A Universe in a box. Tracing the evolution of thousands of galaxies

Using a remarkable new simulation, scientists have studied the formation and evolution of thousands of galaxies in unprecedented detail.

Images of the optical light emitted by the stars of 16 galaxies from the TNG50 simulation. Each galaxy is seen face-on or from the top (top sub-panels), and edge-on or from the side (lower sub-panels). (D. Nelson (MPA) and the IllustrisTNG team)

The results of a recently completed simulation of the Universe over 13.8 billion years, showing the evolution of thousands of galaxies in painstaking detail, have been released to the public by scientists from the US and Germany. The most detailed large-scale cosmological simulation ever created — labelled TNG50 — has allowed the researchers to study how galaxies formed and evolved from a point shortly after the big bang.

The results — published across two papers in the journal Monthly Notices of the Royal Astronomical Society — reveal that the geometry of cosmic gas flows around early galaxies determined their structure after initial structure determined how the flow of the gas progressed.

The formation of a single massive galaxy through time, from early cosmic epochs until the present day, in the TNG50 cosmic simulation. The main panel shows the density of the cosmic gas (high in white, low in black). Insets show large-scale dark matter and then gas (lower left), and small-scale stellar and gaseous distributions (lower right). This TNG50 galaxy will be similar in mass and shape to Andromeda (M31) by the time the movie reaches the current epoch. Its progenitor experiences rapid star formation in a turbulent gas reservoir which settles into an ordered disc after a couple of billion years of cosmic evolution. A rather quiet late time assembly history without major mergers allows the galaxy to relax into an equilibrium balance of gas outflows from supernova explosions and gas accretion from its surroundings. (D. Nelson (MPA) and the IllustrisTNG team)

The first results from TNG50 are published by a team led by Dr Annalisa Pillepich, Max Planck Institute for Astronomy, Heidelberg, and Dr Dylan Nelson, Max Planck Institute for Astrophysics, Garching. The team’s findings reveal several unforeseen physical phenomena.

“Numerical experiments of this kind are particularly successful when you get out more than you put in. In our simulation, we see phenomena that had not been programmed explicitly into the simulation code,” says Nelson.

“These phenomena emerge in a natural fashion, from the complex interplay of the basic physical ingredients of our model universe."

Cosmic gas fountains shaping galaxies.

TNG50 displayed two important examples of the kind of emergent behaviour described by Nelson.

Firstly, by using the simulation as a rudimentary time-machine, the team ‘rewound’ the evolution of cosmic structure to observe how well-ordered, rapidly rotating disc galaxies — such as our own Milky Way — emerged from the chaotic and turbulent clouds of gas and dust that existed during earlier epochs.

As this gas settles down, the simulation showed, newborn stars are found increasingly on circular orbits. Eventually, this leads to the formation of large spiral galaxies.

Evolution over a few hundreds of million years (from top to bottom) of the gas around a galaxy from the TNG50 simulation, with an active supermassive black hole at its centre. The black hole at the centre of this galaxy is consuming gas from its surroundings and in doing so is generating copious amounts of energy. The release of this energy produces ultra-fast winds, which rapidly expand away from the galaxy and grow in size to become thousands of times larger than they started. These black hole driven outflows achieve velocities of tens of thousands of kilometres per second, have temperatures exceeding millions of degrees and carry with them copious amounts of heavy elements such as oxygen, carbon, and iron. The four columns show, from left to right, the evolving velocity, temperature, density, and heavy element content around the galaxy. The galaxy itself is a cold (blue, second column), dense (yellow, third column) disc of star-forming gas visible as a small, vertical slab in the very centre of each image. (D. Nelson (MPA) and the IllustrisTNG team)

“ In practice, TNG50 shows that our own Milky Way galaxy with its thin disc is at the height of galaxy fashion,” explains Pillepich. “Over the past, 10 billion years, at least, those galaxies that are still forming new stars have become more and more disc-like, and their chaotic internal motions have decreased considerably.

“The Universe was much messier when it was just a few billion years old!”

The second significant emergent property of this simulated universe was demonstrated when these galaxies began to ‘flatten out’. This involved high-speed outflows and gas flowing out of these galaxies, kick-started by the explosions of massive stars and the activity of supermassive black holes lurking at the heart of galaxies.

Despite initially spewing out from galaxies chaotically across all different directions, over time these gaseous outflows begin to conjugate along the path of least resistance.

Late in the development of the Universe, these outflows take the form of two cones emerging in opposite directions along the rotational axis of galaxies. These material outflows slow down as they attempt to pass a gravitational well created by galaxies’ haloes of dark matter. This can often result in the gas stalling and falling back onto the galaxy, forming something analogous to a ‘cosmic fountain’ of recycled gas.

This process redistributes gas from the centre of a galaxy to its outer regions, in to turn accelerating the transformation into a more thinly distributed disc. Therefore, from these two emergent properties, we can say that galactic structure shapes galactic fountains, and these fountains go on to influence structure.

TNG50: A pocket Universe in unprecedented detail

The TNG50 simulation avoids a trade-off that has hampered previous cosmological simulations of this nature. As computer power is finite, previous simulations have either been highly detailed or have spanned a large volume of virtual space. This means that simulations with limited volumes can only track a few galaxies making generalisms and statistical findings impossible.

On the flip-side of this, simulations with large volumes lack the details to reproduce small-scale properties observable in the Universe — thus, radically reducing their predictive power.

TNG50 avoids these pitfalls combining large-scale cosmological simulations with the resolution of small-scale efforts. In the process, the researchers have attained a level of detail only previously possible in the study of single galaxies and applied it to a ‘Universe in a box’ scale cube — a simulation that is more than 230 million light-years across. In this simulated cube, TNG50 allows researchers to discern phenomena that occur on a scale one-millionth the size of its impressive volume.

This means the ability to trace the evolution of thousands of galaxies individually over the 13.8 billion year history of the Universe. This is achieved with the use of 20-billion particles representing dark matter, stars, cosmic gas, magnetic fields and supermassive black holes.

The Hazel Hen supercomputer granted researchers the computing power to create a Universe in a box in unprecedented detail. (HPCwire)

The calculation itself required 16,000 cores on the Hazel Hen supercomputer in Stuttgart, working in unison 24 hours a day, for more than a year — the equivalent of fifteen thousand years on a single processor. This makes it one of the most demanding astrophysical computations to date.

And these results promise to be just the first of many. The team of scientists generating the TNG50 universe based at Max-Planck-Institutes in Garching and Heidelberg, Harvard University, MIT, and the Center for Computational Astrophysics (CCA), say that they will eventually release all simulation data to the astronomy community at large, as well as to the public.

This will allow astronomers all over the world to make their own discoveries in the TNG50 universe — and possibly find additional examples of emergent cosmic phenomena, of order emerging from chaos.