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 universe revolving around our little planet. We eventually realized this wasn’t true. Over the centuries, we have found that although Earth and life are scarce, our Sun is very normal, our solar system is relatively nondescript, and even our galaxy is one of billions of spiral galaxies, the type that makes up 60% of the galaxies in the universe.

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

Illustris TNG is an ongoing series of large-scale simulations. The goal is to understand the mechanisms underlying galaxy formation and evolution. The effort is “a series of large scale magnetodynamic cosmic simulations,” according to the Illustris TNG website. So far, the project has produced three first runs, each larger and louder 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 about 300 million megafrescos in size, spanning more than a billion light-years, and containing millions of galaxies.

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

New research based on Illustris TNG shows that the Milky Way is special. But it is not special purely for its intrinsic qualities. She is particular about her surroundings.

The findings come in a new paper based on Illustris TNG 300 published in the Monthly Notices of the Royal Astronomical Society. The title is “An Unusual Plate System of the Milky Way: Implications for Rotational Force and Alignment.” The principal investigator is Miguel Aragon, computational cosmologist and assistant professor at the National Astronomical Observatory, National Autonomous University of Mexico.

Illustris TNG simulates the large-scale structure of the universe. It shows how galaxies are arranged on strands of dark matter that weave their way through the vast cosmic voids. Some of the features it shows are cosmic walls, also called galaxy walls. They’re massive features, 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 across.

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 coalesce to fuel galaxy formation and growth.  Image credit: Illustris TNG 50.
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 coalesce to fuel galaxy formation and growth. Image credit: Illustris TNG 50.

The cosmic walls are made up of galaxies. They are a subspecies of thread, but they are flattened and have voids on either side. The voids seem to crush the walls into their oblate shape. The cosmic wall closest to the Milky Way is called the local wall or local plate.

The local leaf affects 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 showed that the rotation is more orderly than if the galaxy was not near the local plate.

The study also shows that the Milky Way is distinct. While typical galaxies tend to be much smaller in relation to the walls, the Milky Way is surprisingly massive in relation 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. Credit: Illustris TNG

One of the properties cosmologists study is velocity dispersion. It describes the amount of dispersion present in the velocity of a group of astronomical objects. Speeds disperse around medium speed. The velocity of objects in the Milky Way/local wall has a low dispersion, which means that they are not dispersed far from the mean.

This is unusual for a galaxy as large as the Milky Way in an environment like this, so close to the local wall. To better understand this, the researchers looked for the Milky Way Analog (MWA) galaxy in Illustris TNG 300.

They found that MWAs in local lamina analogues are rare. There was only one of them for every 160-200?3 of size in the simulation. Through their research, they show that the cold environment around the local leaf is responsible. “We found that the cold, plate-like environment maintains, amplifies or simplifies environmental influences on the angular momentum of galaxies,” they wrote in their paper.

Specifically, the local sheet affects the rotation of the Milky Way. “…there are particularly strong alignments between the sheet and the rotation of the galaxies,” they explained, adding that in the simulation, the near-walled galaxies have low rotation parameters.

They think all of this affects how galaxies grow and merge over time. It leads to low-mass galaxies in these kinds of cosmic neighborhoods. This is why the Milky Way, with its high mass, is so unusual, and why the simulation has found only one like it in up to 200 megafarsks of space.

يوضح هذا الشكل من الدراسة كيف يرتبط تشتت السرعة بالكتلة ، مع إظهار الكتلة على المحور السيني.  توضح وسيلة الإيضاح الموجودة في الجزء العلوي الأيمن كيف يمثل كل خط في الرسم البياني تشتتًا مختلفًا للسرعة (سيجما v.) الخط الأزرق الصلب هو دالة الكتلة في المناطق الباردة (<25 ، تشتت عالي السرعة) ، والخط الأسود الصلب هو وظيفة الكتلة في المناطق الدافئة (<40 ، تشتت منخفض السرعة).  الخطوط المنقطة هي نفسها ، لكن بالنسبة للمناطق القريبة من الجدران الكونية.  يوضح كيف ينتج تشتت السرعة المنخفضة مجرات أقل ضخامة بالقرب من الجدران.  هناك أيضًا منطقتان مظللتان باللون الرمادي: الرمادي الفاتح والداكن.  تمثل منطقة الضوء كتل مناظير درب التبانة في المحاكاة ، ويظهر اللون الرمادي الداكن مكان وجود مجرة ​​درب التبانة بالفعل.  حقوق الصورة: أراجون وآخرون.  2023.
This figure from the study shows how velocity dispersion relates to mass, showing mass in X-axis. The legend in the upper right shows how each line in the graph represents a different velocity dispersion (sigma Fifth.) The blue solid line is the mass function in cold regions (<25, high-velocity dispersion), and the solid black line is the mass function in warm regions (<40, low-velocity dispersion). The dotted lines are the same, but for the regions near the cosmic walls. Shows how low-velocity scattering produces less massive galaxies near the walls. There are also two areas shaded with gray: light and dark gray. The light region represents the masses of the Milky Way binoculars in the simulation, and the dark gray color shows where the Milky Way actually is. Image credit: Aragon et al. 2023.

The study reminds us of something crucial: context matters. If we consider the Milky Way as a separate object and compare it to other similar separate objects, it does not appear to be exceptional. But as far as its surroundings are concerned, it is. “Our results highlight the importance of accurate characterization of the environment around our galaxy,” the paper states. “The influence of geometry and the cooling of the local plate 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 Copernican bias. “This bias, which describes the successive removal of our own status in the nearly 500 years since Copernicus demoted the Earth from the center of the universe, may come from the assumption that we live somewhere exactly in the middle of the universe,” the press release says. The danger inherent in ignoring the environment of the object under study emerges.

This work also shows a potential flaw in how scientists use simulations like Illustris TNG. It is misleading to think that any point in the simulation is the same as any other. Galaxies close to the cosmic wall can develop very differently from other points.

“So, the Milky Way is, in a way, special,” said research leader Miguel Aragon. “Earth is obviously very special, the only home of life that we know of. 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 like just another spiral galaxy among billions of other galaxies in the visible universe.”

“The Milky Way doesn’t have a particularly special mass or type. There are a lot of spiral galaxies that are almost like it,” said Joe Silk, another researcher. “But it’s rare to take into account its surroundings. 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 Leaf. This is something special in itself. What we have recently discovered is that the walls of other galaxies in the universe such as the Local Plate rarely appear to have a massive galaxy like the Milky Way. “

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

“You may have to travel half a billion light-years from the Milky Way, passing many galaxies, to find another cosmic wall with a galaxy like ours,” Aragon said. “This is a few hundred times farther away from the nearest large galaxy around us, Andromeda,” he adds.

So is it normal to feel special again? We’re obviously special just because we’re alive, and most things we can see aren’t. But that doesn’t necessarily tell us how much other matter might be alive and whether we’re special. From a point of view elsewhere in the universe, there could be a lot of living matter. Before the advent of modern astronomy, we had no idea whether there was life elsewhere or how special Earth could be. We are wise to be careful with the word SpecialAccording to one of the authors.

You do have to be careful, however, when selecting properties that qualify for this ‘Special,’said Dr. Mark Nerink, another member of the team. “If we add a ridiculously restrictive condition to a galaxy, such as it should have in the paper we wrote about this, we would certainly be the only galaxy in the observable universe like that. But we think this property of ‘too big for its wall’ has physical significance.” And relevant enough to watch to be really special.”


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