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Let's talk about...Cosmic magnetism

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When you think about magnetism, your mind may return to school physics lessons, playing around with little magnets and iron filings. While magnetism is apparent to us at those very small scales, it also pervades the entire Universe.

As one of the four fundamental forces of physics like gravity, it helps to govern everything around us on Earth. Our own magnetic field, the magnetosphere, acts like a protective blanket to shield us from harmful radiation from the Sun. The beautiful auroras that we see in the skies of both hemispheres are the result of this interaction – and these auroras have been spotted beyond Earth, too. We’ll come back to that later in the article.

“Magnetic fields are everywhere!” says Dr George Heald of CSIRO Astronomy and Space Science, co-chair of the SKA’s Cosmic Magnetism Science Working Group. “Their influence can be felt in the space between objects, like at home when you feel the fridge door attracting a magnet before they are touching. When we talk about ‘magnetic fields’, it is a way to describe the ability for magnetic forces to influence objects or even light.”

Let’s start with the basics. Magnetic fields are generated whenever a charged particle like an electron or a proton moves, and space is full of moving charged particles – hence cosmic magnetism.

“As far as we can tell, everything in the Universe is magnetic, or at least co-exists with magnetic fields,” George explains. “One of the big outstanding questions in astronomy is why magnetic fields are so strong and so organised across wide areas in space, especially inside objects like galaxies.”

That strength is particularly extreme around the cores of dead stars (neutron stars or pulsars); we’re talking trillions of times stronger than on Earth. But that’s not the case everywhere. Vast regions between galaxies that we might think of as “empty” space still have magnetic fields, but here they may be a billion times weaker than on Earth.

“The study of magnetic fields in all these different environments is of paramount importance to track the full history of how magnetism has evolved in the Universe,” says Dr Valentina Vacca of INAF, co-chair of the working group alongside George. “Magnetic fields in evolved systems, like planets, stars, galaxies and clusters of galaxies, reflect this evolution, while those in more diluted environments such as filaments and voids between clusters of galaxies can give us precious information about more ‘pristine’ magnetic fields.”

While there’s much we don’t yet understand about magnetism’s role in the Universe, we know that it influences star formation, affecting how many stars are created from gas clouds inside galaxies, and how massive they become. It’s also possible magnetic fields affect the evolution of galaxies, for example influencing the formation of the arms of spiral galaxies.

Of course, we can’t see magnetic fields, so how do we study them?

“The only way we have to see them is through their actions,” says Valentina. “There are three physical mechanisms we can observe at radio wavelengths, so radio telescopes allow us to study magnetic fields with great detail compared to other telescopes.”

Spiral galaxy NG 628, around 32 million light years from Earth in the constellation Pisces. The flow lines overlaid on this optical image trace the orientation of the magnetic fields that run through the disk of this galaxy, as traced by radio observations.
Credits: Optical image: Steve Mazlin and Vicent Peris, Calar Alto Observatory; Radio data: Mulcahy et al. 2017, Jansky Very Large Array (VLA)

Time to get technical. The first mechanism is known as “Faraday rotation”, named for 19th Century British scientist Michael Faraday. Radio telescopes are highly sensitive to what we call polarised radiation – radio waves emitted by a distant object which travel to us along a single plane, rather than in lots of different directions.

On their way to Earth, the waves can be “twisted” by the magnetic fields of other objects they encounter, a change that radio astronomers can detect and use to measure the magnetic environment of those objects in between.

Next, we have “Zeeman splitting”, named after Dutch physicist Pieter Zeeman, who shared the 1902 Nobel Prize for his discovery. Here the narrow emission lines produced by atoms and molecules are split by the effects of magnetic fields. Emission lines are like the fingerprints of astronomical objects. When these lines split, it signals a change in energy that astronomers can observe, and enables them to measure the strength of the magnetic fields.

The third mechanism radio astronomers can observe is called synchrotron emission, a type of radiation generated by charged particles spiralling around magnetic field lines at close to the speed of light. That’s been at the heart of two recent discoveries involving the MeerKAT (see page 26) and LOFAR radio telescopes.

With LOFAR, researchers detected emission from a star which had the tell-tale signature of auroras, meaning it was likely being created by an interaction with an orbiting planet (Read our previous article on this story). In other words, the effects of magnetism had potentially opened up a new way of detecting exoplanets. Until now, exoplanet discoveries mostly involved using an optical telescope to measure the dip in a star’s light as its orbiting planet passed by, or the star’s “gravitational wobble” caused by the planet’s orbit, George explains.

“The radio technique can not only help us to understand the environment, including the magnetic field, around known exoplanets, but can possibly help us to find new exoplanets that are too small or far from their parent star to cause much of a gravitational wobble,” he says.

Here the radio emission is shown in blue; the diffuse emission along the filament connecting the two clusters is a direct expression of the magnetic field permeating this region.

Cosmic magnetism, exoplanets and distant galaxies might all seem a bit disconnected from everyday life in these strange times for world health, but important crossovers do occur.

“The signals used to study cosmic magnetism and other phenomena are often very faint from distant objects which require sensitive and high resolution images, a bit like the difficulty doctors have when trying to image parts of the human body,” Valentina notes. “In radio astronomy, techniques such as aperture synthesis – simulating a very large antenna with a collection of small ones – have been developed to tackle this, and medical imaging uses a technology inspired by it.”

The newest generation of radio telescopes, in particular SKA pathfinders and precursors like LOFAR, MWA, ASKAP, and MeerKAT, are especially well-suited to studying cosmic magnetism thanks to the broad range of frequencies they cover and high sensitivity, two key things you need to study faint and distant objects. The SKA will provide another leap in capabilities, allowing astronomers to map the vast magnetic structure which connects the Universe on the grandest scales.

The SKA will enable us to study the magnetic fields in cosmic objects to a level of detail that we have been unable to achieve with any other radio telescope so far

“We want to trace the beautiful ‘cosmic web’ that stretches through space on scales of tens and hundreds of millions of light years and more, and within which clusters of galaxies are embedded,” George says. “The SKA will enable us to study the magnetic fields in cosmic objects to a level of detail that we have been unable to achieve with any other radio telescope so far. We hope to reveal where magnetic fields originally came from, and how they’ve shaped our Universe!”

By Cassandra Cavallaro (SKAO)

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