5 billion-year-old galaxy prompts cosmic magnetism rethink


A distant galaxy has provided a major clue in the mystery of the development of magnetism in the universe, writes Andrew Masterson.


The magnetic field of the Milky Way, looking along the galactic plane.
The magnetic field of the Milky Way, looking along the galactic plane.
ESA / Planck

A magnetic field in a galaxy 5 billion light years away is prompting astronomers to re-examine theories about how magnetism develops on a cosmic scale.

The field was discovered by a team led by Sui Ann Mao of the Max Planck Institute for Radio Astronomy in Bonn, Germany. The galaxy is the most distant and youngest confirmed to have a magnetic field.

Because the galaxy is 5 billion light years away, astronomers now see it as it was 5 billion years ago. This insight into the past is of key significance.

Recent research published in the Monthly Notices of the Royal Astronomical Society suggested that “magnetic fields grow exponentially at early times” in a galaxy’s life.

The study, led by Rüdiger Pakmor of Germany’s Heidelberg Institute for Theoretical Studies, noted that how magnetic fields in galaxies form was so challenging a matter that “most previous attempts to simulate galaxy formation from cosmological initial conditions have ignored [it] altogether.”

The middle one of the three blobs in this Hubble image is the galaxy in which a magnetic field has been detected.
The middle one of the three blobs in this Hubble Space Telescope image is the galaxy in which a magnetic field has been detected. The other two are gravitationally lensed images of a single quasar that allowed the magnetic field measurement.
NASA
The findings by Mao’s team, published in the journal Nature Astronomy, paint a different picture.

The distant galaxy’s magnetic field is estimated to be much the same strength and configuration as that of the Milky Way – evidence suggesting magnetic fields form while galaxies are young and remain relatively stable.

"This means that magnetism is generated very early in a galaxy's life by natural processes, and thus that almost every heavenly body is magnetic," says co-author Bryan Gaensler of the University of Toronto. "The implication is that we need to understand magnetism to understand the universe."

Despite being very large things, galaxies have extremely weak magnetic fields. The entire Milky Way, for instance, has a magnetic field more than a million times weaker than that of the Earth.

Insight into how the fields develop involves measuring ever more distant galaxies. The further away a galaxy is, the longer its light takes to reach Earth. What we are observing now is the galaxy when it was far younger.

Galactic magnetic fields are impossible to detect directly. Instead, astronomers make use of a phenomenon known as Faraday rotation – the degree to which light is altered as it passes through a field.

To make their calculations on the distant galaxy, Mao and colleagues measured the light of a quasar positioned beyond the galaxy. They did this using the Karl G. Jansky Very Large Array in New Mexico, operated by the US National Radio Astronomy Observatory.

"Nobody knows where cosmic magnetism comes from or how it was generated," Gaensler says. "But now we have obtained a major clue needed for solving this mystery, by extracting the fossil record of magnetism in a galaxy billions of years before the present day."

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Andrew Masterson is an author and journalist based in Melbourne, Australia.
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