Milky Way's Magnetic Fields Mapped with Highest Precision

The sky map of the Faraday effect caused by the magnetic fields of the Milky Way. Red and blue colors indicate regions of the sky where the magnetic field points toward and away from the observer, respectively.

The band of the Milky Way (the plane of the galactic disk) extends horizontally in this panoramic view. The center of the Milky Way lies in the middle of the image. The North celestial pole is at the top left and the South Pole is at the bottom right.

With a unique new all-sky map, scientists at MPA have made significant progress toward measuring the magnetic field structure of the Milky Way in unprecedented detail.

Specifically, the map is of a quantity known as Faraday depth, which among other things, depends strongly on the magnetic fields along a particular line of sight.

To produce the map, data were combined from more than 41,000 individual measurements using a novel image reconstruction technique.

The work was a collaboration between scientists at the Max Planck Institute for Astrophysics (MPA), who are specialists in the new discipline of information field theory, and a large 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.

All galaxies are permeated by magnetic fields, including our own Milky Way galaxy. Despite intensive research, the origin of galactic magnetic fields is still unknown.

One assumes, however, that they are built up by dynamo processes in which mechanical energy is converted into magnetic energy.

Similar processes occur in the interior of the earth, the Sun, and - in the broadest sense - in the gadgets that power bicycle lights through peddling.

By revealing the magnetic field structure throughout the Milky Way, the new map provides important insights into the machinery of galactic dynamos.

One way to measure cosmic magnetic fields, which has been known for over 150 years, makes use of an effect known as Faraday rotation.

When polarized light passes through a magnetized medium, the plane of polarization rotates. The amount of rotation depends, among other things, on the strength and direction of the magnetic field.

Therefore, observing such rotation allows one to investigate the properties of the intervening magnetic fields.

To measure the magnetic field of our own galaxy, radio astronomers observe the polarized light from distant radio sources, which passes through the Milky Way on its way to the Earth.

The amount of rotation due to the Faraday effect can be deduced by measuring the polarization of the source at several frequencies.

Each such measurement can only provide information about a single path through the Galaxy. To get a complete picture of the magnetic fields in the Milky Way from Faraday rotation measurements, one must observe many sources distributed across the entire sky.

A large international collaboration of radio astronomers have provided data from 26 different projects to give a total of 41,330 individual measurements. On average, the complete catalogue contains approximately one radio source per square degree of sky.