A ‘Holy Grail’ of astronomy is to provide a clear map of our galaxy –a perspective of Earth’s relationship to the physical universe where our solar system drifts between two spiral arms at its outer edges of the Milky Way, some 27,000 light-years from its opaque central disk. Beyond that, like the maps of ancient sea-faring mariners, is a terra ingognito, the “zone of avoidance” where no space craft has yet to ever travel beyond the opaque central disk.
Astronomers have now discovered a vast structure, the “South Pole Wall,” — a cosmic curtain hidden behind the billions of stars, dust, and dark worlds of the Milky Way Galaxy in the “zone of avoidance” that forms an arc across some 700 million light-years along the southern border of the local cosmos.
Ghosts of the Zone
Lurking inside the zone is an enormous ‘ghost’ galaxy, believed to be one of the oldest in the universe, was detected on the outskirts of the Milky Way in November of 2018 by a team of astronomers who discovered the massive object when trawling through new data from the European Space Agency’s Gaia satellite. One of an untold number of galaxies that clump together in what’s known as the cosmic web, enormous strands of hydrogen gas in which galaxies are strung like pearls on a necklace that surround gigantic, dark empty voids.
The image at the top of the page shows the South Pole Wall near the southernmost part of the sky. ( © D. Pomarede, R. B. Tully, R. Graziani, H. Courtois, Y. Hoffman, J. Lezmy.)
The spectral object, named Antlia 2, avoided detection thanks to its extremely low density as well as a perfect hiding place behind the dusty shroud of the Milky Way’s disc with its concentration of bright, ancient stars near the galactic center. Optically, the Zone of Avoidance is like “trying to look through a velvet cloth—black as black can be,” says Thomas Dame, Director of the Radio Telescope Data Center at the Harvard-Smithsonian Center for Astrophysics and Senior Radio Astronomer at the Smithsonian Astrophysical Observatory in 2019. “In terms of tracing and understanding the spiral structure, essentially half of the Milky Way is terra incognito.”
The New Wall
An international team of astronomers, reports Dennis Overbye for the New York Times, 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, joining the earlier discoveries of other structures of our universe like the Great Wall, the Sloan Great Wall, the Hercules-Corona Borealis Great Wall and the Bootes Void. The paper –based on measurements, performed by Dr. Tully and his colleagues, of the distances of 18,000 galaxies as far away as 600 million light-years–includes a video tour of the South Pole Wall.
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. As a result, writes Overbye, “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 might wonder how such a large and not-so-distant structure remained unnoticed,” said Pomarède in a statement issued by Paris-Saclay University.
The discovery has joined what astronomers call our “long address” writes Overbye: “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, Dr. 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.
But it appears the our adddress doess not stop there –in 2013, astronomers discovered that the Milky Way exists in void –one of the vast holes of the “Swiss-cheese” structure of the cosmos– with a radius measuring roughly 2 billion light years in diameter –the largest void known to science, shaped like a sphere with a shell of increasing thickness made up of galaxies, stars and other baryonic matter. As with other voids, it is not completely empty but contains our own galaxy, the Milky Way (a few hundred million light-years from the void’s center), the Local Group, and a larger part of the Laniakea Supercluster.