In keeping with simulations, the Milky Manner is one in 1,000,000

Mankind is in an interplay with nature. At first we thought we were in the center of everything, with the sun and the entire cosmos revolving around our little planet. At some point we realized that that wasn’t true. Over the centuries we have found that while Earth and life are rare, our sun is fairly normal, our solar system is relatively nondescript, and even our galaxy is one of the billions of spiral galaxies, a type that accounts for 60% of the galaxies in the universe.

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

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

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

New research based on Illustris TNG shows the Milky Way is something special. But it’s not just special because of its intrinsic qualities. It is special in relation to its surroundings.

The results are included in a new article based on Illustris TNG 300 published in the Monthly Notices of the Royal Astronomical Society. The title is “The Milky Way’s unusual local leaf system: Implications for spin strength and orientation”. The lead researcher is Miguel Aragón, a computational cosmologist and assistant professor at the National Astronomical Observatory of the 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 are huge structures, and one of them – a wall called Hercules – Corona Borealis Great Wall – is the largest known structure in the universe and is 10 billion light years long.

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

Cosmological walls are made up of galaxies. They are a subspecies of filaments, but they are flattened and have cavities on either side. The voids seem to squeeze the walls into their flattened shape. The cosmological wall closest to the Milky Way is called the local wall or local sheet.

The local sheet affects how the Milky Way and other nearby galaxies rotate about their axes. The Milky Way takes about 250 million years to rotate, and the study shows that the rotation is more organized than if the galaxy weren’t near the local sheet.

The study also shows that the Milky Way is something special. While typical galaxies tend to be much smaller compared to walls, the Milky Way is surprisingly massive compared to the local wall. According to research, this is a rare cosmic event.

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

One property that cosmologists study is velocity dispersion. It describes how much spread there is in the velocity of a group of astronomical objects. The speeds scatter around an average speed. The velocity of objects in the Milky Way/Local Wall neighborhood has a small spread, which means they are not far from the mean.

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

They found that MWAs are rare in local sheet analogues. In the simulation there was only one of these per 160-200 Mpc3 volume. With their research, they showed that the cold environment around the Local Sheet is to blame. “We find that a plate-like cold environment preserves, enhances, or facilitates environmental influences on the angular momentum of galaxies,” they write in their paper.

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

All of this affects how galaxies grow and merge over time, they think. It leads to lower-mass galaxies in such cosmic neighborhoods. That’s why the Milky Way is so unusual with its high mass and why the simulation has only found one in up to 200 cubic megaparsecs of space.

This figure from the study shows how the velocity spread is related to mass, with mass plotted on the x-axis. The legend at top right shows how each line on the graph represents a different velocity spread (sigma v). The blue solid line is the mass function in cold regions (<25, high velocity propagation) and the black solid line is the mass function in warm regions (<40, low velocity propagation). The dotted lines are the same but for regions near Cosmic Walls. It shows how slower propagation creates less massive galaxies near walls. There are also two shaded gray areas: light and dark gray. The light area represents the masses of the Milky Way analogues in the simulation, and the dark gray shows where the Milky Way actually is. Photo credit: Aragon et al. 2023

The study reminds us of something crucial: context matters. If we consider 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 underscore the importance of carefully characterizing our galaxy’s surroundings,” the paper states. “The effect of the geometry and coldness of the local sheet environment on angular momentum processes can help us better understand current problems in galaxy formation…”

In a press release presenting the research, the authors cite the Copernican bias. “This bias, which describes the gradual removal of our special status in the nearly 500 years since Copernicus demoted the earth from the center of the cosmos, would come from assuming that we are in a perfectly average place in the universe,” according to the press release says. It shows the risk associated with ignoring the surroundings of an inspected object.

This work also reveals a potential flaw in how scientists use simulations like Illustris TNG. It is misleading to think that every point in the simulation is identical to every other point. Galaxies near a cosmic wall can evolve very differently than elsewhere.

“So the Milky Way is special in a way,” said research leader Miguel Aragón. “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.”

“The Milky Way doesn’t have a particularly special mass or type. There are many spiral galaxies that look something like this,” said Joe Silk, another researcher. “But it’s rare considering the environment. If you could easily see the next dozen or so large galaxies in the sky you would see that they all lie almost on top of a ring embedded in the local sheet. That’s a bit special. What we found new is that other galaxy walls in the Universe like the Local Sheet very rarely seem to have a galaxy as massive as the Milky Way within them.”

Spiral galaxies are common. This image shows six spectacular spiral galaxies in images from ESO’s Very Large Telescope (VLT) at the Paranal Observatory in Chile. Photo 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 few hundred times more distant than the nearest large galaxy around, 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 how much other matter might be alive and whether we’re anything special. From another point of view in the universe, there could be much more living matter. Before the advent of modern astronomy, we had no idea if life existed elsewhere or how special Earth might be. According to one of the authors, we should be careful with the word “especially”.

“However, you have to be careful when selecting properties that are considered ‘special,'” said Dr. Mark Neyrinck, another member of the team. “If we put a ridiculously restrictive condition on a galaxy, like having it contain the paper we wrote about it, we’d certainly be the only galaxy in the observable universe of that kind. But we think these are ‘too big for its “wall” property, is physically meaningful and observable enough to be called truly special.”


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