Scientists map galaxy clusters’ largest magnetic fields using synchrotron intensity gradient

In a new study, scientists have mapped magnetic fields in galaxy clusters, revealing the impact of galactic mergers on magnetic field structures and challenging previous assumptions about the effectiveness of turbulent dynamo processes in amplifying of these fields.

Galaxy clusters are large, gravitationally bound systems containing many galaxies, hot gas, and dark matter. They represent some of the most massive structures in the universe. These clusters can be made up of hundreds or even thousands of galaxies, linked together by gravity, and are embedded in vast halos of hot gas called intracluster media (ICM).

The ICM, composed primarily of ionized hydrogen and helium, is held together by the gravitational pull of the cluster itself. Magnetic fields in large-scale structures, such as galaxy clusters, play a central role in shaping astrophysical processes. They influence the ICM, have an impact on the formation and evolution of galaxies, contribute to the transport of cosmic rays, participate in cosmic magnetization and serve as tracers of the evolution of large-scale structures.

Previous studies and simulations have suggested that magnetic fields within clusters evolve, indicating their susceptibility to cluster dynamics and undergoing amplification during merger events.

The study, published in Natural communications, uses a method called synchrotron intensity gradient (GIS) to map magnetic fields in clusters, particularly during galaxy mergers. This method provides a unique perspective on the structures of magnetic fields and provides a tool to compare numerical expectations from simulations with observational data.

The study’s lead author, Professor Alex Lazarian of UW-Madison, spoke to Phys.org about his motivation for studying magnetic fields in galaxy clusters, saying: “The goal of my research lies in understanding the role of magnetic fields in astrophysical environments. , particularly in magnetized and turbulent environments.

“Over the past two decades, I have extensively studied magnetic turbulence and reconnection processes in collaboration with my students. The technique used to map magnetic fields in galaxy clusters relies on the theoretical and numerical knowledge acquired over years of research.”

Synchrotron intensity gradient

Synchrotron intensity refers to the radiation emitted by charged particles, usually electrons, as they spiral along magnetic field lines at relativistic speeds. This phenomenon is known as synchrotron radiation.

The GIS method introduces a unique perspective by mapping magnetic fields through a process anchored in the synchrotron intensity gradient. The basic principle of the applied technique consists of using the interactions between magnetic fields and conductive fluids, in particular ionized gases or plasma.

The key idea is that magnetic fields influence the movement of these fluids and their resistance to bending makes it easier to determine their direction. Professor Lazarian explained: “These movements result in velocity gradients and the magnetic field fluctuations are perpendicular to the magnetic field. By measuring these gradients, we can obtain the direction of the magnetic field.”

This approach represents a new way of measuring magnetic fields, developed by Professor Lazarian’s group based on fundamental studies of magnetohydrodynamics.

“It uses data initially deemed irrelevant to magnetic field studies, allowing us to derive meaningful results from various archival datasets collected for purposes unrelated to magnetic field studies,” said Professor Lazarian.

Magnetic field mapping

The researchers obtained maps of magnetic fields at the largest scales ever studied, particularly in galaxy halos within galaxy clusters.

“We confirmed the accuracy of this technique by comparing the magnetic field directions obtained with our technique with those obtained with the traditional technique based on polarization measurement. We also evaluated the accuracy of GIS with numerical simulations,” said declared Professor Lazarian.

The study demonstrated that GIS opens a new avenue for mapping magnetic fields at unprecedented scales. The complexity of plasma movement within merging galaxy clusters has been revealed by the structure of the magnetic field.

The results have implications for our understanding of cluster dynamics and evolution, offering unique insights into the role of magnetic fields in key processes within galaxy clusters.

Overcoming depolarization

In traditional synchrotron polarization measurements, depolarization challenges the mapping of magnetic fields in galaxy cluster regions, except for relics. Unlike other methods, GIS is not affected by depolarization. This study aimed to test whether GIS and polarization indicate the same magnetic field directions where polarization is present.

First author Ph.D. student Yue Hu, together with Italian scientists Annalisa Bonafede and Chiara Stuardi, successfully tested magnetic field measurements in the relics, confirming the reliability of GIS magnetic field maps. Ph.D. from Professor Lazarian. Student Ka Wai Ho’s fluid dynamics simulations further confirmed the map’s accuracy.

GIS offers a unique way to answer long-standing questions about the origin, evolution, and effects of magnetic fields in galaxy clusters without facing the challenges posed by traditional measurements.

Heat conduction in ICM

GIS also allows researchers to test and validate existing theories regarding heat conduction in ICM and the development of cooling flows, a poorly understood process.

“The heat conduction in the intra-cluster plasma (fully ionized gas) of the ICM is significantly reduced in the direction perpendicular to the magnetic field. Thus, the ability of heat to be transported in different directions depends on the structure of the magnetic field “Heat changes Conductivity controls the formation of streams of cold gases surrounded by hot gases, called cooling flows,” explained Professor Lazarian.

Acceleration of cosmic rays

Cosmic rays are high-energy charged particles that interact strongly with the magnetic fields of the halos of galaxy clusters. Dr. Gianfranco Brunetti, co-author of the paper, is the leading expert on cosmic ray acceleration processes in galaxy clusters. He is enthusiastic about revealing the previous enigmatic structure of magnetic fields.

“It is known that galaxy clusters accelerate cosmic rays through the interaction of cosmic rays with moving magnetic fields. The picture of this acceleration is not yet clear and depends on the dynamics of the magnetic field,” he said. declared Professor Lazarian.

Additionally, cosmic rays follow the trajectories of magnetic field lines, meaning that their exit from clusters is influenced by the specific structure of these magnetic fields.

The dynamics of magnetic fields within clusters can now be mapped using GIS, helping us understand how the largest particle accelerators in the universe work.

Final Thoughts

GIS, with its ability to map magnetic fields in regions where polarization information is lost, offers invaluable information about the halos of galaxy clusters and the even larger synchrotron emitting structures, the recently discovered Megahalos.

As the astrophysics community eagerly awaits the commissioning of the Square Kilometer Array (SKA) telescope in 2027, the future of magnetic field mapping in galaxy clusters looks bright. The SKA will provide synchrotron intensity for the GIS technique as well as polarization that can be used by other techniques developed by Professor Lazarian’s group to study the detailed 3D structure of astrophysical magnetic fields.

Professor Lazarian said: “The gradient technique is the practical outcome of a better understanding of fundamental magnetohydrodynamic processes, pushing us to delve deeper into these essential processes. Although the benefits of fundamental studies are not always immediately apparent, advances in the understanding of key physical processes induce tectonic shifts that affect many aspects of science and engineering.

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