Fundamental to the cosmos are magnetic fields, which orchestrate the movement of elementary particles—the fundamental constituents of celestial bodies ranging from planets to galaxies.
While the genesis of cosmic magnetic fields remains an enigma, their omnipresence is well-established. Our own planet, Earth, possesses a magnetic field to which navigational instruments and avian migrants both orient themselves.
Astronomers leverage radio telescopes to probe the light emitted by remote galaxies, thereby illuminating vast, otherwise imperceptible, regions of space.
Our recent investigation, detailed in the current issue of Publications of the Astronomical Society of Australia, utilized Australia’s most potent radio telescope to generate an unprecedentedly extensive and granular depiction of cosmic magnetic fields.
Galactic Regulators: Cosmic Powerhouses
The intensity and distribution of magnetic fields exhibit considerable variation throughout the universe. Extremely dense astronomical objects, such as neutron stars and black holes, command magnetic fields magnitudes stronger than Earth’s by factors of trillions.
Conversely, within the interstellar medium, magnetic fields have been detected that are a million times less intense than Earth’s. Notwithstanding their attenuated strength, these fields play a critical role in shaping galactic evolution, functioning as immense energy reservoirs that can impede or even forestall the initiation of new stellar formations.
However, magnetic fields are inherently undetectable by direct human observation. To ascertain their presence in space, astronomers rely on analyzing electromagnetic radiation originating from distant stars and galaxies. This is because light itself is a manifestation of oscillating electric and magnetic fields, giving rise to the term “electromagnetic spectrum“.
As light traverses the cosmos, its trajectory is influenced by any intervening magnetic fields, causing a rotation in its plane of oscillation—a phenomenon termed “polarisation.” Consequently, light oscillating vertically exhibits a distinct polarisation from light oscillating horizontally.
Astronomers are adept at detecting this polarised light, particularly within the radio wave portion of the electromagnetic spectrum.
Visualizing the Unseen
Australian astronomical facilities have consistently been at the vanguard of both radio astronomy and the detection of magnetic fields since their initial discovery.
In 1962, CSIRO’s Murriyang, the Parkes radio telescope, achieved the pioneering detection of polarised light rotation attributable to extraterrestrial magnetic fields.
Since that landmark achievement, astronomers have been persistent in identifying an increasing number of cosmic sources exhibiting this characteristic light polarisation. By accumulating a sufficient volume of such measurements, a comprehensive map of the universe’s magnetic fields can be constructed.
Each data point on this map represents a celestial object observed by our telescope, with its emitted light serving as a tracer for the magnetic fields situated between us and that distant source. The greater the number of detected sources, the more refined the resultant map becomes.
The preceding significant magnetic field map was compiled in 2009. The absence of a comparable successor over the subsequent 17 years has constrained the depth and breadth of astronomical investigations previously undertaken.
Across various cosmic domains, including our own Milky Way galaxy, a complete understanding of the magnitude and configuration of cosmic magnetic fields remains elusive.
Not only is their origin subject to ongoing inquiry, but their potential evolution since the Big Bang is also largely unknown.
Addressing these fundamental questions necessitates the development of a new generation of radio telescopes.
A Telescope Engineered for Expediency
The field of radio astronomy is currently experiencing a transformative period with the ongoing construction of the Square Kilometre Array (SKA) Observatory, spanning sites in South Africa and Australia.
In anticipation of this monumental undertaking, a cohort of telescopes, designated as SKA precursors and pathfinders, are already operational worldwide.
The ASKAP radio telescope stands as one such precursor instrument. Situated at Inyarrimanha Ilgari Bundara, CSIRO’s Murchison Radio-astronomy Observatory on Wajarri Yamaji Country in Western Australia, this facility comprises 36 parabolic dishes, each 12 meters in diameter. These dishes possess the capacity to simultaneously observe vast swathes of the celestial sphere, affording astronomers an exceptionally broad perspective of the universe.
The principal initiative dedicated to mapping the universe’s magnetic fields is designated as the Polarisation Sky Survey of the Universe’s Magnetism (POSSUM).
As a preparatory phase for POSSUM, the telescope’s team developed the Rapid ASKAP Continuum Surveys (RACS). This endeavor is akin to compiling a comprehensive celestial atlas.
The most recent iterations of these surveys have identified close to 4 million distant galaxies, with approximately 2 million of these being previously uncatalogued discoveries.
Navigating the Magnetic Sky
Our recently generated map, identified as SPICE-RACS, is the product of a synergistic collaboration between the two survey teams.
Our objective was to examine every galaxy catalogued by RACS, searching for indications of polarisation shifts induced by magnetic fields. Employing the latest data release from the survey, we identified 350,000 galaxies out of the initial 4 million amenable to our analysis.
Our compiled dataset of celestial sources is approximately tenfold larger than the previous record, and fivefold greater than the cumulative total of all prior observational efforts combined.
Consequently, we have achieved the most extensive and detailed map to date.
The map employs colour coding, with red hues indicating magnetic fields directed towards the observer and blue hues signifying fields oriented away, analogous to the North and South poles of a compass.
A significant portion of the intricate, swirling patterns depicted originates from our own Milky Way galaxy. Within the finer details of the map lie the signatures of magnetic fields emanating from even more remote cosmic regions.
This novel map is already facilitating groundbreaking scientific research globally, and its data is freely accessible to the research community via a public online portal.
Looking ahead, our plan is to integrate all RACS survey datasets to produce an even more comprehensive and detailed map.
In parallel, the POSSUM project is projected to conclude its observational phase by 2030. The enhanced magnetic field map derived from this survey promises to unveil new insights into distant cosmic magnetic fields, thereby enabling us to peer further back into the universe’s historical timeline.
