Cosmic Magnetism with the Square Kilometre Array and its Pathfinders
One of the five key science projects for the Square Kilometre Array (SKA) is “The Origin and Evolution of Cosmic Magnetism”, in which radio polarimetry will be used to reveal what cosmic magnets look like and what role they have played in the evolving Universe. Many of the SKA prototypes now being built are also targeting magnetic fields and polarimetry as key science areas. Here I review the prospects for innovative new polarimetry and Faraday rotation experiments with forthcoming facilities such as ASKAP, LOFAR, the ATA, the EVLA, and ultimately the SKA. Sensitive wide-field polarisation surveys with these telescopes will provide a dramatic new view of magnetic fields in the Milky Way, in nearby galaxies and clusters, and in the high-redshift Universe.
💡 Research Summary
The paper outlines a comprehensive roadmap for probing cosmic magnetism using the Square Kilometre Array (SKA) and its precursor facilities—ASKAP, LOFAR, the Allen Telescope Array (ATA), and the Expanded Very Large Array (EVLA). It begins by emphasizing that magnetic fields are a fundamental component of the Universe, influencing galaxy formation, star formation, cosmic‑ray propagation, and the thermodynamics of the intergalactic medium (IGM). Yet, current knowledge of magnetic field strength and geometry is limited to a few nearby objects and narrow frequency ranges. The SKA’s unprecedented sensitivity, wide field of view, and broad frequency coverage (0.1–10 GHz) are poised to transform this landscape by delivering all‑sky, high‑resolution Faraday rotation measure (RM) grids that trace magnetic fields from the Milky Way to the most distant quasars.
The author then reviews each precursor instrument in detail. ASKAP, with its 36‑dish 12 m array operating from 300 MHz to 1.8 GHz, offers a 30 deg² field of view and fine spectral resolution, enabling the construction of RM grids containing thousands of sources per pointing. LOFAR, covering 10–240 MHz, excels at detecting extremely weak magnetic fields in high‑redshift environments because low‑frequency polarization is highly sensitive to small Faraday depths. The ATA provides rapid, wide‑band surveys (0.5–10 GHz) that are ideal for monitoring polarization variability, while the EVLA delivers sub‑arcsecond imaging across 1–50 GHz, allowing detailed studies of magnetic structures in galaxy nuclei and active galactic jets.
A central theme is the synergy among these facilities. By combining ASKAP’s wide‑field, mid‑frequency data with LOFAR’s low‑frequency sensitivity and EVLA’s high‑resolution imaging, researchers can map magnetic fields across a continuum of spatial scales—from parsec‑scale structures in star‑forming regions to megaparsec‑scale fields in galaxy clusters and the IGM. The paper highlights modern analysis techniques such as RM synthesis, which exploits broadband data to reconstruct the Faraday depth spectrum, and QU‑fitting, which directly models the complex Stokes Q and U spectra to disentangle multiple Faraday components along the line of sight.
Scientifically, the proposed observations aim to achieve several milestones. First, a dense RM grid across the Milky Way will resolve the three‑dimensional geometry of the Galactic disk, spiral arms, and halo, refining models of the Galactic dynamo. Second, polarization surveys of nearby galaxies (e.g., M31, M33) and clusters (e.g., Virgo) will reveal how magnetic fields are amplified and ordered by large‑scale flows, turbulence, and mergers. Third, by detecting polarized emission from high‑redshift quasars and radio galaxies (z > 2), the SKA will test theories of primordial seed fields (∼10⁻⁹ G) and their subsequent amplification via small‑scale dynamos or plasma instabilities.
The paper also discusses the data‑processing challenges posed by the massive data rates expected from the SKA. Real‑time calibration, ionospheric correction, and machine‑learning classification of polarized sources will be essential to turn raw visibilities into scientifically useful RM catalogs. The author argues that the combination of deep, high‑resolution SKA observations with the precursor surveys will ultimately produce a “magnetic map of the Universe,” enabling quantitative tests of how magnetic fields influence cosmic structure formation and evolution.
In summary, the article presents a compelling case that the next generation of radio interferometers, culminating in the SKA, will provide the necessary tools to answer long‑standing questions about the origin, amplification, and role of magnetic fields throughout cosmic history. By leveraging complementary frequency coverage, angular resolution, and survey speed, the community will be able to construct unprecedentedly detailed three‑dimensional magnetic field models from the Milky Way to the high‑redshift Universe.