Magnetic fields in multiphase gas: A turbulent tango

November 01, 2023

Space is filled with gases of vastly different temperatures and it is important to understand how these interact. A group of scientists at MPA has now looked into the mixing of gases with and without magnetic fields. Surprisingly, they find that the outcome depends on whether turbulence is already present at the beginning. Without turbulence, magnetic fields can suppress the mixing by suppressing turbulence, while if the turbulence is already present, magnetic fields have a marginal effect.

From the Milky Way and Andromeda in our neighbourhood to the farthest observed galaxy JADES-GS-z13-0, galaxies are islands in the vast expanse of space. But, they are not isolated. They accrete gas from their surroundings, churn and compress it to form stars, before throwing it out again when those stars explode as supernovae. This expulsion of gas from the galactic disk creates gigantic galactic outflows. Such outflows can drain the galaxy of the gas that could have fuelled future star formation, leading to a period of low star formation rate, which is also called “quenching” of the galaxy. However, it is believed that the gas that is thrown out as galactic outflows can later be recycled and accreted back into the galactic disk to fuel further star formation. This cyclic expulsion and accretion of the baryonic gas in and out of the galaxy is called the “Baryon cycle” and is a crucial part of the evolution of galaxies.

The gas as the main ingredient of this cycle can exist in different phases: extremely hot gas (with millions of degrees) along with pockets of much colder gas (of only thousands of degrees). Understanding how these gases of different temperatures mingle and mix is important to our knowledge of the Baryon cycle and the many related astrophysical processes.

Turbulent mixing is a well-studied field, as it is important in a lot of applied areas like meteorology, pollutant transport, combustion engines, etc. But in astrophysics there’s a catch: magnetic fields fill interstellar and circumgalactic space, too. Do these invisible fields affect the mixing among the vastly different phases of gas? Scientists have dived into such questions using computer simulations and found that in some cases magnetic fields can suppress mixing. This can pose a problem to the multiphase nature of the gas. Still, this combination of turbulent, magnetised and multiphase gas remains an enigma. In this study, we investigate how the presence of magnetic fields affects the mixing and the interaction between hot and cold gases.

The researchers at MPA, using computer simulations, first mimic a very simple setup called a mixing layer: hot and cold gases are next to each other with a single interface and a relative velocity between them. First, the gases mix without any magnetic field. In this case, turbulence efficiently stirs and mixes the gases. Next, the team introduces magnetic fields and finds that the mixing is suppressed. The magnetic fields stabilize the boundary between the hot and cold gases, preventing turbulence (see first video). However, in a second, more realistic simulation called a turbulent box, turbulence is already present at the beginning. In this case, the magnetic fields do not affect the mixing.

Mixing gases with magnetic fields

These videos show how two gases with different temperatures (indicated by the colour scale) mix with and without the presence of a magnetic field.

The left-most column shows the case without any magnetic fields. The middle and right columns show the case with different orientations (along each of the major axes, indicated at the top) and with different strengths of the magnetic fields (middle: relatively strong, right: relatively weak).

This is a puzzling outcome, as the mixing layer setup was expected to exist within the bigger turbulent box setup. Why should the magnetic fields make a big difference in the first case, where there is only a marginal effect in the second? Until now, mixing layer simulations were thought to be zoomed-in versions of the turbulent box simulations, to situations similar to the mixing-layer-setup at each cold-gas/hot-gas interface. So, a big effect on mixing in these was hypothesised to have an equally big effect on mixing in the turbulent boxes.

To resolve this dilemma, the team had to examine the root cause of mixing, which is turbulence. The mixing depends only on the turbulent motions and not on how this turbulence formed originally. In mixing layers, the presence of magnetic fields hinders the generation of turbulence; in a turbulent box, the turbulence already exists. If the turbulence in a setup is unchanged by magnetic fields, the mixing remains unchanged, which means that there is no effect on the growth and survival of cold gas.

Even though in the more realistic turbulent box simulations, the magnetic fields do not change the overall mixing of the different phases, magnetic fields still leave their fingerprints. Magnetic fields affect the structure of the cold gas, which is very evident from the evolution in simulations: in the presence of magnetic fields, the gas becomes more elongated and filament-like (see second video). The researchers also examined how these changes in the cold gas structure can affect observations. Mock quasar line-of-sight observations were created from the simulations with and without magnetic fields, and in both cases follow the observed relation between number of spectral features and their strengths – but almost no observable difference could be found between the two cases.

Evolution of the structure of mixing gases

These two videos show the mixing of a cold, dense cloud in a hot turbulent environment with and without magnetic fields. The structure in the two cases looks very different: without magnetic fields (left) the gas remains clumpy, while it becomes more elongated and filament-like with magnetic fields (right).

In conclusion, these numerical experiments show that magnetic fields can inhibit turbulence and a mixing of gases, if there is a shear between layers of hot and cold gas. However, if turbulence is already present in the gas, the magnetic fields become just bystanders and do not affect the efficiency of the mixing. These findings can help better understand the complex multiphase nature of observed astrophysical gases.

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