A group of astronomers have detected the magnetic field of a galaxy five billion light years away from Earth, the most distant observation to date of a galaxy's magnetic field. The discovery shines a light on how magnetism formed in the early universe.

"This finding is exciting. It is now the record holder of the most distant galaxy for which we have this magnetic field information", says Dr. Sui Ann Mao, lead author of the study and astronomer at the Max Planck Institute for Radio Astronomy.

The astronomers' observations, published in the journal Nature Astronomy, show a magnetic field around the same strength as that of our own Milky Way, despite the fact the galaxy is five billion years younger than ours. This suggests that magnetic fields form early in the life cycles of galaxies and remain stable thereafter, which runs counter to one theory of magnetic field formation that says they are weak at first and become stronger and more coherent over time.

"The implication is that we need to understand magnetism to understand the Universe", adds Professor Bryan Gaensler, a co-author of the paper, from the University of Toronto.

Analysing the evolution of galactic magnetic fields requires studying galaxies at varying distances from Earth. These observations show us galaxies of different ages due to the fact they are so distant that even light takes millions of years to reach us. However, making these observations is very difficult because a magnetic field can't be detected directly. Astronomers work around this by observing the magnetic markings that affect any light passing through the field. This phenomenon is known as the Faraday Rotation.

The team were able to observe the magnetic field of the record breaking galaxy due to fortunate coincidence: A quasar – a supermassive black hole surrounded by clouds of gas and one of the brightest objects in the Universe – lay behind the galaxy in question in the same line of sight. This meant that light from the Quasar passed through the galaxy and was marked by its magnetic field on its way to Earth.

"Nobody knows where cosmic magnetism comes from or how it was generated," says Gaensler. "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."