Published: 18.07.08

Galactic magnetic fields stronger than expected

The origin of magnetic fields in galaxies is still a mystery to astronomers. Popular theories suggest continual strengthening over billions of years. The latest results from Simon Lilly’s group, however, contradict this assumption and reveal that young galaxies also have strong magnetic fields.

Samuel Schläfli
What equates to the magnetic field of perigee galaxies for quasars that are billions of light years away (large: “whirlpool” galaxy; small: quasar OC-65)? (Photo:
What equates to the magnetic field of perigee galaxies for quasars that are billions of light years away (large: “whirlpool” galaxy; small: quasar OC-65)? (Photo: (large view)

“There is an astronomer joke that goes ‘to understand the universe, we examine galaxies for radiation, gases, temperatures, chemical constitution and much more. Anything we can’t explain after that we attribute to the magnetic fields’”, explains Simon Lilly, Professor at the Institute of Astronomy at ETH Zurich. The creations of the magnetic fields in galaxies remain a badly researched mystery. Until now, it was deduced that galaxies which formed after the Big Bang 13.8 billion years ago had very weak magnetic fields that then proceeded to grow exponentially in strength over several billions of years. At least that is what the dynamo theory (see box), which is often used to explain the development of magnetic fields, conveys.

Statistical approach for exact proof

In the latest issue of “Nature”, Martin Bernet, Francesco Miniati and Simon Lilly probed into the topic of magnetic fields in young galaxies. The results are astonishing: Contrary to the popular dynamo explanatory model, the team was able to prove that even very young galaxies have a strong magnetic field on the basis of a statistical analysis of existing and new astronomical data. Technically speaking, determining the strength of magnetic fields that are many billions of light years from Earth is difficult and extremely time-consuming. This is probably one reason why the field has hardly been researched. Using the Faraday Rotation (FR) as the parameter, however, the strength of a magnetic field can be deduced from the polarization of the light in the radio field. If linearly polarized light radiates through a magnetized gas cloud, the polarization level of the light rotates. The rotation of the polarization level is all the larger the stronger the magnetic field is. This effect was first described by Michael Faraday in 1845. The researchers used quasars as radiation sources to measure the magnetic fields in the galaxies in question. These are extremely luminous objects whose radiation can in all likelihood be explained through the existence of supermassive black holes at the heart of the galaxies.

Observations in Chile

For their analyses, the scientists around Lilly had recourse to FR quasar measurements conducted by the astronomer Philipp Kronberg from the University of Toronto. Martin Bernet, a PhD student of Lilly’s and Miniati’s, studied the relationship between the Faraday Rotation and the redshift of the quasars’ light for 300 of these FR measurements. In astronomy, the redshift is used to determine the age and distance of galaxies. The researchers developed a thesis from the statistical distribution of values obtained: “The stronger, observed rotation of the quasar light with a higher redshift is on a longer path and can be ascribed to the subsequently greater probability of coming into contact with other galaxies”. To verify this thesis, the astronomers selected 76 quasars from the original Kronberg sample and, using the Very Large Telescope (VLT) in Chile, observed how many magnesium absorption lines are contained in quasar spectra. We know from earlier studies that almost every galaxy along a quasar’s line of sight (path of the light between quasar and telescope) displays magnesium absorption. The researchers were thus able to determine how many galaxies there are between ours and the quasar and ascertain the galaxies’ magnetic field by comparing the FR values of the line of sight with and without magnesium absorption. For the magnetic fields of the galaxies, the calculations yielded a value of approximately 10 μGauss, in other words a field that is about a million times weaker than the Earth’s magnetic field. This more or less corresponds to the values of our own galaxy, the Milky Way. The results enabled the researchers to prove that young, distant galaxies also have a strong, large-scale magnetic field. This at a time when the universe was only a third as old as it is today. The realization contradicts the popular dynamo theory, according to which magnetic fields build themselves up exponentially over billions of light years through constant reinforcement. “A galaxy’s magnetic field has to develop much more rapidly during its evolution than we previously assumed. A lasting equilibrium then appears at a relatively early stage”, explains Lilly.

Magnetic fields slip into the consciousness of astronomers

The scientific community has had doubts about the dynamo theory for some time. Kronberg also repeatedly expressed misgivings with regard to the existing model during his thirty-year FR measurements. The proof, however, has been lacking until now. According to Lilly, the latest issue of “Nature” above all reveals the high quality of the VLT measurements and the clear response to the original question that is somewhat rare for astronomy. “I could imagine that the methodology we recently introduced and the combination of Faraday Rotation measurements and data from telescopes like the VLT might open a new window into the distant universe”, Lilly speculates.

As the authors point out in the conclusion of the “Nature” article, the results should also lead to the reconsideration of the existing astronomical practice, where the magnetic fields are often ignored. Lilly and his team will continue to peer out of the window that has just opened and probe into the secrets of the magnetic fields. The researchers have already been granted additional observation time with the telescope. The next steps towards a more comprehensive understanding of the “mystery of the magnetic field” will be to increase the quasar sample and localize the magnetic fields precisely in the galaxies.

The Dynamo Theory

A dynamo converts mechanical energy into magnetic energy. The dynamo theory is an attempt at explaining the mechanism with which bodies in the sky can develop a magnetic field. In astronomical objects like planets, stars or galaxies, the dynamo effect occurs if there are turbulent currents and a non-uniform (differential) rotation prevails. This so-called alpha-omega dynamo can generate large-scale magnetic fields – even if the initial field was chaotic.

Bernet M., Miniati F., Lilly S., Kronberg P. & Dessauges–Zavadsky M.: Strong magnetic fields in normal galaxies at high redshift. Nature (17 July 2008) 454: 302-304, doi:10.1038/nature07105