Yonsei News

There are an "astronomical" number of celestial bodies in the universe. Our solar system consists of one star, eight planets, more than a hundred satellites, and numerous small asteroids; there are about 100 billion solar systems like ours in our galaxy, the "Milky Way" alone, and the number of galaxies identified so far is over 100 billion. Some galaxies gather in hundreds to form a city of "galaxy clusters." These clusters would form a "filament" when closely connected to one another, as if they were woven together like a string. Among all these celestial bodies, including stars, galaxies, galaxy clusters, filaments, and the universe itself, the most difficult things to explain are galaxies; they look too unnatural.

The calculations for the models of stars, galaxy clusters, and the universe can be done with the idea of "spherical symmetry."  Technically speaking, they are not necessarily spherical, but such assumptions can be made in an "astronomical" sense. However, galaxies are different. Most of the galaxies are spiral galaxies, just like our own, and they would have similar appearances to that of Figure 1. A spiral galaxy is a giant spinning disk. From the top, we can see several wonderful spiral arms formed by rotation, and from the side, it is flat like a pancake. This is not natural.

You have probably seen the shape of a snowflake. Based on the properties of water, where ice crystals of various shapes are formed depending on temperature and humidity, we would see a flat yet beautiful pattern from the top. It might not receive much attention when a multi-millimeter snow crystal shows such a geometric figure, but if the clouds in the autumn sky were to display such regular yet flat shapes, the entire scientific community would probably rush in to reveal its origin. It is much more challenging to science if a galaxy floating alone in space, which is a thousand times emptier than the vacuum capable of being created in the world's best laboratory, appears so unnatural. For this reason, understanding when, how, and why galaxies were formed in this shape emerges as the greatest riddle in the astronomical community.
 Figure 1) A typical appearance of a spiral galaxy.
(Left: View of the galaxy NGC 4565 from the side / Right: View of the galaxy NGC 5457 from the top.)
The 21st Century: Beginning of Cosmological Understanding

The hope of the possibility of understanding the galaxy came with the understanding of cosmology. In order to theoretically reveal the formation process of a giant galaxy, it is necessary to be able to realistically reproduce how matter clumps and scatters over a long period of time, such as the history of the universe, in space much larger than the galaxy. To do so, cosmological understanding, such as how old the universe is, how space has expanded, and how the energy of the universe is composed, including dark matter and ordinary matter, should be preceded. This became only possible at the beginning of the 21st century when humanity's holistic cosmological understanding began.

The universe's expansion was discovered early in the 1920s through the observation of galaxies (the Hubble–Lemaître law) and the discovery of cosmic microwave background radiation (CMBR) in the 1960s. The finding that there is six times more cold dark matter with mass than baryons or ordinary matter that consists of atoms in our universe was made from observations of various galaxies, galaxy clusters, and filaments from the 1930s to the 1990s. Finally, by the beginning of the 21st century, we reached the hypothesis that most of the net energy in the universe today is in the form of dark energy. There are still big questions, such as why the universe began with a big explosion called the "Big Bang," why the universe has such an energy density that seems to be "designed," and the identities of dark matter and dark energy if they really exist, but we reached some point to have a cosmological understanding, or a "concordance model," that can provide explanations to a lot of accumulated observation data to some extent.

The important characteristic of the concordance model is its energy composition. In the current universe, approximately 4% of the total energy density is taken up by ordinary matter composed of atoms. This includes all the stars, galaxies, and gases we usually know. The remaining 24% is dark matter, and 72% is dark energy. Though the composition of dark matter and dark energy are measured relatively consistently in various ways, we do not know their exact identities. Although it may sound unconvincing to the general public, the concordance model draws keen attention among astronomers due to the much circumstantial evidence to support it. The most important of all the evidence is the observational data of the CMBR.
 

CMBR Explains the Concordance Model

The early universe's light particles, which were hot at an absolute temperature of 3,000 degrees at 380,000 years after the Big Bang, have since expanded a thousand times – 1 billion times in volume – and have been discovered as light "fossils" with 3 degrees of energy today. This CMBR has been found countless times and supports the concordance model. CMBR tells us the distribution of matter density in the early universe. According to calculations, the universe had a density difference of 1/100,000 per region. In other words, the distribution of matter in the universe was almost uniform, with only minor differences.

According to the concordance model, our universe is now approximately 13.7 billion years old. The universe has been continuously expanding since the Big Bang. There are three essential pieces of information we need to understand the formation of galaxies. First, what density distribution did the universe start with in the beginning? The CMBR told us the answer. Second, how long has the universe been expanding? In other words, it is about the universe's age, which tells how much time the initial density distribution can change over time. Lastly, we should know in what way the universe has expanded. That is the energy composition of the universe, in particular, the properties and quantity of dark matter and dark energy. Accordingly, the matter may or may not gather more easily, even if the universe expands during the same time. Just as a giant snowman requires a lot of easy-to-clump snow, a large galaxy requires a lot of cold dark matter that is good at gravitational aggregation. All these three pieces of information are crucial to galaxy formation. Now that we have a cosmological model that we can rely on to some extent, so let's look at how the concordance model universe presents galaxy formation.
 

Recreating the Galaxy Formation Using a Supercomputer

Large galaxies, such as the Milky Way, are about 60,000 light-years in diameter, and various matters have flocked from every corner of the universe to become the galaxies of the present day. In order to understand the formation history of such large galaxies, we must study the movement of matter from the beginning of the universe in areas that are about a million times the volume that galaxies currently occupy. In simple terms, if we want to know the origins of 10 million residents living in Seoul, which is approximately 600 square kilometers wide as of 2021, we would have to find out about the human movements across the entire planet over the last tens of thousands of years. To come up with a collective explanation for the formation of galaxies, there is a need to recreate the formation of as many galaxies as possible.

