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Planck takes magnetic fingerprint of the Milky Way

Jenny Winder, News Writer
May 6, 2014, 9:50 UTC

Sen—A new image compiled from the first all-sky observations of 'polarised' light emitted by interstellar dust in the Milky Way reveals our Galaxy's magnetic field.

Launched in 2009, European Space Agency’s (ESA) Planck satellite mapped the sky in nine frequencies, before being switched off on 19 October 2013.

The map presented here was obtained using detectors on Planck that acted as the astronomical equivalent of polarised sunglasses. Swirls, loops and arches in this new image trace the structure of the magnetic field in our home galaxy, the Milky Way.

Light can be described as a series of waves of electric and magnetic fields that vibrate in directions that are at right angles to each other and to their direction of travel. If they vibrate preferentially in certain directions, we say the light is 'polarised'.

In space, the light emitted by stars, gas and dust can also be polarised in various ways. By measuring the amount of polarisation in this light, astronomers can study the physical processes that caused the polarisation to reveal the existence and properties of magnetic fields in the medium the light has travelled through.

In addition to its hundreds of billions of stars, our Galaxy is filled with a mixture of gas and dust. Even though the tiny dust grains are very cold, they do emit light but at very long wavelengths. The dust grains are almost always spinning rapidly, tens of millions of times per second, due to collisions with photons and rapidly moving atoms.

Because interstellar clouds in the Milky Way are threaded by magnetic fields, the spinning dust grains become aligned preferentially with their long axis perpendicular to the direction of the magnetic field. As a result, there is a net polarisation in the emitted light, which can then be measured to study the structure of the Galactic magnetic field and, in particular, the orientation of the field lines projected on the plane of the sky.

Planck takes magnetic fingerprint of the Milky Way

An artist's impression of the Planck telescope observing in deep space. Image credit: ESA/D. Ducros

In the new Planck image, darker regions correspond to stronger polarised emission, and the striations indicate the direction of the magnetic field projected onto the plane of the sky. The image shows that there is large-scale organisation in some parts of the Galactic magnetic field.

The dark band running horizontally across the centre corresponds to the Galactic Plane. Here, the polarisation reveals a regular pattern on large angular scales, which is due to the magnetic field lines being predominantly parallel to the plane of the Milky Way.

The data also reveal variations of the polarisation direction within nearby clouds of gas and dust. This can be seen in the tangled features above and below the plane, where the local magnetic field is particularly disorganised.

Hidden behind the foreground emission from our Galaxy is the primordial signal from the Cosmic Microwave Background (CMB), the most ancient light in the Universe. Later this year, scientists from the Planck collaboration will release data based on Planck’s observations of polarised light covering the entire sky at seven different frequencies.

This will enable a much more detailed investigation of the early history of the cosmos, from the accelerated expansion when the Universe was much less than one second old to the period when the first stars were born, several hundred million years later.