Unraveling the origin of magnetic fields in galaxies

Sergio Martin-Alvarez, Harley Katz, Debora Sijacki, Julien Devriendt, and Adrianne Slyz

Find the full publication here: https://ui.adsabs.harvard.edu/abs/2021MNRAS.tmp..955M/abstract

This work has also been discussed by various outreach articles scientific magazines:




What is this about?

Primordial magnetic fields are likely the most unknown component of our Universe just after dark matter and dark energy. Contrarily to these two, their physical nature is well understood. It is instead 'how much' of this primordial magnetic does exist what we don't really know about. As pristine, unprocessed magnetic fields that are the remnants of this primordial magnetism would only be potentially found in regions of our Universe with low matter density and negligible activity (say kinematics or temperature), they are only found is cosmic voids. Magnetic fields are generally observed by studying radio emission and how it is polarised. Due to the same low density and activity, it is practically impossible to observe magnetic fields in voids.

Current bounds on the existence of primordial magnetic fields allow for magnetic fields strong enough to magnetise our current Universe, and even affect the growth of galaxies (see this other post). We know, for example, that galaxies are magnetised. And while there are various 'astrophysical' (i.e. driven by specific astrophysical objects - let's say for the sake of simplicity, those that take place inside the galaxy) mechanisms that could generate their magnetic fields, we cannot rule out a primordial origin. As astrophysical channels are going to be at play with different degrees of success (did someone said turbulent dynamo?), we generally assumed that primordial magnetic fields got 'forgotten', or in the most generous case, that we could not tell them apart from the astrophysical ones.

But the two types are generated by extremely different causes, and have considerably different properties. So how true have been these assumptions?

Well, that's what we precisely tried to address with this work!

Alright, this is interesting now, but how could we ever find an answer?

If I managed to convince you that it is interesting to know whether these magnetic fields exist and are responsible for the magnetisation of pretty much anything bigger than say ~stars, you'd be wondering how can we tackle this anyway.

The answer is simple, we use numerical simulations of galaxy formation! Some years ago we modified the magneto-hydrodynamical code RAMSES so that it would allow us to follow each type of magnetic field separately. Using this code, we generate various realistic cosmological simulations of a spiral galaxy, magnetising it through either a solely astrophysical mechanism, an exclusively primordial origin, or a combination of both. As all cosmological 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).

By considering how the magnetic fields in the galaxy evolve in each of these cases, we can explore whether primordial magnetic fields get erased by their astrophysical counterpart, and whether they are distinguishable from each other. The image below shows the different simulations. In the rightmost panels, you can see the two types of magnetic energy: astrophysical in red, and primordial in green; where the bottom two galaxies are only magnetised by one of these two each.

Figure: rows present an individual simulation, each magnetising the galaxy through different mechanisms and combinations. Leftmost column shows a synthetic optical observation of the galaxy. Central column corresponds to a combination of gas density (cyan) temperature (orange) and magnetic energy (green). The rightmost column separates the magnetic energy into a primordially originated (green), astrophysically generated (red), or emerging through the interaction of both (blue).

What do we find out?

The very first thing the image above shows is that when you combine astrophysical and primordial magnetic fields to magnetise the galaxy (first row) the green magnetic fields survive the 'onslaught' of those pesky astrophysical fields, and remain present in the galaxy. This is an exciting result, because it confirms that if primordial magnetic fields are strong enough, they won't be 'forgotten' by galaxies.

This poses a next question, which opens a fascinating new window: if we are able to discriminate astrophysical and primordial magnetic fields in galaxies, we could use galaxies to study the primordial magnetic field of our Universe. But, can we discriminate the two types of fields?

The short answer is yes. By looking at the different properties of each magnetisation channel in our simulation, we find that the primordial fields are mostly found in the warm, unprocessed, metal poor gas of our galaxy. My favourite however is their concentration on large galactic scales, which is illustrated by the graph below.

Magnetic energy spectra for tracer magnetic fields
Figure: distribution of each type of magnetic energy across spatial scales. The spatial scale decreases as lines move towards the right of the graph. Each separated form of energy in the galaxy is shown by dashed lines. The primordial magnetic energy (green dashed line) dominates at large scales.

The fact that we can differentiate these two field origins shouldn't be that surprising: primordial magnetic fields are generated early in the history of our Universe, and become associated with cold, pristine gas distributed at large scales. On the contrary, astrophysical fields are generated in (or close to) galaxies, which are enriched with metals and bustling with activity.

Knowing this, maybe galaxies will be our next window to primordial magnetism.

Where can I read more?

Well, if you'd like to find out more, I'd recommend reading the full paper. On top of the main findings discussed here, we also discuss the emergence of a cross-term magnetic energy and go into more detail in how the two considered origins of the magnetic field can be differentiated. And obviously, on top of extra material, it also has much more scientific detail.

Here is a link to it:

Feel free to contact me with any questions you might have! https://ui.adsabs.harvard.edu/abs/2021MNRAS.tmp..955M/abstract

Un saludo!

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