A three-phase amplification of the cosmic magnetic field in galaxies

Updated: Sep 30, 2020

Sergio Martin-Alvarez, Julien Devriendt, Adrianne Slyz, and Romain Teyssier

Find the full publication here: http://adsabs.harvard.edu/abs/2018MNRAS.479.3343M

What is this about?

Magnetic fields in the interstellar medium (ISM) of galaxies are known to be in equipartition with the turbulent energy. How these strong magnetic fields are attained from the weak magnetic fields expected in proto-galaxies at the moment of their collapse remains an open question.

However, there are some mechanisms that could serve to bridge the gap between these weak primordial magnetic fields and the strong magnetic fields observed in galaxies nowadays. One of the possible mechanisms is a turbulent dynamo, which converts kinetic energy from the turbulence in the ISM into magnetic energy. This dynamo is a particularly good candidate to solve this problem due to the ubiquity of turbulence in the Universe and its fast amplification time scale, comparable to the e-folding time of turbulence. Rieder & Teyssier (2016) were able to capture this turbulent amplification on an isolated spiral galaxy, where the turbulence was driven by feedback. Particularly interesting is that their study employs the code RAMSES, with a Constrained Transport solver for the magnetic field. This kind of solver has the convenient characteristic of fulfilling the magnetic field solenoidal constraint down to numerical precision.

In this study, we extend their work to a cosmological context. This allows us to check whether a turbulent dynamo operates when galaxies are modelled in a realistic environment, and how does this environment affect the evolution of galactic magnetic fields. To run these simulations, we require large super computers. We used the DiRAC Complexity system, operated by the University of Leicester IT Services, which forms part of the STFC DiRAC HPC Facility (www.dirac.ac.uk).

"These are thus the three phases of magnetic amplification we find in our galaxy: an initial phase when the collapse of the galaxy compresses magnetic energy, a second phase when turbulent amplification is driven by accretion, and a final phase when turbulent amplification is driven by stellar feedback."

What do we find?

We find that once a threshold spatial resolution is reached in our simulations, the specific magnetic energy (i.e. the magnetic energy per unit of mass) commences to grow. This growth increases as we repeat the simulation with better spatial resolutions, and displays other signals of the presence of a turbulent dynamo (e.g. the shape of its magnetic energy spectrum).

When we review the growth of the magnetic energy, it can be separated into three different phases, each intimately linked with a different stage in the evolution of the galaxy. The evolution of this specific magnetic energy is shown in the image below under three different feedback prescriptions:

Figure: evolution of the specific magnetic energy in the simulated spiral galaxy, under three different feedback prescriptions. Amplification is found to occur, with a tri-phasic behaviour.

A first phase correspond to the collapse of the primordial perturbation, where the growth is triggered by compression of magnetic field lines. Afterwards, the galaxy grows mainly through cold accretion from its nurturing filaments. The turbulent amplification occurring during this phase is driven by this accretion. There is a large agreement that cold accretion drives turbulence through gravitational instabilities, but other possibilities are not discarded. All of linked to accretion. Therefore, we find a second phase where the turbulent amplification is dominated by accretion. In the final phase, the galaxy has its gravitational potential stellar dominated, develops a spiral disk, and accretion progressively losses relevance. We now find that magnetic amplification requires the presence of stellar feedback. These are thus the three phases of magnetic amplification in our galaxy: an initial phase during which the collapse of the galaxy compresses magnetic energy, a second phase when turbulent amplification is driven by accretion, and a final phase when this turbulent amplification is driven by stellar feedback.

Where can I read more?

Well, if you are interested there are many other interesting things I didn't write about here that we describe in the full paper. And obviously, much more scientific detail.

Here is a link to it:


Feel free to contact me with any questions you might have!

Un saludo!

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Institute of Astronomy & Kavli Institute for Cosmology

Churchill College Postdoctoral By-Fellow

University of Cambridge

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