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How primordial magnetic fields shrink galaxies

Sergio Martin-Alvarez, Adrianne Slyz, Julien Devriendt, and Carlos Gómez-Guijarro

And some of the various outreach articles in magazines and blogs about it here:


Magnetic fields galaxy galaxies shrink shrinking compact compatification astrophysics cosmology
Figure: visualisations of the same spiral galaxy simulated with different primordial magnetic fields. (Top) mock optical observations, with the magnetic field increasing from the top left to the bottom right. (Bottom) visualisation of the gas density in the galaxy, with magnetic fields increasing from left to right. Magnetic fields affect the properties of the galaxy and cause it to shrink

What is this about?

We have known for a long time that the typical magnetic fields observed in galaxies observed in galaxies are as important for their interstellar medium as thermal or turbulent energy. However, something less clear is what is the impact of magnetic fields in shaping galaxies.

In fact, this is something quite hard to pin down, as we don't really know what is the origin of magnetic fields in galaxies. One of the possibilities is that magnetic fields are primordial in nature, meaning that they were generated during the early stages of our Universe. These magnetic fields would evolve with the Universe, being amplified to their currently observed strengths as gas and dark matter collapse to form the first galaxies.

Unfortunately, we currently struggle to figure out whether the primordial magnetic field of our Universe is weak or strong. This is because observing it directly would require finding the most pristine magnetic fields in the Cosmos, most likely residing in large voids of the Universe, where the signal of magnetic fields is also the weakest. The telescopes capable of making this observation are way beyond our technical abilities. While we can look for signatures of primordial magnetic fields for example in observations of the Cosmic Microwave Background, the significant noise from other sources makes their specific signal imperceptible.

But we can observe galaxies, and we can similarly observe their magnetic fields. So what can galaxies tell us about the primordial magnetic field of the Universe? And how do magnetic fields change galaxies as they evolve?

How do we find out?

We use the magneto-hydrodynamical code RAMSES to generate cosmological simulations of the formation of a spiral galaxy. These simulations combine a realistic simulation of the environment around galaxies (which requires very large scales to be accounted for), with extremely high spatial resolutions in the simulated galaxy of interest. 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 (

We simulate the same galaxy several times, varying parameters such as the stellar feedback (meaning whether and how stars explode at the end of their lives). But most importantly, we perform the same simulation over and over, exclusively changing the primordial magnetic field at the beginning of the simulation. This controlled experiment gives us a good insight of what is the role played by magnetic fields in shaping the growth of the galaxy.

"After collapse, strong primordial magnetic fields can reduce the radial scale length of the gas disc significantly as the galaxy grows. By redshift z = 2, the strongest primordial magnetic field studied (of 10^−10 Gauss) brings the gas radial scale length down to half the size measured when no significant magnetic fields are present. As a consequence, the stellar disc size is also drastically reduced. Both of these reductions are accompanied by a large outward transfer of angular momentum, reflected in the reduction of the spin parameter λrot."

What do we find out?

Here comes the interesting part. Interestingly, characteristic properties of the galaxy such as the amount of stars formed are unaffected by the primordial magnetic fields we studied. Other researchers pointed this out before, so it wasn't unexpected. It indicates that the process of formation of stars in our simulated galaxy is regulated at first order by the interplay between the mass falling into the galaxy and the supernova explosions taking place in the galaxy, with magnetic fields not affecting the final stellar mass.

However, we also find some evidence that with stronger primordial magnetic fields, stars are formed closer to the centre of the galaxy. Furthermore, we do find that depending on the primordial magnetic field, some properties of the galaxy change. In particular, the galaxy appears to shrink in size and to lose specific angular momentum (meaning that the amount of rotation per mass of the galaxy is reduced). You can see this reduction in radius, of up to almost 50% in the plot below.

Galaxy galaxies cosmology magnetic fields shrink shrinking compression
Observed compression of the galaxy (y-axis) versus the radial scale of the galactic disk (x-axis). Each data point correspond to a different primordial magnetic field simulation, with darker colours representing stronger fields. The galaxies simulated with stronger primordial magnetic fields show a higher concentration towards their centre and a smaller size.

This suggests that strong primordial magnetic fields can affect the evolution and properties of galaxies. Using this, perhaps one day we will be able to use galaxies to learn more about how strong the primordial magnetic field of our Universe was.

Where can I read more?

Well, if you'd like to find out more, I'd recommend reading the full paper. There are many interesting findings I didn't write about here. 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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