As such, in 2014, researchers from the teams "Eagle (UK and the Netherlands)," "Horizon-AGN (France)," and "Illustris (Germany and US)" experimented with how galaxies were formed in a cuboid universe with about 300 million light-years of width independently. To pull this off, it took a year of calculations using a supercomputer that calculates with thousands of computer cores in parallel. In the end, each team succeeded in recreating 100,000 different-looking galaxies within their experimental universe. It was the first officially recorded moment humans had grasped galaxies' origin. This confirmed that the current cosmological understanding provides a plausible background for the formation of galaxies.

However, as these simulations were aimed at creating as many galaxies as possible in such ample space, the appearance or characteristics of each galaxy could not be elaborately recreated. Figuratively speaking, although we managed to take pictures of buildings across the Han River to gain awareness of the locations and types of buildings, there were difficulties in knowing precisely how they were built. This was an urgent matter to solve.

This is how the "New Horizon" research team was born. The Institut d'Astrophysique de Paris (IAP) and Yonsei University's international joint research team has conducted parallel supercomputing by tying 4,800 cores over three years from 2017, reproducing the creation of more than 200 galaxies in a relatively small space of 60 million light-years in diameter. The spatial resolution of the experiment is 100 light-years, which is an eight-fold increase from the previous experiment to form the world's highest-resolution galaxy and a five-hundred-fold increase in spatial resolution.


Stars and Galaxies Formed by Gravitational Fields

During the first few billion years in the universe, the gravitational phenomena of dark matter governing the universe's mass can be noticed. Initially, there was only a slight difference in density, but over a long period of time, dense areas become denser. The combination of gravity and time is genuinely frightening. About two billion years later, dark matter would gather gravity throughout the universe to create a pool known as a gravitational field, the so-called "dark halo." As time passes, the dark halo grows, and structures in the universe that were not present in the past begin to form. Of course, they are dark matter structures that are not visible to the eyes or telescopes. The gases, which are ordinary matter, had no idea where to go and get sucked into the gravitational field dug by dark matter, like people flocking towards a city that was already built.

The baryon gases, which have flocked to the center of the dark halo, go through cooling through the process of transition among various energy levels of simple atoms that existed at that time, and they would eventually form stars when the temperature drops sufficiently. The larger the mass of the newborn star, the higher the temperature of the core, which causes explosive fusion and creates complex elements that are heavier than helium. Stars that are ten times larger than the sun's mass would end up with an extremely short life span - in the concept of astronomy - of about 10 million years, after which they would explode in various ways, one of which being a supernova. From this process, all kinds of heavy elements, such as oxygen, neon, silicon, magnesium, and iron, are dispersed into space. Since heavy elements contain more electrons, they are easier to cool due to having far more possible energy transitions, making it easier for new gases with heavy elements to form new stars. This phenomenon, almost like a bullet train out of control, leads to the birth of galaxies.

Figure 2) One of the 200 recreated galaxies from the simulation experiment of New Horizon.
It is very similar to the actual spiral galaxies found today, as shown in Figure 1.
This signifies that the origin of spiral galaxies, which account for more than 70 percent of the universe, has been factually revealed.
The primitive galaxies born through such processes have undetermined and chaotic appearances. As in the beginning, gases would enter from all different directions, and the clumped gases merge through the process of chaotic collisions. During the galaxy's formation, gases continue to flow in from the outside. These external gases would first enter the dark halo and flow in the footsteps of the gases that entered beforehand, just like rainwater descends the already-made pathway in the mountain on a rainy day. Each galaxy would have a small number of such "waterways," for which the vector sum is difficult to completely offset and bound to have a particular direction. Eventually, gases gathered would reach the galaxy and rotate in one direction according to the sum of the vectors. The stars formed from that rotating gaseous disk would also rotate at the same angular momentum. We can compare it to how citizens living in Seoul might have countless areas of origin, but a few major branches of origin and their characteristics form the image of the city we know today. This is how the flat disk-shaped spiral galaxy was born (Figure 2). Galaxies have been considered objects of appreciation so far and now have become science subjects.

If the secret of the galaxy's formation is an onion, it feels like the first shell has been peeled off at this moment. There are still many problems to solve, such as the function of a black hole, which is hundreds of millions of times the mass of the sun and located in the center of the galaxy, the efficiencies through which stars are formed as gases gather, and the interactions of energy circulations between stars formed and the galaxy. Over the past three decades or so, the Hubble Space Telescope has discovered young galaxies that existed 10 billion years ago through the "Ultra Deep Field project." The "James Webb Space Telescope," which is set to be launched into space before this Christmas to replace the Hubble Space Telescope, is much larger and more capable, giving us a more detailed view of the smaller, darker celestial bodies. Perhaps, we might have the chance to observe the actual formation of galaxies by observing the young universe only 100 million years after the Big Bang. When that happens, will we be able to find out for sure if the galaxy formation model created by our research team is accurate? I am nervous yet excited.
 

*Images sources

Figure 1) NGC 4565 (Howard Trottier)
http://annesastronomynews.com/photo-gallery-ii/galaxies-clusters/the-needle-galaxy-ngc-4565/

Figure 1) NGC 5457 NASA/ESA (K. Kuntz, F. Bresolin, J. Trauger and others)
https://hubblesite.org/contents/media/images/2006/10/1865-Image.html?keyword=NGC%205457

Figure 2) Yonsei University Center for Galaxy Evolution Research (Jaekyoung Jang, Yi Sukyoung)