Our galaxy is larger than should be expected for galaxies within our “cosmological wall,” a local flat cluster of galaxies known as the Local Sheet, as described in a paper published in the journal Monthly Notices of the Royal Astronomical Society: Letters on December 23, 2022.
These cosmological walls are characterized by a flattened plane of galaxies with similar velocities relative to the expansion of the universe, with empty “voids” on either side of them. Galaxies in a cosmological wall influence each other’s rotation.
The Milky Way is also in a cosmological wall called the Local Sheet. Our home galaxy is a barred spiral galaxy consisting of between 100 and 500 billion stars and measuring roughly around 100,000 light years in diameter. It was named due to its pale and milky appearance across the night sky.
Galaxies within a wall tend to be a certain size relative to the overall size of the wall. The Milky Way, however, is much larger than would be expected based on the size of our Local Sheet, despite not being an uncommonly large galaxy on a universal scale.
“So the Milky Way is, in a way, special,” said Miguel Aragón, lead author and astronomer at Instituto de Astronomía in Mexico, in a statement.
“The Earth is very obviously special, the only home of life that we know. But it’s not the center of the universe, or even the solar system. And the sun is just an ordinary star among billions in the Milky Way. Even our galaxy seemed to be just another spiral galaxy among billions of others in the observable universe.”
“You might have to travel half a billion light years from the Milky Way, past many, many galaxies, to find another cosmological wall with a galaxy like ours,” Aragón said. He added: “That’s a couple of hundred times farther away than the nearest large galaxy around us, Andromeda.”
In the paper, the researchers described how they simulated millions of galaxies across a volume of space about a billion light-years across. They discovered that a galaxy as large as the Milky Way existing in a cosmological wall of the scale of our Local Sheet was uncommon.
“The Milky Way doesn’t have a particularly special mass, or type. There are lots of spiral galaxies that look roughly like it,” Joe Silk, another of the researchers and astronomer at Sorbonne University’s Institut d’Astrophysique de Paris in France, said in the statement.
“But it is rare if you take into account its surroundings. If you could see the nearest dozen or so large galaxies easily in the sky, you would see that they all nearly lie on a ring, embedded in the Local Sheet. That’s a little bit special in itself. What we newly found is that other walls of galaxies in the universe like the Local Sheet very seldom seem to have a galaxy inside them that’s as massive as the Milky Way.”
They found that to find another galaxy just as outsized in its own wall as the Milky Way is, you’d have to travel 160–200 megaparsecs, or between around 521,600,000 and 652,000,000 light years. In relative terms, the distance between the Earth and the sun is 0.000015565 light years—8 light minutes—and the distance between us and our nearest star system, Alpha Centauri, is around 4 light years.
The researchers did not include the presence of Andromeda, our closest galactic neighbor, in the analysis. Since Andromeda is also in our Local Sheet, and is actually even larger than the Milky Way, this further exemplifies how rare our galactic neighborhood is.
“Note that after the two galaxies merge, the situation will be the same as we analyze here, except with a larger combined mass. A cold wall with a galaxy of this larger mass would be extremely rare,” the authors wrote in the paper.
The Milky Way and Andromeda are due to collide and combine to form a mega-galaxy nicknamed Milkdromeda in around 4.5 billion years’ time.
“You do have to be careful, though, choosing properties that qualify as ‘special,’” Neyrinck, another author and astronomer at the Basque Foundation for Science in Spain, said. “If we added a ridiculously restrictive condition on a galaxy, such as that it must contain the paper we wrote about this, we would certainly be the only galaxy in the observable universe like that. But we think this ’too big for its wall’ property is physically meaningful and observationally relevant enough to call out as really being special.”
The researchers note that their study combats our “Copernican bias,” which is the assumption that our home galaxy is a completely average and uninteresting place.
Clearly, that isn’t quite as true as we had previously thought.
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References
M A Aragon-Calvo et al, The unusual Milky Way-local sheet system: implications for spin strength and alignment, Monthly Notices of the Royal Astronomical Society: Letters, 2022. doi: 10.1093/mnrasl/slac161
