Galactic fountains and carousels

Order emerging from chaos

November 07, 2019

Scientists from Germany and the United States have unveiled the results of a newly-completed, state of the art simulation of the evolution of galaxies. TNG50 is the most detailed large-scale cosmological simulation yet. It allows researchers to study in detail how galaxies form, and how they have evolved since shortly after the Big Bang. For the first time, it reveals that the geometry of the cosmic gas flows around galaxies determines galaxy structure, and vice versa. 

Astronomers running cosmological simulations face a fundamental trade-off: with finite computing power, typical simulations so far have been either very detailed or have spanned a large volume of virtual space, but not both. Detailed simulations with limited volumes can model no more than a few galaxies, making statistical deductions difficult. Large-volume simulations, in turn, typically lack fine details on smaller scales, which are important for describing individual galaxies. The TNG50 simulation, which has just been published, manages to avoid this trade-off. For the first time, it combines the idea of a large-scale cosmological simulation – a Universe in a box – with the computational resolution of “zoom” simulations, at a level of detail that had previously only been possible for studies of individual galaxies. 

In a simulated cube of space that is more than 230 million light-years across, TNG50 can discern physical phenomena that occur on scales one million times smaller, tracing the simultaneous evolution of thousands of galaxies over 13.8 billion years of cosmic history. It does so with more than 20 billion particles representing dark matter, stars, cosmic gas, magnetic fields, and supermassive black holes. The calculation itself required 16,000 cores on the Hazel Hen supercomputer in Stuttgart working together, 24/7, for more than a year – the equivalent of fifteen thousand years on a single processor, making it one of the most demanding astrophysical computations to date.

Emerging phenomena in a simulated universe

The first scientific results from TNG50, presented in two articles that have just been published in the Monthly Notices of the Royal Astronomical Society, by a team led by Dr. Annalisa Pillepich (Max Planck Institute for Astronomy, Heidelberg) and Dr. Dylan Nelson (Max Planck Institute for Astrophysics, Garching), have revealed unforeseen physical phenomena. According to Nelson: “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. These phenomena emerge in a natural fashion, from the complex interplay of the basic physical ingredients of our model universe.”

TNG50 features two prominent examples for this kind of emergent behavior. The first concerns the formation of “disk” galaxies like our own Milky Way. Using TNG50 as a time machine to rewind the evolution of cosmic structure, researchers have seen how the well-ordered, rapidly rotating disk galaxies (which are common in our nearby Universe) emerge from chaotic, disorganized, and highly turbulent clouds of gas at earlier epochs. 

As the gas settles down, newborn stars are typically found on more and more circular orbits, eventually forming large spiral galaxies – galactic carousels. Annalisa Pillepich explains: “In practice, TNG50 shows that our own Milky Way galaxy with its thin disk is at the height of galaxy fashion: over the past 10 billion years, at least those galaxies that are still forming new stars have become more and more disk-like, and their chaotic internal motions have decreased considerably. The Universe was much more messy when it was just a few billion years old!” 

Interplay of gas flows and galaxies

As these galaxies flatten out, researchers found another emergent phenomenon, concerning high-speed outflows and winds of gas flowing out of galaxies. Such outflows and winds are launched as a result of supernovae explosions and supermassive black hole activity. Galactic gaseous outflows are initially also chaotic and flow away in all directions, but over time, they begin to become more focused along a path of least resistance. In the late universe, outflows are oriented within two conical volumes, emerging from the galaxy in opposite directions – like two ice cream cones placed tip to tip, with the galaxy swirling at the center. 

These winds slow down as they attempt to leave the gravitational well of the dark matter halo, and can eventually stall and fall back onto the galaxies, forming a galactic fountain of recycled gas. This process redistributes gas from the center of a galaxy to its outskirts, further accelerating the transformation of the galaxy itself into a thin disk: galactic structure shapes galactic fountains, and vice versa.

Just as for the other simulations of the TNG family, the team of scientists creating TNG50 (based at Max Planck Institutes in Heidelberg and Garching, Harvard University, MIT, and the CCA) will eventually release all simulation data to the astronomy community at large and to the public. Then, astronomers all over the world will be able to make their own discoveries in the TNG50 universe – and possibly find additional examples of emergent cosmic phenomena, of order emerging from chaos.

Formation and evolution of a massive galaxy in the TNG50 simulation. Main image: cosmic gas density. Insets, from left to right: large-scale dark matter density, large-scale gas density, then zoomed-in images of stellar distribution (luminosity) and gas density in the central region of the galaxy. The resulting TNG50 galaxy is similar in mass and shape to the Andromeda galaxy (M31). After a turbulent beginning, the galaxy experiences no major disturbances and can settled down into an equilibrium state.

D. Nelson (MPA) and the illustris TNG team more

Background information

The first scientific results from the TNG50 simulation were published as D. Nelson et al. 2019, “First Results from the TNG50 Simulation: Galactic outflows driven by supernovae and black hole feedback” and A. Pillepich et al. 2019, “First Results from the TNG50 Simulation: The evolution of stellar and gaseous disks across cosmic time,” both in the Monthly Notices of the Royal Astronomical Society. Both manuscripts have been online as “Accepted manuscripts,” and are now being published in the current volume of MNRAS.

The MPIA researchers involved are Annalisa Pillepich and Arjen van der Wel (now University of Ghent), in collaboration with Dylan Nelson, Volker Springel, and Rüdiger Pakmor (all Max Planck Institute for Astrophysics), Paul Torrey (University of Florida), Rainer Weinberger (Harvard-Smithsonian Center for Astrophysics), Mark Vogelsberger and Federico Marinacci (both Kavli Institute for Astrophysics and Space Research, MIT), Shy Genel (Flatiron Institute) and Lars Hernquist (Harvard-Smithsonian Center for Astrophysics).

Additional figures and download

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