The Invisible Web: Astronomers Unveil the Largest Map of the Universe's Magnetic Fields

To turn radio signals into a magnetic map, scientists exploit the Faraday rotation effect. When linearly polarized radio waves from a distant galaxy travel through magnetized plasma, their plane of polarization twists. The amount of twisting—called the Faraday rotation measure (RM)—is proportional to the strength of the magnetic field and the density of electrons along the line of sight .

By measuring this twist for every polarized source, researchers can reconstruct the magnetic fields that the light passed through. SPICE-RACS did this for nearly 4 million galaxies with a median RM error of just 2.2 rad/m², creating a grid with an effective resolution of about 23 arcminutes .

What the Map Reveals: A Hidden Cosmic Skeleton

The resulting map makes the invisible visible. Key findings include:

• Galactic Magnetic Filaments: The map resolves striking linear filamentary structures in the RM grid across the northern and southern Galactic poles. These filaments extend across the entire poles and resemble structures seen in neutral hydrogen and interstellar dust, suggesting a common underlying framework .
• A "Zipper-Like" Pattern along the Galactic Plane: Near the inner Milky Way, the team found a quasi-periodic, antisymmetric RM pattern about the Galactic plane. This unusual structure suggests line-of-sight reversals in the Galactic magnetic field, possibly tied to the Galactic bar or center .
• Magnetized Gas at High Redshift: A companion study combined SPICE-RACS data with the DESI quasar catalog to probe magnetized circumgalactic gas around galaxies at a redshift of roughly 1, revealing that substantial magnetic fields exist in the halos of galaxies billions of years ago .
• An Interacting System: The map provides the magnetic context for the interaction between the Milky Way and its satellite galaxies, the Large and Small Magellanic Clouds—offering a new way to study how galactic-scale magnetic fields influence cosmic collisions .

A Leap Beyond Two Decades of Data

To grasp the scale of this release, you have to understand what came before it. According to SKAO Chief Scientist Professor Naomi McClure-Griffiths, for the last 20 years the field was stuck with essentially the same dataset, which left the southern sky uncharted . SPICE-RACS represents a transformation measured in orders of magnitude:

• Sky Coverage: Five times larger than all previous such maps combined, covering from the South celestial pole to declination +30° .
• Source Density: Roughly 40 times more sources than earlier RM catalogs—nearly 4 million versus about 100,000 .
• Data Quality: The broadband observation strategy resolves ambiguities that plagued earlier narrow-band surveys, delivering a much cleaner RM grid .

From the POSSUM Collaboration to the SKA Era

SPICE-RACS is an early product of the Polarisation Sky Survey of the Universe's Magnetism (POSSUM) collaboration, a major ASKAP project dedicated to studying cosmic magnetism . While SPICE-RACS provides an early look, the formal POSSUM survey aims to produce an RM grid over approximately 30,000 square degrees with up to a million sources .

The ASKAP site in Western Australia is also the future home of the Square Kilometre Array's low-frequency telescope (SKA-Low). When the SKA begins operations later this decade, cosmic magnetism—one of the SKA's five Key Science Projects—will be studied with a sensitivity and resolution that make ASKAP look like a preview . Observations at higher frequencies (1–10 GHz) will trace magnetic fields in the disks and central regions of distant galaxies in unprecedented detail .

Why Cosmic Magnetism Matters

Magnetic fields are a fundamental force in the universe, but their origin and large-scale role remain a major unsolved problem in astrophysics. They influence how galaxies grow, how matter flows through space, and how the universe evolved over billions of years .

For decades, big questions—"When did magnetic fields first appear?" or "How do fields in the cosmic web shape galaxy formation?"—seemed almost impossible to answer empirically . SPICE-RACS turns them into tractable research questions. The data, freely available via CSIRO's data portal, gives the worldwide scientific community a new foundation for studying the magnetized universe .

In charting this invisible skeleton, the map marks the end of a 20-year data drought and the start of an era where the magnetic universe is finally coming into view.