Faraday caustics: Light patterns from cosmic magnetism

Similar to the light pattern at the bottom of a swimming pool on a sunny day, recent work by scientists at the Max Planck Institute for Astrophysics predicts that future images of cosmic magnetic fields will contain networks of bright structures. These structures should play a key role in the three-dimensional cartography of galactic magnetic fields, such as the one permeating the turbulent plasma in the Milky Way. Studying these in detail has been difficult in the past but will strongly benefit from the new generation of radio telescopes which permit 3-dimensional mapping of magnetic structures. The MPA scientists have now predicted that among the first structures to be seen by this novel method will be bright sheets of polarized emission, which they named 'Faraday caustics'. These Faraday caustics are images of magnetic field reversal in our galaxy and elsewhere, and therefore open a new window to study cosmic magnetism in greater detail.

Fig. 1: Optical caustics at the bottom of a swimming pool. Ripples on the surface of the pool refract the sunlight to produce a bright network of features on the floor. Similarly with Faraday caustics, particular magnetic field configurations produce bright features in polarized radio emission.
Image credit: linkPfeilExtern.gifGregory Massal

Fig. 2: a) A simulated Faraday spectrum, i.e. the polarized intensity separated by the amount of Faraday rotation suffered. The spectrum shows examples of Farday caustics, circled in red and green.
b) The Faraday rotation as a function of distance along the line of sight (LOS).
c) The LOS component of the magnetic field as a function of LOS distance. This effects the Faraday rotation. Faraday caustics appear when this is zero.
d) The magnetic field in the plane of the sky as a function of line of sight distance. The caustics, circled in red or green in the Faraday spectrum, are associated with features in the magnetic field distribution marked by the like colored lines.

Imagine a sunny day at the pool. Looking down at the bottom of the pool reveals a network of ridges of bright light that is constantly in motion. These structures, known as optical caustics, are an effect of the sunlight being focused to a single point as it is refracted by the wavy surface of the water. The rippling surface causes light to "pile up" in certain regions at the bottom of the pool instead of filling all the space equally.

What does this have to do with astrophysics? Recently, Michael Bell, Henrik Junklewitz and Torsten Enßlin have shown that similar features, which they are calling "Faraday caustics", can be seen in images of polarized radio emission produced using next generation radio telescopes. Just as the caustics at the bottom of the pool trace conditions at the surface of the water, Faraday caustics trace specific properties of magnetic fields in the universe. In the same way that one might study the properties of the pool's surface by observing the network of light patterns at the bottom, the authors propose that Faraday caustics may be very useful for learning about the distribution of magnetic fields in the universe and to help shed light on their yet unknown origins.

Magnetic fields can be found everywhere in the cosmos. They are generated by planets, like the Earth, stars or other celestial objects, and permeate the vast space of the largest structures in the universe, such as galaxy clusters. Nevertheless, although we know of their existence, it is often difficult to measure their exact properties. For those observing radio waves, an effect known as Faraday rotation can be a good tracer of some of the magnetic fields' properties. The effect of Faraday rotation is that the plane of polarization of a radio wave is rotated as it passes through a magnetized plasma. The amount of rotation depends, among other things, on the properties of the magnetic field and the observed frequency. Since this rotation can be calculated from polarization sensitive observations at different frequencies, Faraday rotation has been a very useful tool for studying cosmic magnetism.

Astronomers face the problem that the radiation from a single direction might have been emitted by two different radio sources. The radiation from each source would have traveled through different magnetic fields and been rotated by different amounts. How can astronomers distinguish between these sources? To overcome this problem a new measurement technique was devised in recent years called "Rotation Measure Synthesis". This technique uses the same mathematical approach used to analyze the different frequency components that produce a complicated acoustic signal, like a song. After measuring polarized radio emission at many different frequencies, we can reconstruct the 'Faraday spectrum', i.e. separate the polarized emission into components that are rotated by different amounts due to Faraday rotation. In this spectrum, Faraday caustics are predicted to leave a tell-tale trace.

The scientists of the Max Planck Institute have shown that Faraday caustics are caused by reversals of the magnetic field orientation along a line of sight. Such reversals are quite common in turbulent astrophysical environments, therefore Faraday caustics are predicted to appear in many observations. The behavior and statistics of the Faraday caustics will reveal structural and statistical properties of cosmic magnetic fields.

The authors show that the new European radio telescope LOFAR, in whose construction the Max Planck Institute for Astrophysics is participating, will be ideally suited to observing Faraday caustics. Using LOFAR and other telescopes, future observations specifically designed to look at Faraday caustics will greatly improve our understanding of the magnetic fields in our own and other galaxies, and help to unravel their yet unknown origins. This is the aim of the German research unit "Magnetisierung des interstellaren und intergalaktischen Mediums“ (Magnetization of the interstellar and intergalactic medium) funded by the German Society of Research (DFG) of which Michael Bell, Henrik Junklewitz and Torsten Ensslin are members.

Michael Bell, Henrik Junklewitz, and Torsten Enßlin

Related Links

linkPfeilExtern.gifLOFAR Telescope Array
linkPfeilExtern.gifLOFAR in Deutschland
linkPfeilExtern.gifMPA LOFAR Project

linkPfeilExtern.gifResearch group about "Magnetisation of Interstellar and Intergalactic Media"

Related Publication

M. R. Bell, H. Junklewitz, T. A. Enßlin, "Faraday caustics: Singularities in the Faraday spectrum and their utility as probes of magnetic field properties", submitted to A&A. linkPfeilExtern.gifhttp://arxiv.org/abs/1105.2693