Kavli Institute for Particle Astrophysics and Cosmology
KIPAC - Stanford University
What is the magnetic field of our Universe?
The cosmic magnetic field of our Universe remains undetected. Its direct detection in the intergalactic medium would provide us with a vast amount of information about our Universe. It the detected magnetic field resides in a quiet region of space such as voids, they could provides us with information about the primordial magnetic field of our Universe. The strength and spectral shape of primordial magnetic fields are cosmological parameters barely constrained by our current knowledge. However, magnetic fields on large scales will change depending on which region of the Universe we are looking at. On top of the spectral variation of the primordial magnetic field, depending on the structures present in a given region of the Cosmos, the local magnetic field will be affected by astrophysical phenomena such as magnetised outflows from sources in galaxies, plasma physics, etc. Cosmological simulations are the best tool at our disposal to improve our understanding on how these cosmological may leave an imprint on structure formation and equally aid us in interpreting future observations.
Some of my ongoing work uses large cosmological simulations to tackle various questions related with the cosmic magnetic field and its detection. However, we can also gain understanding of the cosmic magnetic fields by studying galaxies.
Background image: projection of one of my cosmological simulations (500 Mpc per side box) magnetised by a red spectrum primordial magnetic field. Left panel: density (cyan) and magnetic energy (green). Right panel: different colours identify magnetic fields of different coherence lengths.
On the origin of magnetic fields in galaxies
While we have no detection of the cosmic magnetic field, there are plenty of observations of magnetic fields in galaxies around us. These indicate that magnetic fields in galaxies have energies comparable to the thermal or turbulent energy budgets. Therefore, they are important agents in the dynamics of the interstellar medium of galaxies and could contribute to shape galaxies during their formation and evolution.
Some of my work explores how magnetic fields affect the evolution of galaxies (Martin-Alvarez et al. 2020). However, one question that I find particularly fascinating is what is the origin of these observed magnetic fields. Did our Universe have such a strong primordial magnetic field in its early stages that galaxies already reached their current strengths by retaining the magnetisation of the pristine gas that formed them? Were galaxies born instead with extremely weak magnetic fields and amplify them dynamically through dynamical processes? Or where these channels of overpowered by the heavily magnetised gas escaping from stars and black holes in the form of winds, supernova explosions and AGN feedback? Numerical simulations of galaxy formation might be the perfect tool to figure out what is the origin of magnetic fields in galaxies (Katz & Martin-Alvarez et al. 2019, Martin-Alvarez et al. submitted).
Three main channels of magnetisation
I am still writing this part. However, you can check my paper on how strong primordial magnetic fields affect the properties of galaxies to learn more about the topic: Martin-Alvarez et al. 2020. My more recent publication (Martin-Alvarez et al. submitted) shows instead that if the primordial magnetic field of our Universe is strong enough, it will overpower the other channels of magnetisations. In that scenario, the magnetic fields observed in galaxies around us are the remnant of magnetic fields produces in the Early Universe.
I am still writing this part. However, you can check out my paper on how a turbulent dynamo should be capable of producing the observed magnetic fields in galaxies, and how its efficiency depends on the properties of galaxies and their evolutionary stage: Martin-Alvarez et al. 2018.
I am still writing this part. However, you can check out my paper on how the magnetic fields escaping from supernovae lead to magnetisations comparable with those observed in galaxies around us (Martin-Alvarez et al. submitted). This paper also shows that if the primordial magnetic field of the Universe is relatively mild, the ones return by supernova explosions to the interstellar medium of galaxies will overcome them and erase their signatures.