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The Origin and Evolution of Cosmic Magnetism

The giant radio galaxy Hercules A.

Figure 1. The giant radio galaxy Hercules A. Radio synchrotron jets emerging from the optical host of the galaxy mark the presence of magnetic fields roughly 1 million light-years in scale. Credit: NASA, ESA, S. Baum and C. O'Dea (RIT), R. Perley and W. Cotton (NRAO/AUI/NSF), and the Hubble Heritage Team (STScI/AURA)

By Melanie Johnston-Hollitt

Understanding the origin and evolution of magnetic fields in the universe is one of the great challenges of modern astrophysics. The unique capabilities of the SKA will provide astronomers with the best tools to explore how, when and where magnetic fields in the cosmos formed.

Most people are familiar with magnets, usually in the form of the ubiquitous fridge magnet, which is bound tightly to your refrigerator by a magnetic field. Many people will also know that the Earth possesses a global magnetic field that is generated by the motion of iron-rich material in the depths of the planet.

What is perhaps more surprising is that the entire universe is filled with magnetic fields, that these fields are in many ways still a mystery, and that we have yet to address fundamental questions about their origin, strength and evolution. Fortunately, the mysteries of these cosmic magnetic fields are about to be solved as the Square Kilometre Array (SKA) radio telescope provides us with the first glimpses into the depths of the hidden magnetised universe.

The origin and evolution of cosmic magnetism is one of the great unsolved problems of modern astrophysics. In fact, so important and fundamental are the questions concerning magnetism that they touch almost every aspect of modern astronomy. For example, we do not yet know how magnetic fields first arose in the universe, nor if these fields arose over the entire universe simultaneously, allowing individual objects to become more magnetised over time through a process known as amplification, or if small pockets of magnetism formed in individual astronomical objects that then spread magnetism out into the vast regions of intervening space.

We believe magnetism may have played an important role in the formation of structure in the early universe, but we don’t know for sure what that role was or how influential it might have been. We know there are a plethora of bodies in the cosmos that contain magnetic fields on a variety of scales, from the stars to vast collections of gravitationally bound galaxies, and yet we can only speculate about how important those magnetic fields are in the ongoing life cycle of these objects.

For more than 100 years we have known that magnetic fields are associated with the production of ultra high-energy cosmic rays. These are highly charged particles of unknown astrophysical origin that must have been accelerated to high energies in regions of space with concentrated magnetism. Ironically, additional magnetic fields in intervening space bend the paths of these charged particles away from their original trajectory and confound our efforts to determine the origin of cosmic rays.

We know that magnetic fields are integral to the processes that create and sustain unimaginably vast jets of electrons emerging from active galactic nuclei (Fig. 1) – galaxies with supermassive black holes at their cores that light up in the radio spectrum – but the precise details of just how these jets arise in the depths of the galaxies, close to the black hole, is still unknown.

We also see evidence of the effects of magnetic fields on the shapes of spiral galaxies, which exhibit both an ordered overall field and a turbulent, disturbed field as a result of generations of supernova explosions twisting and compressing the surrounding material. While the presence and strength of these fields is known, understanding the mechanisms that create, amplify and sustain them requires much more data.

There are several ways that astronomers can gain information about the magnetic fields present in regions of the universe, but the best and most direct methods make use of radio emission. In particular, synchrotron emission, which is generated by electrons moving along helical paths in a magnetic field, allows radio astronomers to make maps of magnetic fields in the plane of the sky and to probe the strength of the magnetic field perpendicular to the plane of the sky. The direction of the magnetic field in the plane of the sky is measured by observing the polarisation of the synchrotron radiation with a radio telescope.


Figure 2. The spiral galaxy M51 showing the magnetic field lines measured by synchrotron emissions (overlaid). Note that the magnetic field of the galaxy follows the spiral pattern. Credit: R. Beck (MPIfR) and A. Fletcher (Newcastle University)

Radio telescopes are formed by a pair of cross-dipole receivers that allow us to measure the electric field of the radiation in two perpendicular directions. From these measurements we can construct the direction in which the electric field is linearly polarised and this, in turn, enables the direction of the magnetic field associated with that radiation to be inferred. For radiation emitted with wavelengths of the order of centimetres, the magnetic field inferred from the synchrotron emission should be parallel to the magnetic field in the original object. This powerful technique for directly probing the structure of objects has resulted in spectacular images of the magnetic fields of spiral galaxies such as M51 (Fig. 2).

