Within the cosmic web forming the Universe, gigantic structures are embedded in this network of matter and gas. With the increase in the precision of observation instruments, astrophysicists are constantly discovering new large-scale structures. The most recent, the South Pole Wall, spanning 1.4 billionlight years, was recently discovered by the same researchers who had demonstrated the existence of Laniakea. The discovery and study of such structures allows astrophysicists to test and test current cosmological models.
Spectacular 3D maps of the Universe have revealed one of the largest cosmic structures ever found – a wall spanning 1.4 billion light years in diameter and containing hundreds of thousands of galaxys. The South Pole Wall, as it has been dubbed, has so far been hidden because large parts are half a billion light years behind our brilliant galaxy, the Milky Way. The South Pole Wall rivals in size with the Great Wall of Sloan, the sixth largest cosmic structure discovered.
Astronomers have long observed that galaxies are not scattered randomly throughout the Universe, but rather clump together in what is called the cosmic web, huge strands of gaseous hydrogen in which the galaxies are integrated and surrounded by gigantic voids.
Cosmography assaulting large-scale cosmic structures
The cartography of these intergalactic threads belongs to the field of cosmography, which is the cartography of the cosmos, explains Daniel Pomarede, cosmographer at Paris-Saclay University in France. Previous cosmographic work has mapped the extent of other galactic assemblages, such as the current structural record holder, the Great Wall of Hercules-Corona Borealis, which spans 10 billion light years, or more one-tenth the size of the visible Universe.
The South Pole Wall is located in the center of the map, bordering the Caméléon stellar complex. The Galactic Obscuration Zone is visible as a large light line across the map. Credits: Daniel Pomarède et al. 2020
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In 2014, Pomarède and his colleagues unveiled the Laniakea supercluster, a galactic collection in which our own Milky Way resides. Lanaikea is 520 million light years wide and contains around 100 million billion suns. For their new map, the team used newly created sky surveys to scan an area called the Galactic Darkening Zone. It is an area in the southern part of the sky in which the bright light of the Milky Way blocks much of what is behind and around it.
Zoomed map of the sky area housing the South Pole Wall (in the center in red), showing the Galactic Obscuration Zone. Credits: Daniel Pomarède et al. 2020
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Cosmographs generally determine the distance to objects using the redshift, the speed at which an object moves away from Earth due to the expansion of the Universe, which depends on their distance. The further away an object is, the faster it will appear to move away from Earth, an observation made for the first time by astronomer Edwin Hubble in 1929.
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Towards a better understanding of current cosmological models
But he and his colleagues used a slightly different technique, examining the particular speed of the galaxies. This measurement includes the red shift but also takes into account the movement of the galaxies around each other. The advantage of the method is that it can detect a hidden mass which gravitatively influences the movement of galaxies and therefore discover dark matter, this invisible substance which does not emit light but exerts a gravitational pull on everything that is enough close.
Map showing the South Pole Wall, bordering the constellations of the Whale (Lepus) and the Bird of Paradise (APus). Credits: Daniel Pomarède et al. 2020
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By running algorithms examining particular movements in the galactic catalogs, the team was able to trace the three-dimensional distribution of matter in and around the galactic darkening zone. Their results are detailed in the review The Astrophysical Journal.
The resulting map shows a bubble of material more or less centered on the southernmost point of the sky, with a large wing extending north on one side towards the constellation of the Whale and another arm more squat in front of her in the direction of the constellation of the Bird of Paradise. Knowledge of the Universe on such large scales confirms our current cosmological models, says Neta Bahcall, astrophysicist at Princeton University. But it’s hard to pinpoint where exactly these huge criss-crossing structures start and end.
Sources : The Astrophysical Journal
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