milky way

According to Simulations, the Milky Way is One in a Million

Humanity is in a back-and-forth relationship with nature. First, we thought we were at the center of everything, with the Sun and the entire cosmos rotating around our little planet. We eventually realized that wasn’t true. Over the centuries, we’ve found that though Earth and life might be rare, our Sun is pretty normal, our Solar System is relatively non-descript, and even our galaxy is one of the billions of spiral galaxies, a type that makes up 60% of the galaxies in the Universe.

But the Illustris TNG simulation shows that the Milky Way is special.

Illustris TNG is an ongoing series of large-scale simulations. The goal is to understand the mechanisms behind galaxy formation and evolution. The effort is a “series of large, cosmological magnetohydrodynamical simulations,” according to the Illustris TNG website. So far, the project has produced three primary runs, each one larger and higher resolution than the previous one: TNG 50, TNG 100, and TNG 300. Each run also focuses on various aspects of galaxy formation. TNG 300 is the largest, simulating a region of almost 300 million megaparsecs, over a billion light-years across, and containing millions of galaxies.

TNG 50, TNG 100, and TNG 300. Image: IllustrisTNG

New research based on Illustris TNG shows that the Milky Way is special. But it’s not special purely for its intrinsic qualities. It’s special in relation to its surroundings.

The findings are in a new paper based on Illustris TNG 300 published in the Monthly Notices of the Royal Astronomical Society. The title is “The unusual Milky Way-local sheet system: implications for spin strength and alignment.” The lead researcher is Miguel Aragón, a computational cosmologist and assistant professor at the National Astronomical Observatory, Universidad Nacional Autonoma de Mexico.

Illustris TNG simulates the large-scale structure of the Universe. It shows how galaxies are arranged on filaments of dark matter that weave their way through vast cosmic voids. Some of the features it shows are cosmological walls, also called galaxy walls. They’re enormous features, and one of them—a wall called the Hercules–Corona Borealis Great Wall—is the largest known structure in the Universe and is 10 billion light-years long.

This image from TNG 50 shows the large-scale structure of cosmic gas in the early Universe at redshift three. It shows a region of space 15 megaparsecs across, where the cosmic web of gas filaments come together to fuel galactic formation and growth. Image Credit: Illustris TNG 50.

Cosmological walls are made up of galaxies. They’re a subtype of filaments, but they’re flattened and have voids on either side. The voids seem to squash the walls into their flattened shape. The cosmological wall nearest the Milky Way is called the Local Wall or Local Sheet.

The Local Sheet influences how the Milky Way and other nearby galaxies rotate on their axes. The Milky Way takes about 250,000,000 years to rotate, and the study shows that the rotation is more organized than if the galaxy wasn’t near the Local Sheet.

The study also shows that the Milky Way is special. While typical galaxies tend to be much smaller in relation to walls, the Milky Way is surprisingly massive in relation to the Local Wall. According to the research, this is a rare cosmic occurrence.

This video is from Illustris TNG 50 and shows the formation of an elliptical galaxy. Credit: Illustris TNG

One characteristic that cosmologists study is velocity dispersion. It describes how much dispersal there is in the velocity of a group of astronomical objects. The velocities are dispersed around a mean velocity. The velocity of objects in the Milky Way/Local Wall neighbourhood has low dispersion, meaning they’re not dispersed far from the mean.

This is unusual for a high-mass galaxy like the Milky Way in an environment like this, close to the Local Wall. To understand this better, the researchers looked for Milky Way Analogue (MWA) galaxies in Illustris TNG 300.

They found that MWAs in Local-Sheet analogues are rare. There was only one of them per 160–200?Mpc3  of volume in the simulation. With their research, they showed that the cold environment around the Local Sheet is responsible. “We find that a Sheet-like cold environment preserves, amplifies, or simplifies environmental effects on the angular momentum of galaxies,” they write in their paper.

Specifically, the Local Sheet affects the spin of the Milky Way. “… there are particularly strong alignments between the sheet and galaxy spins,” they explain, adding that in the simulation, the galaxies near walls have low spin parameters.

This all affects how galaxies grow and merge over time, they think. It leads to lower-mass galaxies in these types of cosmic neighbourhoods. That’s why the Milky Way, with its high mass, is so unusual and why the simulation found only one like it in up to 200 cubic megaparsecs of space.

This figure from the study shows how velocity dispersal relates to mass, with mass shown on the x-axis. The legend in the upper right shows how each line in the graph represents a different velocity dispersal (sigma v.) The blue solid line is the mass function in cold (<25, high velocity dispersal) regions, and the black solid line is the mass function in warm (<40, low velocity dispersal) regions. The dotted lines are the same, but for regions near Cosmic Walls. It shows how lower velocity dispersal produces less massive galaxies near walls. There are also two shaded grey areas: light and dark grey. The light area represents the masses of Milky Way Analogues in the simulation, and the dark grey shows where the Milky Way actually is. Image Credit: Aragon et al. 2023.

The study reminds us of something critical: context is important. If we look at the Milky Way as a discrete object and compare it to other similar discrete objects, it doesn’t appear to be exceptional. But in relation to its surroundings, it is. “Our results highlight the importance of carefully characterizing the environment around our galaxy,” the paper states. “The effect of the geometry and coldness of Local Sheet environment on angular momentum processes may help us better understand current problems in galaxy formation…” 

In a press release presenting the research, the authors refer to the Copernican bias. “This bias, describing the successive removal of our special status in the nearly 500 years since Copernicus demoted the Earth from being at the centre of the cosmos, would come from assuming that we reside in a completely average place in the Universe,” the press release says. It shows the risk inherent in ignoring the environment of an object being studied.

This work also shows a potential flaw in how scientists use simulations like Illustris TNG. It’s misleading to think that any point in the simulation is the same as any other point. Galaxies close to a Cosmic Wall can evolve quite differently than at other points.

“So, the Milky Way is, in a way, special,” said research lead Miguel Aragón. “The Earth is very obviously special, the only home of life that we know. But it’s not the centre 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.”

“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, said. “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.”

Spiral galaxies are common. This image shows six spectacular spiral galaxies in images from the ESO’s Very Large Telescope (VLT) at the Paranal Observatory in Chile. Credit: ESO

“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 adds, “That’s a couple of hundred times farther away than the nearest large galaxy around us, Andromeda.”

So is it okay if we feel special again? We’re obviously special just because we’re alive, and most matter we can see isn’t. But that doesn’t necessarily tell us about how much other matter might be alive and if we’re special. From a vantage point elsewhere in the Universe, there could be much more living matter. Before the rise of modern astronomy, we had no idea if there was life elsewhere or how special Earth might be. We’re wise to be cautious with the word special, according to one of the authors.

“You do have to be careful, though, choosing properties that qualify as ‘special,’” Dr. Mark Neyrinck, another member of the team, 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.”


Evan Gough

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