Scientists map the largest magnetic fields in galaxy clusters 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 efficiency of turbulent dynamo processes in the amplification of these fields.

Galaxy clusters are large, gravitationally bound systems containing numerous galaxies, hot gas, and dark matter. They represent some of the most massive structures in the universe. These clusters can consist of hundreds to thousands of galaxies, bound together by gravity, and are embedded in vast halos of hot gas called the intracluster medium (ICM).

ICM, consisting mainly of ionized hydrogen and helium, is held together by the gravitational pull of the cluster itself. Magnetic fields in large-scale structures, like galaxy clusters, play pivotal roles in shaping astrophysical processes. They influence the ICM, impact galaxy formation and evolution, contribute to cosmic ray transport, participate in cosmic magnetization, and serve as tracers of large-scale structure evolution.

Prior studies and simulations have suggested that magnetic fields within clusters evolve, indicating their susceptibility to the dynamics of the cluster and experiencing amplification during merging events.

The study, published in Nature Communications, uses a method called synchrotron intensity gradient (SIG) to map magnetic fields in clusters, especially during galaxy mergers. This method provides a unique perspective on magnetic field structures and offers a tool to compare numerical expectations from simulations with observational data.

Lead author of the study, Prof. Alex Lazarian from UW-Madison, spoke to Phys.org about his motivation to study magnetic fields in galaxy clusters, saying, “The focus of my research lies in understanding the role of magnetic fields in astrophysical environments, particularly in magnetized and turbulent media.”

“Over the past two decades, I’ve extensively studied magnetic turbulence and reconnection processes in collaboration with my students. The technique used to map magnetic fields in galaxy clusters is grounded in the theoretical and numerical insights gained from years of research.”

Synchrotron intensity gradient

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

The SIG method introduces a unique perspective by mapping magnetic fields through a process rooted in the synchrotron intensity gradient. The basic principle behind the applied technique involves utilizing the interactions between magnetic fields and conductive fluids, specifically ionized gas or plasma.

The key idea is that magnetic fields influence the motion of these fluids, and their resistance to bending makes it easier to discern their direction. Prof. Lazarian explained, “These motions result in velocity gradients, and magnetic field fluctuations are perpendicular to the magnetic field. By measuring these gradients, one can obtain the direction of the magnetic field.”

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

“It utilizes data initially deemed irrelevant for magnetic field studies, allowing us to derive significant results from diverse archival datasets collected for purposes unrelated to magnetic field investigations,” said Prof. Lazarian.

Mapping magnetic fields

The researchers obtained maps of magnetic fields at the largest scales ever studied, specifically in the halos of galaxies 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 one based on measuring polarization. We also gauged the accuracy of SIGs with numerical simulations,” said Prof. Lazarian.

The study demonstrated that SIGs open a new avenue to map magnetic fields over unprecedentedly large scales. The complexity of plasma motion within merging galaxy clusters was revealed through the structure of the magnetic field.

The findings 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 mapping magnetic fields in galaxy cluster regions, except for relics. Unlike other methods, SIGs remain unaffected by depolarization. This study aimed to verify if SIGs and polarization indicate the same magnetic field directions where polarization is present.

First author Ph.D. student Yue Hu, with Italian scientists Dr. Annalisa Bonafede and Dr. Chiara Stuardi, successfully tested magnetic field measurements within relics, confirming the reliability of SIG magnetic field maps. Prof. Lazarian’s Ph.D. student Ka Wai Ho’s fluid dynamics simulations further affirmed map accuracy.

SIGs provide a unique way to address longstanding questions about the origin, evolution, and effects of magnetic fields in galaxy clusters without facing the challenges that traditional measurements do.

Heat conduction in ICM

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

“Heat conduction in intracluster plasma (fully ionized gas) of 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. The changes in heat conductivity control the formation of cold gas streams surrounded by hot gas, the so-called cooling flows,” explained Prof. Lazarian.

Cosmic ray acceleration

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

“Clusters of galaxies are known to accelerate cosmic rays through the interaction of cosmic rays with moving magnetic fields. The picture of this acceleration is still unclear and depends on magnetic field dynamics,” said Prof. Lazarian.

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

The dynamics of the magnetic fields within the clusters can now be mapped using the SIG technique, helping us understand the operation of the largest particle accelerators in the universe.

Concluding thoughts

SIGs, with their ability to map magnetic fields in regions where polarization information is lost, offer invaluable insights into the halos of galaxy clusters and even larger synchrotron-emitting structures, the recently discovered Megahalos.

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

Prof. Lazarian said, “The gradient technique is a practical fruit of a better understanding of fundamental magnetohydrodynamical processes, propelling us to delve deeper into these essential processes. While the benefits of fundamental studies may not always be immediately apparent, advances in understanding key physical processes induce tectonic changes that affect many aspects of science and engineering.”

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