Since it was first theorized in the 1970s, astrophysicists and cosmologists have done their best to resolve the mystery that is Dark Matter. This invisible mass is believed to make up 85% of the matter in the Universe and accounts for 27% of its mass-energy density. But more than that, it also provides the large-scale skeletal structure of the Universe (the cosmic web), which dictates the motions of galaxies and material because of its gravitational influence.
Unfortunately, the mysterious nature of Dark Matter means that astronomers cannot study it directly, thus prevented them from measuring its distribution. However, it is possible to infer its distribution based on the observable influence its gravity has on local galaxies and other celestial objects. Using cutting-edge machine-learning techniques, a team of Korean-American astrophysicists was able to produce the most detailed map yet of the local Universe that shows what the “cosmic web” looks like.
A galaxy’s main business is star formation. And when they’re young, like youth everywhere, they keep themselves busy with it. But galaxies age, evolve, and experience a slow-down in their rate of star formation. Eventually, galaxies cease forming new stars altogether, and astronomers call that quenching. They’ve been studying quenching for decades, yet much about it remains a mystery.
A new study based on the IllustrisTNG simulations has found a link between a galaxy’s quenching and its stellar size.
Since time immemorial, philosophers and scholars have sought to determine how existence began. With the birth of modern astronomy, this tradition has continued and given rise to the field known as cosmology. And with the help of supercomputing, scientists are able to conduct simulations that show how the first stars and galaxies formed in our Universe and evolved over the course of billions of years.
Until recently, the most extensive and complete study was the “Illustrus” simulation, which looked at the process of galaxy formation over the course of the past 13 billion years. Seeking to break their own record, the same team recently began conducting a simulation known as “Illustris, The Next Generation,” or “IllustrisTNG”. The first round of these findings were recently released, and several more are expected to follow.
Using the Hazel Hen supercomputer at the High-Performance Computing Center Stuttgart (HLRS) – one of the three world-class German supercomputing facilities that comprise the Gauss Centre for Supercomputing (GCS) – the team conducted a simulation that will help to verify and expand on existing experimental knowledge about the earliest stages of the Universe – i.e. what happened from 300,000 years after the Big Bang to the present day.
To create this simulation, the team combined equations (such as the Theory of General Relativity) and data from modern observations into a massive computational cube that represented a large cross-section of the Universe. For some processes, such as star formation and the growth of black holes, the researchers were forced to rely on assumptions based on observations. They then employed numerical models to set this simulated Universe in motion.
Compared to their previous simulation, IllustrisTNG consisted of 3 different universes at three different resolutions – the largest of which measured 1 billion light years (300 megaparsecs) across. In addition, the research team included more precise accounting for magnetic fields, thus improving accuracy. In total, the simulation used 24,000 cores on the Hazel Hen supercomputer for a total of 35 million core hours.
As Prof. Dr. Volker Springel, professor and researcher at the Heidelberg Institute for Theoretical Studies and principal investigator on the project, explained in a Gauss Center press release:
“Magnetic fields are interesting for a variety of reasons. The magnetic pressure exerted on cosmic gas can occasionally be equal to thermal (temperature) pressure, meaning that if you neglect this, you will miss these effects and ultimately compromise your results.”
Another major difference was the inclusion of updated black hole physics based on recent observation campaigns. This includes evidence that demonstrates a correlation between supermassive black holes (SMBHs) and galactic evolution. In essence, SMBHs are known to send out a tremendous amount of energy in the form of radiation and particle jets, which can have an arresting effect on star formation in a galaxy.
While the researchers were certainly aware of this process during the first simulation, they did not factor in how it can arrest star formation completely. By including updated data on both magnetic fields and black hole physics in the simulation, the team saw a greater correlation between the data and observations. They are therefore more confident with the results and believe it represents the most accurate simulation to date.
But as Dr. Dylan Nelson – a physicist with the Max Planck Institute of Astronomy and an llustricTNG member – explained, future simulations are likely to be even more accurate, assuming advances in supercomputers continue:
“Increased memory and processing resources in next-generation systems will allow us to simulate large volumes of the universe with higher resolution. Large volumes are important for cosmology, understanding the large-scale structure of the universe, and making firm predictions for the next generation of large observational projects. High resolution is important for improving our physical models of the processes going on inside of individual galaxies in our simulation.”
This latest simulation was also made possible thanks to extensive support provided by the GCS staff, who assisted the research team with matters related to their coding. It was also the result of a massive collaborative effort that brought together researchers from around the world and paired them with the resources they needed. Last, but not least, it shows how increased collaboration between applied research and theoretical research lead to better results.
Looking ahead, the team hopes that the results of this latest simulation proves to be even more useful than the last. The original Illustris data release gained over 2,000 registered users and resulted in the publication of 130 scientific studies. Given that this one is more accurate and up-to-date, the team expects that it will find more users and result in even more groundbreaking research.
