Cosmic magnetic fields in ancient galaxies surprisingly strong
Magnetic fields influence the physics of exploding stars, galaxies, and even the cosmic large-scale structure. One of the most central questions is how they reached their current strength. It is believed that the amplification of galactic magnets happens through the so-called “dynamo effect”. The simplest version of this theory predicts that magnetic field strengths in galaxies should be continually evolving and should be much weaker in the cosmic past. Observational tests of the dynamo model could therefore hold the key to answering basic questions of fundamental physics during the history of the Universe.
Testing the dynamo model has been a long-standing problem because normal galaxies are very faint and mostly invisible to radio astronomers. CAASTRO Affiliate Dr Jamie Farnes at the University of Sydney (soon moving to Radboud University in the Netherlands) and colleagues, however, used a novel “line-of-sight” technique to put the model to the test. This technique combines optical and radio data from bright quasars in the distant Universe. The optical data from over 35,000 sources served to determine the magnesium absorption lines in the spectra of sources, caused by the light passing through a foreground intervening galaxy. Sources with more magnesium lines, and hence more galaxies along the line-of-sight, should have larger magnetic fields. The radio data from over 25,000 polarised background sources allowed for measuring the magnetic field along the entire line-of-sight with measurements of their spectral index and rotation measure.
For their resulting sample of 599 sources, the researchers found – somewhat unexpectedly – that their flat spectrum sub-sample was correlated to the rotation measure at 1.4GHz. Having eliminated all potentially confounding factors in the data, the team concluded that magnetic fields seem to be the same strength 7 billion years ago as they are today. This result argues strongly that the simplest form of dynamo model cannot be operating, and that more exotic, fast-acting dynamos must be at play throughout the Universe. For example, some dynamo models argue that all the magnetic field amplification occurred early in a galaxy’s history, and has now plateaued – in essence these dynamos “lived-fast and died-young’. CAASTRO continues to have an involvement in this research with follow-up studies using the Very Large Array in the US.
J. S. Farnes, S. P. O’Sullivan, M. E. Corrigan, B. M. Gaensler in ApJ (2014): “Faraday Rotation from Magnesium II Absorbers towards Polarized Background Radio Sources”