Magnetogenesis around the first galaxies and its impact on galaxy formation

May 01, 2021

Magnetic fields are ubiquitous in the Universe today, from stars to clusters of galaxies. Their origin, however, remains a mystery. MPA researchers have now simulated in great detail a variety of proposed mechanisms for magnetogenesis – i.e. how magnetic fields might be created – in high-redshift galaxies. They also studied their impact on the formation and evolution of galaxies, providing guidance to both future observations and simulations. Their work demonstrates that high-redshift galaxies may hold the key to understanding the origin of cosmic magnetic fields. It also provides the first-ever investigation on galactic scales of a novel magnetogenesis mechanism.

Magnetic fields have been detected in most cosmic structures in the nearby Universe, from individual stars to entire galaxies and galaxy clusters. These fields are often strong enough to play an important role in the evolution of the material in these structures. At the same time, the Cosmic Microwave Background radiation shows that magnetic fields at the origin of the Universe – if they existed at all – must have been extremely weak.

There are many physical mechanisms that can produce tiny magnetic fields during the evolution of galaxies. The chaotic motion of gas inside galaxies can then amplify these seed fields to the strength observed today. What is the most important source of seed fields for the evolution of the Universe? And are different mechanisms for magnetogenesis affecting the evolution of galaxies in the same way? Can we use galaxies to learn something about the origin of cosmic magnetic fields?

These questions prompted a small team of MPA researchers to perform a series of advanced numerical simulations using different recipes for the generation of initial seed magnetic fields: residual fields left over from the Big Bang, fields produced by supernovae, or different plasma physics processes (including a new type of ‘magnetic battery’ never studied before on these scales). These simulations are of two kinds. The first type follows the evolution of a representative part of the Universe, allowing researchers to study the creation and evolution of magnetic fields in and around a large number of cosmic structures (see movie). The second type of simulation focuses on a single forming galaxy, reaching a high level of accuracy and enabling the researchers to study the details of the physical processes inside the zoomed-in galaxy.

The influence of early magnetic fields

Evolution of the cosmic gas density (top left), magnetic field strength (top right), metal fraction (bottom left) and temperature (bottom right) in a slice through a cosmological simulation. Panels are split in four quadrants, each one showing the results from a different magnetic seed model. While the gas density, metal fraction and temperature are very similar in all models, the properties of magnetic fields change drastically for different seed processes.

The simulations show that by today, the magnetic field at the very center of galaxies has lost every memory of its origin, but may have done so at different times in the past, depending on the exact physical process that created the original magnetic field (see figure). This opens up the possibility of using high-redshift galaxies to understand which magnetogenesis processes are dominant in the Universe.

Evolution of the mean magnetic field strength inside a certain radius (top: 1 kpc, middle: 10 kpc, bottom: 100 kpc) of a simulated galaxy. Each line shows the results for a different magnetic seed model. Today (right hand side of the plot) the strength is the same for all models, but this value is reached at different times by each model.

The researchers also used simulations to investigate where in the Universe there may be hints about the origin of cosmic magnetic fields. They discovered that the diffuse and smooth gas filling the space between galaxies (known as the inter-galactic medium, or IGM) retains a memory of the original strength of its magnetic field. The IGM therefore may provide crucial information about the origin of cosmic magnetic fields.

Finally, this study simulates, for the first time within a realistic cosmological galaxy formation model, a new magnetic battery, which is at play at the ionizing radiation fronts surrounding the first galaxies. The researchers were able to show that this process in principle constitutes a viable mechanism for creating cosmic magnetic fields, although these are generally expected to be much weaker than those produced by competing physical processes active at the same time.

This new comprehensive study not only paves the way to more targeted observations of magnetic fields, but also demonstrates the crucial role that the diffuse IGM and high-redshift galaxies might play in pushing forward our understanding of cosmic magnetic fields.

Researchers: Enrico Garaldi (MPA), Rüdiger Pakmor (MPA), Volker Springel (MPA)

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