Detected Magnetic Field of an Ancient Galaxy


A magnetic field spanning 16,000 light-years wide has been detected in an ancient galaxy whose light is amplified by a gravitational lens. Since it is so distant from us, we see this galaxy as it existed over 11 billion years ago. Although all galaxies contain a gigantic magnetic field, astronomers have never before detected galactic magnetism at such a remote time.

The Discovery

Discovery was made using the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, by a multinational team of astronomers led by astrophysicist Jim Geach from the University of Hertfordshire in the United Kingdom. The galaxy, known as ASW0009io9, was discovered in 2014 as part of a citizen science project called Space Warps.

Project produced 7.5 million classifications of lensed galaxies in images taken by the Legacy Survey of the Canada–France–Hawaii Telescope (CFHT). The galaxy’s magnification is distorted, appearing as what is known as an Einstein ring. This phenomenon occurs when the distant galaxy, a foreground object like a galaxy cluster, and Earth are all in nearly perfect alignment.

Geach, who initially led the observations of ASW0009io9 in 2014, led a team that used ALMA to detect the light emitted by interstellar dust grains, which often contain metals. These dust grains align with the galaxy’s magnetic field. The alignment causes the polarization of long-wavelength light emitted by the dust. When light is polarized, it means the photons oscillate in a preferred direction. It’s a bit like looking at the galaxy through sunglasses, which only allow through light with a certain polarization.

Do All Galaxies Have a Magnetic Field?

All galaxies, including our Milky Way, contain a diffuse magnetic field woven into the fabric of a galaxy’s molecular gas and dust clouds. However, the origin of these magnetic fields is a mystery. “We actually know very little about how these fields form, despite being crucial for the evolution of galaxies” said Enrique Lopez Rodriguez of Stanford University, a member of Geach’s team.

We see the galaxy ASW0009io9 at a time when the universe was just 2.5 billion years old. “The discovery provides us with new clues about how galactic-scale magnetic fields form” Geach said. Indeed, this ancient galaxy tells astronomers that whatever the formation process of galactic magnetic fields is, it occurs relatively early and rapidly.

The magnetic field of this ancient galaxy is not particularly strong and measures 500 microGauss. It is a thousand times weaker than Earth’s magnetic field, which ranges from 25 to 65 Gauss. The value is quite typical of galactic magnetic fields. For example, the Milky Way’s magnetic field is even weaker, at only 20-40 microGauss. The reason for their weakness is the immense size of these objects.

Geach’s team suspects that intense periods of star formation in the early life of ASW0009io9 contributed to propagating the magnetic field throughout the galaxy. In turn, it is known that diffuse galactic magnetic fields permeating molecular gas clouds influence subsequent star formation. Where flows meet, the density and temperature of the gas increase until a star is born.

Stefano Gallotta
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