929 by astronomer Edwin Hubble and confirmed for almost a century, distant galaxies are flying away from us as if they were dots on an inflating balloon; the farther they are, the faster they recede from us, according to a relation called the Hubble law.
That motion away from Earth causes their light to be shifted to longer, redder wavelengths and lower frequencies, like retreating ambulance sirens. Astronomers use this “redshift,” which is easily measured, as a proxy for relative distance in the universe. By measuring the galaxy distances independently, the “Cosmicflows” team, as Pomarède and his colleagues call themselves, was able to distinguish the motion caused by the cosmic expansion from motions caused by gravitational irregularities.
As a result, they found that the galaxies between Earth and the South Pole Wall are sailing away from us slightly faster than they otherwise should be, by about 30 miles per second, drawn outward by the enormous blob of matter in the wall. And galaxies beyond the wall are moving outward more slowly than they otherwise should be, reined in by the gravitational drag of the wall.
One astonishing aspect of the wall is how big it is compared to the volume that the team was surveying: a contiguous filament of light 1.4 billion light-years long, packed into a cloud maybe 600 million in radius. “There is hardly room in the volume for anything bigger!” Tully said in an email. “We’d have to anticipate that our view of the filament is clipped; that it extends beyond our survey horizon.”
On the largest scales, cosmologists attest, the universe should be expanding smoothly, and the galaxies should be evenly distributed. But on smaller, more local scales, the universe appears lumpy and gnarled. Astronomers have found that galaxies are gathered, often by the thousands, in giant clouds called clusters and that these are connected to one another in lacy, luminous chains and filaments to form superclusters extending across billions of light-years. In between are vast deserts of darkness called voids.
From all of this has emerged what some astronomers call our “long address”: We live on Earth, which is in the solar system, which is in the Milky Way galaxy. The Milky Way is part of a small cluster of galaxies called the Local Group, which is on the edge of the Virgo cluster, a conglomeration of several thousand galaxies.
In 2014, Tully suggested that these features were all connected, as part of a giant conglomeration he called Laniakea — Hawaiian for “open skies” or “immense heaven.” It consists of 100,000 galaxies spread across 500 million light-years.
All this lumpiness has distorted the expansion of the universe. In 1986, a group of astronomers who called themselves the Seven Samurai announced that the galaxies in a huge swath of the sky in the direction of the constellation Centaurus were flying away much faster than the Hubble law predicted, as if being pulled toward something — something the astronomers called the Great Attractor. It was the beginning of something big. “We now see the Great Attractor as the downtown region of the supercluster that we live in — an overall entity that our team has called the Laniakea Supercluster,” Tully said. All the different parts of this supercluster are tugging on us, he added.