The magnetic sky

On May 6, the team behind the now-inoperative Planck space telescope released a map of the magnetic field pervading the Milky Way galaxy.


Titled ‘Milky Way’s Magnetic Fingerprint’, the map incorporates two textures to visualize the magnetic field’s dual qualities: striations for direction and shading for intensity.

Planck was able to measure the polarization by studying light. Light is a wave (apart from being a particle, too). As a wave, it is composed of electric and magnetic fields vibrating perpendicular to each other. Overall, however, the two fields could vibrate in any direction. So when they choose to vibrate in a particular direction, the light is said to be polarized.

Such light is emitted by dust grains strewn in the space between Milky Way’s stars. As Dr. Chris Tibbs, an astrophysicist from Caltech, told me over Twitter, “Dust grains absorb light from stars, which heats up the grains, and [they] then radiate away this heat producing the emission.”

The grains are oriented along the Milky Way’s magnetic field, so the light they emit is polarized along the magnetic field. Because the grains are so small, the light they emit is of very low intensity (i.e. very long wavelength), so it takes a powerful telescope like Planck, perched on its orbit around the Sun, to study it.

It used a technique that’s the opposite of polarized sunglasses, which use filters to eliminate polarized light and reduce glare. The telescope, on the other hand, used filters to eliminate all but the polarized light, and then studied it to construct the map shown above.

As the astrophysicist Katie Mack pointed out on her Facebook page, the Planck team that released this image has carefully left out showing the magnetic fields in the region of the sky studied by the BICEP2 telescope at the South Pole which, on March 17, announced the discovery of evidence pointing to cosmic inflation. According to Katie,

The amount of polarized dust emission in the region where BICEP2 made its observation is unknown, but if it turns out to be a lot, it could mean that the signal BICEP2 saw was not entirely primordial.

This means we’ve to wait until the end of the year to know if the BICEP2 announcements were all they were made out to be.