August 30, 2017
Magnetic fields occur as regulatory forces in galaxies. They determine, for example, whether stars cluster in spiral arms or are grouped in an elliptical fashion. However, magnetic fields also influence the flow of matter into a black hole at the heart of a star system. Astronomers can now study in greater detail how the magnetic fields in galaxies of the younger universe evolved over time and became strong enough to unfold these effects. An international research team led by Sui Ann Mao from the Max Planck Institute for Radio Astronomy in Bonn has succeeded in measuring the magnetic field in a galaxy 4.6 billion light years away. To date, this galaxy is the most distant in which a coherent magnetic field has been observed, which means it had already evolved quite considerably when the universe was about two-thirds of its current age
It is still one of cosmology's great mysteries: despite all the advances made in this discipline, astrophysicists do not yet fully understand how magnetic fields in the universe have evolved over time. The originally very weak magnetic fields do not resemble those we observe in today's galaxies. To find out more about their development, Sui Ann Mao and her colleagues examined the magnetic field of a galaxy 4.6 billion light-years away. To this end, the astronomers resorted to a trick - using the Very Large Array radio telescope, they observed a quasar through the gigantic cosmic lens CLASS B1152 + 199.
When a background quasar and a foreground distant galaxy are closely aligned along the line of sight as in the system CLASS B1152+199, light from the quasar is gravitationally lensed by the foreground galaxy, forming two separate images as seen from Earth. One can use the light from the quasar passing through different parts of the lensing galaxy to study magnetic fields in a galaxy we otherwise cannot see. The team measured a property of the radio waves called polarization that changes when passing through the magnetic field of the foreground galaxy. The astronomers measured this change, the so-called Faraday rotation effect, of the two lensed quasar images to show that the distant lensing galaxy hosts a coherent large-scale magnetic field.
The detection of a strong coherent magnetic field in a galaxy when the universe was about two-thirds of its current age allows the team to measure how fast these fields grow in galaxies. “Although this distant galaxy had less time to build up its magnetic field compared to local galaxies, it still managed to do so”, says Sui Ann Mao, leader of a Minerva research group at the Max Planck Institute for Radio Astronomy in Bonn, the lead author of the study. “The results of our study support the idea that galaxy magnetic fields are generated by a dynamo process.” she adds.
Despite great progress in cosmology, how the Universe became magnetized remains an unsolved problem. It is generally recognized that the original magnetic fields in no way resemble the fields we see today in galaxies, but have been amplified and reconfigured by dynamo processes tied to circulation and turbulence within the interstellar gas. Describing the dynamo, particularly how it imparts large-scale structures to the magnetic field, is itself a largely unsolved problem. “Our measurements have provided the most stringent test to-date of how dynamos operate in galaxies”, says Ellen Zweibel from the University of Wisconsin Madison, USA.
"This finding is exciting - it is the first time we can reliably derive both the magnetic field strength and its configuration in a distant galaxy,” says Sui Ann Mao. The strong lensing system CLASS B1152+199 is now the record holder of the highest redshift galaxy for which this magnetic field information is available. “Our work demonstrates the power of strong gravitational lensing and broadband radio polarimetric observations in revealing magnetic fields in the high redshift universe,” she concludes.