FM08: New Insights in Extragalactic Magnetic Fields

Artist’s impression of the Square Kilometre Array at night featuring all four elements. The low frequency aperture array antennas (bottom right) and precursor ASKAP dishes (background right) will be located in Western Australia. The SKA-mid (front left) dishes and precursor MeerKAT dishes (background left) will be located in South Africa, with some remote stations in other African partner countries. The SKA will revolutionise studies of cosmic magnetic fields. (SKA Organisation)

Magnetic field contours in the Whirlpool Galaxy, also known as Messier 51, in the northern constellation Canes Venatici. (Max Plank Institute for Radio Astronomy)

Magnetic fields are keys ingredients of the extragalactic Universe on many different spatial scales, from individual galaxies and active galactic nuclei (AGN) to clusters of galaxies and large-scale structure. They play an important role in the formation of large-scale structure and the enrichment of the intergalactic medium, affecting turbulence, cloud collapse, large-scale motions, heat and momentum transport, convection, viscous dissipation and more. Magnetic fields are of utmost importance in the growth of radio galaxies and AGN and are crucial for the formation of spiral arms in galaxies, outflows and star-formation processes.

Despite their importance and ubiquity, magnetic fields remain poorly understood. The origins of the fields that we observe remain largely uncertain. A commonly accepted hypothesis is that extragalactic magnetic fields result from the amplification of much weaker pre-existing “seed” fields via shock/compression and/or turbulence/dynamo amplification during merger events, and different magnetic field scales survive as the result of turbulent motions. The origin of seed fields is unknown. They could be either primordial, i.e., generated in the early Universe prior to recombination, or produced locally at later epochs of the Universe, in early stars and/or (proto)galaxies, then injected into the interstellar and intergalactic medium.

The cosmic origin of magnetic seed fields and the subsequent processes through which they are amplified give us critical information on the growth of structure in the Universe. The history of these processes can be uncovered through accurate knowledge of the strength and structure of magnetic fields in galaxy clusters and at the boundaries between them, in the intergalactic medium, in the filamentary cosmic web and in the relation of magnetic fields to gas flows in spiral galaxies, radio galaxies and AGN.

The coming years and decades will see a burst in our knowledge and understanding of extragalactic magnetic fields due to the next generation of radio astronomy facilities, especially the Square Kilometre Array (SKA) and its pathfinders and precursors, as well as major advances in magnetohydrodynamic numerical simulations and algorithmic improvements to extract magnetism information from observational databases.

Focus Meeting 8 will bring together the scientific community to discuss the challenges and opportunities for understanding the magnetized Universe from scales of galaxies to the cosmic web and connect information across the spectrum. It is coordinated by Commission B4, Radio Astronomy, in Division B, Facilities, Technology and Data Science, with support from Division J, Galaxies and Cosmology. In FM8 we’ll address the following fundamental questions:

  • How did magnetic fields form and evolve and how are they maintained?
  • How do magnetic fields control the acceleration and dynamics of relativistic particles in astrophysical plasmas?
  • How do magnetic fields affect the evolution of thermal plasmas in galaxies and clusters?
  • How do magnetic fields illuminate otherwise invisible processes in the thermal plasma?
  • How do the insights from magnetic field studies contribute to larger questions about the origin and evolution of structures in the universe, from galactic to cosmic web scales?

Invited speakers include Kandu Subramanian (India) on the origins of cosmic magnetism, Hiroshi Nagai (Japan) on magnetism across the electromagnetic spectrum, Tina Kahniashvili (USA) on magnetism in the early Universe, Annalisa Bonafede (Italy) on magnetic fields in galaxy clusters, Marcus Brüggen (Germany) on magnetic fields in the intergalactic medium and in the cosmic web, Robert Laing (UK) on magnetic fields in spiral galaxies and radio galaxies, Jeroen Stil (Canada) on the capabilities of next-generation telescopes for investigating cosmic magnetism, Valentina Vacca (Italy) on techniques and algorithmic advances in the SKA era and Ann Mao (Germany) on studies of magnetism in the SKA era.

Most of what we know about cosmic magnetic fields comes from sensitive radio observations, which prove the existence of relativistic electrons gyrating around magnetic field lines. In addition, measurements of the Faraday rotation effect on the polarized emission of radio galaxies crossing a magneto-ionic medium gives information on both the intensity and structure of the magnetic field. Besides studies at centimeter and millimeter wavelengths, magnetism can be investigated in other parts of the spectrum. For example, the density of thermal gas in galaxy clusters obtained from X-ray data is crucial for the interpretation of rotation measure data. In addition, upper bounds to the magnetic fields in voids and large-scale structure can be obtained from studies of anisotropies and dust polarization in the cosmic microwave background radiation and of the multi-TeV gamma-ray flux of distant blazars, whose emission is deflected by extragalactic magnetic fields.

Given the huge shift in processing regimes required by new instruments, the discussion in FM8 will engage the observational and theoretical communities to analyse the results in hand, present new algorithms and numerical techniques for the interpretation of the observations and address theoretical issues. With this approach, we accelerate our ability both to explore the massive volumes of data that will be delivered by new instruments and to achieve our ultimate quest to obtain a deeper understanding of the magnetised Universe.

LUIGINA FERETTI is an Associate and former Director at the Istituto Nazionale di Astrofisica (INAF) Istituto di Radioastronomia, Bologna, Italy, a core member of the Square Kilometre Array (SKA) Science Working Group (WG) on Cosmic Magnetism and a member of the SKA Board.


GEORGE HEALD is Science Leader at CSIRO Astronomy and Space Science (CASS), Perth, Australia, Co-Chair of the SKA Cosmic Magnetism Science Working Group and Chair of the Board of the Murchison Widefield Array (MWA).


MELANIE JOHNSTON-HOLLITT is MWA Director, Chief Executive Officer of Peripety Scientific Ltd., a core member of the SKA Cosmic Magnetism Science WG and a professor of radio astronomy at Curtin University, Perth, Australia.


FEDERICA GOVONI is a researcher at INAF Osservatorio Astronomico, Cagliari, Italy, a core member of the SKA Cosmic Magnetism Science WG and Director of INAF National Qualifying Division II for Radio Astronomy.