The Milky Way and the Local Sheet form a peculiar galaxy in terms of the unusually low velocity dispersion in our neighborhood and the apparent high mass of the Milky Way for such an environment. Using the TNG300 simulation, an international team of astronomers searched for Milky Way analogues (MWA) in the cosmological walls.
A cosmological wall is an oblate arrangement of galaxies that surrounds other galaxies and is characterized by exceptionally empty regions on either side of it, known as “voids”. The flattened arrangement is produced by these cavities compressing the galaxies into a pancake-like structure.
This wall environment, known in this case as the Local Sheet, has a more organized effect on the rotation of the Milky Way and neighboring galaxies around their axes than if we were at any random location in the Universe without a wall.
Usually, galaxies are significantly smaller than this so-called wall. The study revealed that the Milky Way is too big for its “cosmological wall,” something not yet seen in other galaxies. This is a rare cosmic event.
As part of the IllustrisTNG project, the team simulated a volume of the Universe, nearly a billion light-years in diameter, containing millions of galaxies. There were very few galaxies in the simulation as “special” as the Milky Way, as large and cosmic-walled as the Local Leaf, or even one millionth of all galaxies.
The team believes it may be critical to consider the unique environment surrounding the Milky Way when running simulations to avoid what is known as a “Copernican bias” in inferring the galaxies near us. This bias would be due to the assumption that our location in the universe is fully averaged. It describes the gradual erosion of our unique status over the nearly 500 years since Copernicus relegated the Earth from the center of the cosmos.
To simulate observations, astronomers sometimes assume that every point in a simulation like IllustrisTNG is just as good as any other point. Still, the team’s findings indicate that using precise locations to make such measurements may be essential.
Research leader Miguel Aragon said: “So the Milky Way is special in a way. Earth is obviously special, the only home of life we know. But it is 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.”
Joe Silk, another of the researchers, said: “The Milky Way has no particularly special mass or type. There are many spiral galaxies that look something like this. But it’s rare when you consider the environment. If you could easily see the nearest dozen large galaxies in the sky, you would see that almost all of them lie on a ring embedded in the Local Sheet. That in itself is a bit special. We recently discovered that other galaxy walls in the Universe, such as the Local Sheet, very rarely seem to contain a galaxy as massive as the Milky Way.”
Aragon said, “You might have to travel half a billion light-years from the Milky Way, past lots of galaxies, to find another cosmological wall with a galaxy like ours. That’s several hundred times further away than the closest large galaxy to us, Andromeda.”
Dr. Mark Neyrinck, another team member, said: “However, you have to be careful about choosing properties that qualify as ‘special’. If we were to add some ridiculously restrictive condition to a galaxy, like that it must contain the article we wrote about it, then surely we would be the only galaxy in the observable universe that does. But this ‘too big for the wall’ property is physically meaningful and observationally relevant enough to call it truly special.”
- Aragon-Calvo et al. The unusual galaxy-local leaf system: implications for spin force and alignment. Monthly Notices from the Royal Astronomical Society: Letters. DOI: 10.1093/mnrasl/slac161