Magnetogenesis at the cosmic dawn

I just put on the arxiv a paper I am quite proud of, because I took a step outside of my scientific comfort zone and investigated the production mechanisms of magnetic fields.

It started in 2017, when I first learnt that photo-ionisation can produce tiny magnetic fields (not the Biermann battery, mind you, but photo-ionization alone). I remember thinking "Cool! But how strong are they compared to other magnetic seeds?". When I looked it up, I found nothing, so I decided to check myself.

Long story short, I implemented this and few other popular magnetic seeding models (the Biermann battery, injection from SNe) in the Arepo code, and run radiation-magneto-hydrodynamics simulations, using the Auriga galaxy formation model to have realistic galaxies, employing different seeding mechanisms. And since one answers begs for ten questions, I ended up with an entire suite of cosmological and zoom-in simulations.

We found that all magnetic seeds investigated produce magnetic fields that at z=0 are indistinguishable and consistent with the observed values in galaxies, but the time at which they are amplified is very different. We also showed that looking at the entire halo population, we can still see the signature of the original seed in small haloes, and even more so in the IGM.

Overall, our results show that the "photo-magnetisation" is a viable seeding mechanisms, but in practice produces very similar features to the Biermann battery, although with lower strength. Hence, in a realistic scenario where both processes are active, it is likely to be subdominant.

Above: The movie shows the gas density (top left), B field strength (top right), metallicity (bottom left) and magnetic strength radial profile (bottom right) in a suite of zoom in simulations. Each quadrant shows a run employing a different magnetic field seeding prescription.

Above: The movie is identical to the first one, but refers to a full cosmological box. In addition, the bottom right panel hows now the gas temperature.