Our home, the Milky Way, doesn’t seem particularly odd for a galaxy. Medium-sized, spiral, with a few kinks suggests a destructive past.
But astronomers have just identified a quirk never seen before in any galaxy studied so far: the Milky Way is too large for its surroundings.
Specifically, it seems too big for the neighborhood it’s in local paper. This oblate array of galaxies shares similar velocities, bounded by relatively empty Space called Voids on either side.
Our local newspaper, as an example of a ‘cosmological wall’, separates the local emptiness in one direction from the southern void to the other.
The relationship between the galaxies in the Local Sheet seems to have a strong influence on their behavior; for example their similar speeds relative to the expansion of the universe. Outside the cosmological wall environment, these velocities would have a much greater range.
To determine how the environment affects the galaxies around us, a team of astronomers led by Miguel Aragón of the National Autonomous University of Mexico conducted an analysis using simulations from a project called IllustrisTNGthat models the physical universe.
They didn’t expect to find anything particularly extraordinary.
“The Milky Way is special in a way” Says Aragon. “Earth is obviously special, the only home of life that we know. But it is not the center of the universe or even the solar system. And the Sun is just one ordinary star among billions in the Milky Way. Even our galaxy appeared to be just another spiral galaxy among billions of others in the observable universe.”
But when they simulated a volume of space about a billion light-years across containing millions of galaxies, the picture was different: only a handful of Milky Way-sized galaxies could be contained within a cosmological wall structure.
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“The Milky Way doesn’t have a particularly specific mass or type. There are many spiral galaxies that look something like this.” says astronomer Joe Silk from the Institut d’Astrophysique de Paris of the Sorbonne University in France.
“But it’s rare considering its surroundings. If one could easily see the next dozen or so large galaxies in the sky, one would see that they all lie almost on a ring embedded in the local sheet. This is little special in itself. What we found new is that other walls of galaxies in the Universe like the Local Sheet very rarely seem to contain a galaxy as massive as the Milky Way.”
The team’s analysis did not take into account Andromeda, the Milky Way’s largest galactic neighbor. Also a feature of the Local Sheet – and therefore part of the same cosmological wall – is a galaxy similar in size to the Milky Way. Since it would be even rarer to have two heavyweights in a cosmological wall, their conclusions still hold.
However, research shows that when studying the Milky Way, we may need to consider our local environment rather than assuming that our home hangs in an average way in an average place in the universe.
Because the team’s simulations only considered the context of the Milky Way within a cosmological wall, future work might consider more galaxies within the Local Group. The researchers also suggest that the environmental context may help explain some previously unexplained phenomena, such as the unusual arrangement of satellite galaxies around Andromeda and the peculiar lack of them around the Milky Way.
“You have to be careful … in choosing properties that are considered ‘special'”, says astronomer Mark Neyrinck the Basque Foundation for Science in Spain.
“If we put a ridiculously restrictive condition on a galaxy, like having it contain the paper we wrote about it, surely we’d be the only galaxy in the observable universe that way. But we think this is ‘too big for your wall’. Property is physically meaningful and observably relevant enough to qualify as truly special.”
The research was published in Monthly Bulletins of the Royal Astronomical Society.
