The Universe is not just a random scattering of galaxies scattered across an expanding void. The closer we look, the more we see that there are structures, some of which are incomprehensibly vast clusters and clusters of galaxies that are gravitationally bonded.
Such a structure has just been discovered arching at the southern edge of the sky, and it is a colossus, spanning an immense amount of 1.37 billion light years from end to end. Its discoverers have called it the wall of the South Pole.
Although the size is remarkable, it is one of the largest structures in space that we have seen, we know exactly what the South Pole Wall is. It is a galaxy filament, a large galaxy formation that forms a border between the cosmic void voids that together form the cosmic lattice. Therefore, we call it a wall.
Other larger walls are known. The largest is the Hercules-Boreal Crown Great Wall, spanning 9.7 billion light years. But the South Pole Wall is special, because it is incredibly close to the Milky Way galaxy, just 500 million light-years away. In other words, it is the most massive structure we have seen so far.
You are probably wondering how in Laniakea we managed to lose one of the Universe’s largest galaxy strands right on our doorstep. And there is actually a very good answer to that question: It was hidden behind what astronomers sometimes call the Zone of Avoidance or Galactic Darkening Zone: the galactic plane of the Milky Way.
This is the disk of our natal galaxy, a thick and bright region with dust, gas and stars. So thick and bright, in fact, that it obscures a large part of what’s behind it, making that region of the Universe unexplored compared to the rest.
If this avoidance zone concealed the South Pole wall so effectively, how have astronomers found it now? The answer to that question is a little more complex, but it basically depends on how galaxies move around the sky.
A team of researchers led by cosmographer Daniel Pomarède of the Paris-Saclay University used a database called Cosmicflows-3, which contains distance calculations to almost 18,000 galaxies. These are calculated using redshift, which measures how fast something moves away based on the extent of its light waves.
Last year, a different team of researchers used this database to calculate another parameter, something known as peculiar speed, which is the speed of a galaxy in relation to its movement due to the expansion of the Universe.
With these two parameters, the team could calculate the movements of the galaxies with each other, and these movements revealed the gravitational influence of a much larger mass. With the help of algorithms, the team was able to use these movements to map the distribution of material on the South Pole Wall in three dimensions, even beyond the Avoidance Zone.
(Pomarède et al., ApJ, 2020)
The densest section is above the South Pole: this part is the section that is 500 million light years away. Then it curves north and toward us, reaching 300 million light-years from the Milky Way.
Along the curved arm, galaxies are moving toward the group at the South Pole; And from there, they are moving towards another giant structure, the Shapley Supercluster 650 million light years away.
Because there are parts of the South Pole wall that we cannot see, it is possible that the structure is even larger than we can see right now. But we can be sure that astronomers are eager to find out.
On the one hand, it could have interesting cosmological implications, affecting the expansion rate of the local Universe. This may or may not play a role in Hubble Stress, the difference between the rate of expansion in the local Universe and the rate of expansion in the early Universe.
It can also help us understand the evolution of our local corner of space, which includes Laniakea, the supercluster of galaxies to which the Milky Way belongs, also discovered by Pomarède and colleagues (led by Brent Tully of the University of Hawaii at Manoa) in 2014.
This is a magnificent discovery, and we can’t wait to see what else it will reveal.
The research has been published in The Astrophysical Journal.
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