An international team of astronomers has measured the magnetic field of a star-forming galaxy nearly 4.6 billion light-years away.
An image obtained using the NASA/ESA Hubble Space Telescope: the two brightest objects (upper right and lower left) are lensed images of the same, distant quasar. The dimmer object is the lensing disk galaxy in which a magnetic field was detected. Image credit: NASA / ESA / Hubble Team.
The galaxy in question is a star-forming disk galaxy that acts as a gravitational lens in the lensing system CLASS B1152+199.
The galaxy is at a redshift of 0.439, putting it about 4.6 billion years in the past.
It lies directly between a more-distant quasar and Earth, and its strong gravity splits the quasar’s image into two separate images. Importantly, the radio waves coming from this quasar, nearly 7.9 billion light-years away, are preferentially aligned, or polarized.
“The polarization of the waves coming from the background quasar, combined with the fact that the waves producing the two lensed images traveled through different parts of the intervening galaxy, allowed us to learn some important facts about the galaxy’s magnetic field,” explained lead author Dr. Sui Ann Mao, of the Max Planck Institute for Radio Astronomy in Bonn, Germany.
According to astronomers, galaxies have their own magnetic fields, but they are incredibly weak — a million times weaker than the Earth’s magnetic field.
One theory suggests that the magnetic field of a young galaxy starts off weak and tangled, becoming stronger and more organized over time.
But, because the magnetic field of the observed galaxy is not much different from the fields scientists observe in our own Milky Way Galaxy and nearby galaxies, the detection is evidence that galactic magnetism appears relatively early, rather than growing slowly over time.
“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,” said co-author Professor Bryan Gaensler, Director of the Dunlap Institute for Astronomy & Astrophysics at the University of Toronto.
“The implication is that we need to understand magnetism to understand the Universe.”
Schematic view of the lensing system CLASS B1152+199: the distant quasar located 7.9 billion light-years away is gravitationally lensed by the foreground galaxy 4.6 billion light-years away. Sightlines toward images A and B probe different magnetic fields and gas conditions through different parts of the lensing galaxy. Image credit: Sui Ann Mao.
The observation was made using NSF’s Karl G. Jansky Very Large Array in the New Mexico desert.
“This finding is exciting. It is now the record holder of the most distant galaxy for which we have this magnetic field information,” Dr. Mao said.
“This measurement provided the most stringent tests to date of how dynamos operate in galaxies,” said co-author Dr. Ellen Zweibel, from the University of Wisconsin-Madison.
“Nobody knows where cosmic magnetism comes from or how it was generated,” Professor Gaensler added.
“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.”
The findings appear in the journal Nature Astronomy (arXiv.org preprint).
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S.A. Mao et al. Detection of microgauss coherent magnetic fields in a galaxy five billion years ago. Nature Astronomy, published online August 28, 2017; doi: 10.1038/s41550-017-0218-x
