Mapping the Milky Way’s magnetic field

Astronomers at Victoria University were part of an international team led by Germany’s Max Planck Institute for Astrophysics (MPA), which has pooled their radio astronomy observations of the Milky Way to create the highest-ever precision map of the Milky Way galaxy’s magnetic field.

Milky Way’s magnetic fields

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

Rights: Max Planck Institute for Astrophysics.

The Milky Way and all other galaxies possess magnetic fields. Scientists believe the magnetic fields are created when mechanical energy is converted into magnetic energy, similar to the kinds of processes that occur in the interior of Earth and the Sun.

Over 41,000 measurements used to create map

Dr Melanie Johnston-Hollitt and student Luke Pratley, who is about to begin an honours degree in physics, are part of the team led by Niels Oppermann of MPA. The 30 plus researchers used more than 41,000 measurements to create their map, but as Dr Johnston-Hollitt observes, such datasets have only become available in very recent times.

In 2004, Dr Johnston-Hollitt produced the first such map of the Milky Way with Dr Christopher Hollitt from the School of Engineering and Computer Science at Victoria University.

“Back then, there were only 800 data points to use. Now we have 41,000, so we are able to map for the first time the magnetic field of the Milky Way in detail like we’ve not seen before,” says Dr Johnston-Hollitt.

Using radio telescopes

“The map shows the rapid advance of radio telescopes that work in a different way to optical telescopes.

“You can’t directly ‘see’ other galaxies by looking through a radio telescope, but we can use computers to translate radio waves into images. There are parts of space where a radio telescope will allow us to ‘see’ further than optical telescopes.”

In making the new map, the research team measured the light from background galaxies and how the light changes as it passes through the Milky Way to produce a 3D view of the galaxy.

In particular, the researchers were able to use the light that is affected by the galaxy’s magnetic fields (polarised light) to map the galaxy’s structure. This technique is known as the ‘Faraday rotation’.

“The Milky Way is hard to map because we are sitting on the edge of the galaxy looking through it. However, we know that spiral galaxies like the Milky Way have magnetic fields that follow a particular pattern so we were able use polarised light to map the magnetic fields.”

New generation of radio telescopes

Dr Johnston-Hollitt says that having many data sources helped create such an accurate map. The 41,330 individual measurements equates to approximately one radio source per square degree of sky. Despite this, there are still some large areas, especially in the southern sky, where only a few measurements have been recorded, and the researchers had to extrapolate between the data points. The team intends to gather more data for the future and are waiting eagerly for the new generation of radio telescopes – such as LOFAR, eVLA, ASKAP, MeerKAT and the much talked about SKA that may or may not be part-hosted in New Zealand – to come online.

In the meantime, the new map not only shows the structure of the galaxy’s magnetic field on a large scale, it also reveals small-scale features that help scientists better understand other aspects such as turbulence in galactic gas – bringing us a step closer to understanding the origin of galactic magnetic fields.