The second way that we can probe magnetic fields along the line-of-sight uses a phenomenon called Faraday rotation, in which the orientation of the linearly polarised synchrotron radiation rotates as it passes through intervening clouds of magnetised material. The angle through which it rotates is a function of the wavelength of the emitted light and the properties of the medium through which it passes, including the density of free electrons and the strength of the embedded magnetic field. In cases where we understand the density and size of the intervening magnetised material, we can then obtain the strength and average direction of the magnetic field along the path that the radiation takes.

This second technique allows us to use very distant background radio galaxies as sources of polarised light. By observing the change in polarisation of that light as it passes through intervening magnetised objects, such as the Milky Way galaxy, we can reconstruct the magnetic field of the intervening objects.

However, because we can obtain information only along those lines of sight where light from a background galaxy is detected, this is akin to trying to reconstruct the picture on a jigsaw puzzle when you have only a few pieces in place. Obviously the more pieces of the jigsaw we have, the better we will be able to reconstruct the image.

One of the greatest challenges we face in understanding magnetism has been exactly that – the lack of pieces of our jigsaw. This is a consequence of the fact that while it is possible to detect large numbers of radio galaxies in the distant universe, only about 5–10% retain the polarised emission that we need to use them for measurements of Faraday rotation.

In order to get more polarised background sources, we need a more sensitive radio telescope, a broad range of observing frequencies and a wide field of view. This is where the SKA becomes important. To illustrate this point, we can look at recent attempts to reconstruct the magnetic field of our own galaxy using Faraday rotation of background sources.


Figure 3. Images of the magnetic field of the Milky Way. Shades of blue indicate that the magnetic field is going into the page, while shades of red show regions where the field is coming out of the page. The top image is an interpolated map derived from the Faraday rotation measures of about 1000 polarised background sources. The middle figure is a more sophisticated statistical analysis of about 40,000 points, revealing a dramatic improvement in the detailed field of our galaxy. The bottom panel indicates that the SKA will produce about 7,000,000 background sources for this work. Image from Johnston-Hollitt et al. (2015)

About a decade ago astronomers attempted to determine the large-scale magnetic field of the Milky Way using only about 1000 Faraday rotation measurements spread across the 41,000 square degrees of the full sky (Fig. 3). Last year a new project repeated this effort using about 40,000 background sources. Whereas the original work showed only the large-scale features of the Milky Way’s magnetic field, the more recent effort, which improves the sampling rate by a factor of 40, highlights the complex internal structures of the Milky Way and indicates that large-scale features within the galaxy are related to magnetic fields. The improvement in these two images is entirely due to the increased ability of instruments to detect fainter radio emission.

In its first phase, the SKA is expected to increase the current sampling by a factor of 150 and detect around seven million sources spread out across the entire sky. Additionally, the broader frequency range over which the SKA will observe will dramatically improve the accuracy of individual measurements of Faraday rotation, providing more confidence in the interpolated results. This phenomenal increase in the detection of polarised background sources will not only allow us to map the magnetic field of the Milky Way in unprecedented detail, but will also allow the magnetic field of other large-scale objects to be measured.

With the SKA we will be able to undertake a polarisation survey of the entire sky in parallel with other radio observations. Such a survey will explore the origin of magnetic fields in objects on all scales in the universe, ranging from small scales (e.g. pulsars, supernova remnants, high velocity clouds, bubbles and shells in the Milky Way and other nearby galaxies) to intermediate scales (e.g. galactic magnetic fields and jets of active galactic nuclei) and finally large scales (e.g. clusters of galaxies, merger shocks in galaxy clusters and possibly filaments of the cosmic web). Additionally, we will be able to investigate these magnetic phenomena as a function of redshift and for the first time test how magnetic fields in different objects evolve over cosmic time.

Such a survey and the expected leap forward in understanding will allow us to finally put together the jigsaw pieces and see the complete picture of the origin and evolution of cosmic magnetism.

Melanie Johnston-Hollitt is Director of Astronomy & Astrophysics at Victoria University of Wellington, and co-chair of the SKA Working Group on Cosmic Magnetism.