The magnetic field of another galaxy detected with the VLA radio telescope

Scheme of the detection of a galaxy's magnetic field through a gravitational lens (Image Bill Saxton, NRAO/AUI/NSF; NASA, Hubble Heritage Team, (STScI/AURA), ESA, S. Beckwith (STScI). Additional Processing: Robert Gendler)
Scheme of the detection of a galaxy’s magnetic field through a gravitational lens (Image Bill Saxton, NRAO/AUI/NSF; NASA, Hubble Heritage Team, (STScI/AURA), ESA, S. Beckwith (STScI). Additional Processing: Robert Gendler)

An article published in the journal “Nature Astronomy” describes the measurement of the magnetic field of a galaxy that is nearly 5 billion light-years away from Earth, the farthest of which such a detection has been made. A team of researchers used the Very Large Array (VLA) to detect it not directly but thanks to a kind of magnetic footprint called Faraday effect that reached the Earth thanks to the light coming from a quasar that appears to us behind the galaxy studied.

The study of the magnetic fields of other galaxies is useful, together with that of other characteristics, to better understand their evolution. In the specific case, the galaxy in this study is similar to the Milky Way but we see it the way it looked when it was 5 billion years younger. The fact that traces of a magnetic field were found means that those of galaxies are generated very soon during their lifetime.

This type of observation is complicated by the fact that it can’t be performed directly. In this case, the researchers exploited the Faraday effect, also known as Faraday’s rotation, which in extremely simplified terms is the alteration of light passing through a magnetic field. In this case, the light comes from a quasar about 8 billion light years from Earth. Luckily, the galaxy to be studied is between the quasar and the Earth and acts as a gravitational lens bending the light and doubling the quasar’s image.

The quasar’s light has a preferential alignment, in technical terms is polarized. This characteristic is useful in the observations of the two quasar’s images, detected at radio frequencies by the VLA radio telescope. Their analysis shows significant differences in the way the polarization of radio waves was altered. In essence, different regions within the galaxy 5 billion light-years away have a different influence on the radio waves.

Sui Ann Mao of the Max Planck Institute for Radio Astronomy, the first author of the article, explained that the differences found in the analysis indicate that the galaxy studied has a coherent and large-scale magnetic field similar to those of the nearer galaxies we see as they were in relatively recent times.

The observations support the idea that galactic magnetic fields are generated by a dynamo effect similar to that of the Sun. However, the researchers are aware that there are other processes that can generate them so to determine with certainty which one is at work they have to examine farther galaxies that appear to us as they were billions of years ago.

The study of galactic magnetic fields is an important part in the physics of the tenuous gas that permeates interstellar space. Understanding the way they work can provide important clues about the evolution of galaxies and also of the stars and planets that are born and die inside them.

Gravitational lensing effect seen by the Hubble Space Telescope (Image Mao et al., NASA)
Gravitational lensing effect seen by the Hubble Space Telescope (Image Mao et al., NASA)

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