Who knows? Perhaps someday, we may create a simulation that captures the formation and evolution of our Universe with complete accuracy. In the meantime, be sure to enjoy this video of the first Illustris Simulation, courtesy of team member and MIT physicist Mark Vogelsberger:
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The first results of the IllustrisTNG Project have been published in three separate studies, and they’re shedding new light on how black holes shape the cosmos, and how galaxies form and grow. The IllustrisTNG Project bills itself as “The next generation of cosmological hydrodynamical simulations.” The Project is an ongoing series of massive hydrodynamic simulations of our Universe. Its goal is to understand the physical processes that drive the formation of galaxies.
At the heart of IllustriousTNG is a state of the art numerical model of the Universe, running on one of the most powerful supercomputers in the world: the Hazel Hen machine at the High-Performance Computing Center in Stuttgart, Germany. Hazel Hen is Germany’s fastest computer, and the 19th fastest in the world.
Our current cosmological model suggests that the mass-energy density of the Universe is dominated by dark matter and dark energy. Since we can’t observe either of those things, the only way to test this model is to be able to make precise predictions about the structure of the things we can see, such as stars, diffuse gas, and accreting black holes. These visible things are organized into a cosmic web of sheets, filaments, and voids. Inside these are galaxies, which are the basic units of cosmic structure. To test our ideas about galactic structure, we have to make detailed and realistic simulated galaxies, then compare them to what’s real.
Astrophysicists in the USA and Germany used IllustrisTNG to create their own universe, which could then be studied in detail. IllustrisTNG correlates very strongly with observations of the real Universe, but allows scientists to look at things that are obscured in our own Universe. This has led to some very interesting results so far, and is helping to answer some big questions in cosmology and astrophysics.
How Do Black Holes Affect Galaxies?
Ever since we’ve learned that galaxies host supermassive black holes (SMBHs) at their centers, it’s been widely believed that they have a profound influence on the evolution of galaxies, and possibly on their formation. That’s led to the obvious question: How do these SMBHs influence the galaxies that host them? Illustrious TNG set out to answer this, and the paper by Dr. Dylan Nelson at the Max Planck Institute for Astrophysics shows that “the primary driver of galaxy color transition is supermassive blackhole feedback in its low-accretion state.”
“The only physical entity capable of extinguishing the star formation in our large elliptical galaxies are the supermassive black holes at their centers.” – Dr. Dylan Nelson, Max Planck Institute for Astrophysics,
Galaxies that are still in their star-forming phase shine brightly in the blue light of their young stars. Then something changes and the star formation ends. After that, the galaxy is dominated by older, red stars, and the galaxy joins a graveyard full of “red and dead” galaxies. As Nelson explains, “The only physical entity capable of extinguishing the star formation in our large elliptical galaxies are the supermassive black holes at their centers.” But how do they do that?
Nelson and his colleagues attribute it to supermassive black hole feedback in its low-accretion state. What that means is that as a black hole feeds, it creates a wind, or shock wave, that blows star-forming gas and dust out of the galaxy. This limits the future formation of stars. The existing stars age and turn red, and few new blue stars form.
How Do Galaxies Form and How Does Their Structure Develop?
It’s long been thought that large galaxies form when smaller galaxies join up. As the galaxy grows larger, its gravity draws more smaller galaxies into it. During these collisions, galaxies are torn apart. Some stars will be scattered, and will take up residence in a halo around the new, larger galaxy. This should give the newly-created galaxy a faint background glow of stellar light. But this is a prediction, and these pale glows are very hard to observe.
“Our predictions can now be systematically checked by observers.” – Dr. Annalisa Pillepich (Max Planck Institute for Astrophysics)
IllustrisTNG was able to predict more accurately what this glow should look like. This gives astronomers a better idea of what to look for when they try to observe this pale stellar glow in the real Universe. “Our predictions can now be systematically checked by observers,” Dr. Annalisa Pillepich (MPIA) points out, who led a further IllustrisTNG study. “This yields a critical test for the theoretical model of hierarchical galaxy formation.”
IllustrisTNG is an on-going series of simulations. So far, there have been three IllustrisTNG runs, each one creating a larger simulation than the previous one. They are TNG 50, TNG 100, and TNG 300. TNG300 is much larger than TNG50 and allows a larger area to be studied which reveals clues about large-scale structure. Though TNG50 is much smaller, it has much more precise detail. It gives us a more detailed look at the structural properties of galaxies and the detailed structure of gas around galaxies. TNG100 is somewhere in the middle.
IllustrisTNG is not the first cosmological hydrodynamical simulation. Others include Eagle, Horizon-AGN, and IllustrisTNG’s predecessor, Illustris. They have shown how powerful these predictive theoretical models can be. As our computers grow more powerful and our understanding of physics and cosmology grow along with them, these types of simulations will yield greater and more detailed results.