The magnetic field pattern of the Milky Way

New all-sky map shows complex structure of magnetic fields in unprecedented detail

December 06, 2011

Like all galaxies, our Milky Way is awash with magnetic fields. For the first time, scientists have now measured the complex structure of such fields in detail.  Using a novel image reconstruction technique, they combined data from more than 41,000 individual measurements. The work was a collaboration between scientists at the Max Planck Institute for Astrophysics who are specialists in the new discipline of information field theory, and an international team of radio astronomers. The new map not only reveals the structure of the galactic magnetic field on large scales, but also small-scale features that provide information about turbulence in the galactic gas.

The uncertainty in the Faraday map. Note that the range of values is significantly smaller than in the Faraday map. In the area of the celestial south pole, the measurement uncertainties are particularly high because of the low density of data points.  

To account for such problems, scientists at MPA have developed a new algorithm for image reconstruction called the "extended critical filter". To derive this algorithm, the team makes use of the tools provided by the new discipline known as information field theory. Information field theory incorporates logical and statistical methods applied to fields, and is a very powerful tool for dealing with inaccurate information. The approach is quite general and can be of benefit in a variety of image and signal-processing applications, not only in astronomy, but also in other fields such as medicine or geography.

In addition to the detailed Faraday depth map, the algorithm provides a map of the uncertainties. Especially in the galactic disk and in the less well-observed region around the south celestial pole (bottom right quadrant), the uncertainties are significantly larger.

To better emphasize the structures in the galactic magnetic field, in Figure 3 the effect of the galactic disk has been removed so that weaker features above and below the galactic disk are more visible. This reveals not only the conspicuous horizontal band of the gas disk of our Milky Way in the middle of the picture, but also that the magnetic field directions seem to be opposite above and below the disk. An analogous change of direction also takes place between the left and right sides of the image, from one side of the centre of the Milky Way to the other.

A particular scenario in galactic dynamo theory predicts such symmetrical structures, which is supported by the newly created map. In this scenario, the magnetic fields are predominantly aligned parallel to the plane of the galactic disk in a circular or spiral configuration. The direction of the spiral is opposite above and below the galactic disk. The observed symmetries in the Faraday map stem from our position within the galactic disk.

In this map of the sky, a correction for the effect of the galactic disk has been made in order to emphasize weaker magnetic field structures. The magnetic field directions above and below the disk seem to be diametrically opposed, as indicated by the positive (red) and negative (blue) values. An analogous change of direction takes place across the vertical centre line, which runs through the centre of the Milky Way.

In addition to these large-scale structures, several smaller structures are apparent as well. These are associated with turbulent eddies and lumps in the highly dynamic gas of the Milky Way. The new map making algorithm provides, as a by-product, a characterization of the size distribution of these turbulent structures, the so-called power spectrum. Larger structures are more pronounced than smaller, as is typical for turbulent systems. This spectrum can be directly compared with computer simulations of the turbulent gas and magnetic field dynamics in our galaxy, thus allowing for detailed tests of galactic dynamo models.

The new map is not only interesting for the study of our galaxy. Future studies of extragalactic magnetic fields will draw on this map to account for contamination from the Galactic contribution. The next generation of radio telescopes are expected in the coming years and decades, and with them will come a wealth of new measurements of the Faraday effect. New data will prompt updates to the image of the Faraday sky. Perhaps this map will show the way to the hidden origin of galactic magnetic fields.


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