Scientists have spotted a gargantuan bow-like structure made of galaxies that stretches for 1.4 billion light years across the skies above the South Pole.
This “South Pole Wall” is one of the biggest structures ever glimpsed by humans, and its discovery was announced in a study published on Friday in The Astrophysical Journal.
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“When our visualizations indicated something going on at the celestial South Pole, we were surprised: indeed there were no reports of a large scale structure in this region,” said Daniel Pomarède, a cosmographer at Paris-Saclay University in France who led the new study, in an email.
The South Pole Wall is comparable in size to the Sloan Great Wall, the sixth-largest known structure in the universe, though the South Pole Wall is located about 500 million light years from Earth, twice as close as the Sloan structure.
The universe contains many of these enormous belts that connect objects in space to a fundamental matrix known as the cosmic web. Galaxies tend to cluster at the crossroads of these long filaments, which are made of hydrogen gas and a mysterious non-luminous substance called dark matter. The largest known structure is the Hercules-Corona Borealis Great Wall, which is 10 billion light years wide—equal to about a tenth of the diameter of the entire observable universe.
The South Pole Wall is not quite that big, but it is the largest structure ever spotted within a radius of about 650 millions light years around Earth, according to Pomarède. The crooked shape of the structure enabled the team to detect the wall’s extent, relative to a straight-line structure, because so much of it fit within the observational aperture.
“It is because it is shaped like a bow that its 1.4 billion light-years length can fit the observed sphere,” Pomarède explained, though he added that the map the team used “fades away just beyond the wall.”
“So, perhaps we are not seeing the whole of it, if it happens to bend away from us beyond our observational limit,” he said.
You might think that it would be hard to miss a filament as colossal as the South Pole Wall, given that it appears to be some 14,000 times longer than the diameter of our galaxy, the Milky Way, and contains thousands of galaxies within its boundaries. It is so big, in fact, that if it were to magically get outlined in the night sky, like a celestial chalk drawing, skywatchers on Earth would not be able to see it all from one hemisphere.
However, this particular structure is located in the “Zone of Avoidance,” which is the region of space right behind the dusty center of the Milky Way from our perspective on Earth. As a result, our galaxy’s bulk has blocked it from view—until now.
The discovery occurred when Pomarède and his colleagues noticed that the gravitational influence of some gigantic structure was pulling galaxies towards it in all directions.
Pomarède and his colleagues used Cosmicflows-3, a comprehensive catalog that visualizes the distances and motions of 18,000 galaxies, to indirectly peer at the knots and filaments that make up the cosmic web. One of the team’s main goals—and a major quest for astronomers in general—is to figure out how the gravitational heft of the web’s large-scale structures influences the motions of galaxies, including our own Milky Way.
Decades of research has revealed that the Local Group, a cluster of galaxies that includes the Milky Way, is moving at about 630 kilometers per second, at least in part due to gravitational attraction from large-scale structures as well as repulsion from empty regions of space known as voids.
The Cosmicflows-3 map recorded peculiar velocities of galaxies on one side of the obscured South Pole Wall that seemed slower than expected, while galaxies on the side closer to Earth moved a bit faster than expected.
“Our study told us that the South Pole Wall, due to its gravitational attraction, is acting upon us, giving us a velocity of about 40 km/s,” Pomarède said. “We would like to know whether other structures are contributing: in particular, is there something hidden in the Zone of Avoidance? There might be filaments crossing this zone. We are looking for them.”
Following those breadcrumbs, the researchers were able to reconstruct some of the South Pole Wall using models and algorithms, though they said they will not be sure of its full size until more detailed versions of Cosmicflows become available in the coming years. In other words, the South Pole Wall may be much larger than its current estimated dimensions of 1.4 billion light years across, and some 600 million light years deep, but we would need a bigger map to know for sure.
“Our scientific discipline is called cosmography, a branch of cosmology, that aims at establishing a cartography of the cosmos around us (our local universe),” Pomarède said. “Like cartographers who mapped Earth, we map the structure of the sky, and thus we learn more about the structures we live in, and we study whether what we find is compatible with the current theory of structure formation (in the context of the standard model of cosmology).”
Finding one of the top-ten biggest structures in the universe is pretty wild by itself, but these discoveries also reveal threads of the larger cosmic tapestry that undergirds our surroundings and reality. The more we learn about these elongated bubbles filled with dark matter and galaxies, the more we will understand how we came to live in this weird and ever-evolving universe.
