Lessons from Simulating Intergalactic “Pancakes”: Studying the Nature of Matter Between Galaxies

“We are stardust,” sang Joni Mitchell at Woodstock. Yes, with the exception of dark matter, the atoms that make up the vastness of space and all the galaxies within it are essentially the same as the atoms that make up you and me. Today, despite advances in our understanding of space, many questions still plague scientists regarding how matter is distributed throughout the universe. Almost all visible matter has atomic nuclei made of protons and neutrons, both of which fall into the class of particles called baryons. To study the behavior of baryons, a team of researchers set their sights on the intergalactic medium (IGM), the spaces between galaxies, for answers. Thanks to a recent study conducted by Yale postdoctoral associate Nir Mandelker and Yale professor of astronomy Frank van den Bosch in collaboration with colleagues at the Max Planck Institute for Astrophysics and Heidelberg Institute for Theoretical Studies in Germany, a few of these mysteries were uncovered by conducting what is currently the most detailed simulation of the IGM ever reported.

When most people think of the universe, they likely envision thousands of large spiraling galaxies. What is not immediately clear is that these galaxies are mostly occupied by empty space. Despite the billions of stars each galaxy contains, only a small proportion of all baryons can be found within these interstellar units. The vast majority of baryons—likely upwards of eighty percent—are housed within the IGM and the circumgalactic medium (CGM), which is the gaseous space immediately outside a galaxy that extends for tens or even hundreds of light years. “Most of the work focuses on the center of dark matter halos,” the dark matter that surrounds every galaxy, and “what happens when you resolve the intergalactic medium at higher resolutions,” van den Bosch said. This large reservoir of baryons between galaxies means that studying the IGM is essential to further our understanding of galactic evolution, as baryons are a primary source of matter that contribute to the formation of stars, solar systems, and in turn, ourselves. But because the matter between IGM does not shine like stars, it is nearly impossible to observe it directly. Instead, researchers must turn to computer simulations. 

A Simulation of Cosmic Proportions

Using what is known as a cosmological simulation, the researchers began their investigation by approximating the conditions of the early universe using the cosmic microwave background radiation (CMBR). The CMBR—also known as electromagnetic “relic radiation”—exists as a faint buzz filling all space and provides valuable insight into the physical properties of the early universe. With these initial conditions, the simulation can then account for the expansion of the universe, gravity, the existence of dark matter and dark energy, star formations, the explosion of supernovae, and other large-scale physical and chemical phenomena with time. “The simulation allows us to make predictions for the structures of galaxies, or in our case, the structure of the intergalactic medium,” Mandelker said.

There exist two basic approaches to these cosmological simulations. The first is to simulate a large volume of the universe by grossly simplifying each galaxy, so that only a large statistical sample of galaxies and intergalactic space is obtained. This method gives bulk properties and accounts for statistical variation present between galaxies; however, it does not provide detailed information on any individual galaxy or area of space. The second method, known as a zoom-in simulation, involves a high-resolution analysis of only one or two galaxies, enabling researchers to capture a single object or region of space in great detail. However, as a result, the galaxy studied becomes isolated from its low-resolution surroundings.

“What we did is a hybrid approach between the two methods. We want high resolution not for a single galaxy, but for the intergalactic medium,” Mandelker said. The simulation was run in a one-billion-year-old universe, as opposed to the fourteen-billion-year-old universe we find ourselves in today. The initial, lower resolution model contained hundreds of halos. To study the space between galaxies, two of them were randomly selected based on size and other criteria. Each selected galaxy had a mass a trillion times that of our Sun. These two galaxies were one megaparsec—or about three-million light years—apart. The halos were observed to co-exist within a “pancake” sheet structure, a distribution of matter in a flattened shape.

Non-Metallic Pancakes

Beginning in a universe less than one billion years old, the simulation revealed two large sheets of gas present between the two galaxies. Inclined towards one another, these sheets would soon collapse and later merge around the universe’s one-billionth birthday. Collision between the two sheets ultimately led to a shock and period of drastic temperature fluctuations, causing the sheets to fall apart. Within these shattered sheets arose collections of dense gas clouds with temperatures above ten thousand Kelvin, resulting in a cosmic fog. “Low density gas can clump together and form little cloudlets, like water vapor in the air. Looking through this gas results in the absorption of the light, explaining…quasar spectra,” van den Bosch said. Quasars are massive, energetic objects in space that shine approximately one hundred times brighter than the host galaxies in which they are found, so their emissions can therefore be measured.

These cloudlets were previously thought to form only within the CGM because the density of the gas was expected to increase as it approached the galaxy. Contrary to this hypothesis, the simulation by Mandelker and van den Bosch demonstrates that the formation of clouds of matter can also occur in the IGM as well, which is lower in density and further away from any galaxy. According to the simulation, rapid cooling of the gas results in thermal instability, ultimately leading the medium to shatter into small clouds. This shattering allows the gas to take on a more energetically favorable state and equalize its pressure with its surroundings.

The simulation tracked the production and distribution of nine elements produced in supernovae: hydrogen, helium, carbon, nitrogen, oxygen, neon, magnesium, silicon, and iron. This simulated cosmic fog did not contain any heavy metal atoms because it is too far away from either galaxy to have any heavy metal pollution, confirming recent and puzzling findings from observed IGM systems. Astronomers previously had difficulty explaining the presence of these large, non-metallic, isolated regions of space. The simulation suggests that these regions exist due to natural formation processes. “Initially when we saw this in our simulations, we thought, ‘something is going wrong numerically,’ but then we realized that this [process] is actually predicted—it is probably a physical process,” van den Bosch said.

Looking forward into the universe

When modeling such a large astronomical system, limitations must be acknowledged, especially regarding the numerical accuracy of calculations. “A lot of what’s going on is highly nonlinear, and therefore the analytical calculations we can do are highly oversimplified,” van den Bosch said. However, researchers appreciate that simulations often produce unexpected data depending on the numerical properties of the simulation.

Moving forward, the research team hopes to use higher resolution simulations to better understand how often temperature shocks occur, what triggers the gaseous phase transition, and what fraction of the gas they expect to form in these foggy cloudlets. According to van den Bosch, the researchers have only marginally resolved the process, and there is still speculation on possible outcomes depending on how the sheets of matter collide. “The universe is full of surprises: even very well-known and simple physics can continue to surprise us to this very day,” van den Bosch said.