If you live in or have ever visited a place with clear, dark skies, you have likely seen the hazy band of the Milky Way stretching across the night sky. Now a European space telescope has looked deeper into the Milky Way’s light, revealing our galaxy’s unique ‘fingerprint’ created by vast magnetic fields.
The Milky Way’s faint glow comes from the light of billions of distant stars inside the plane of our disk-shaped galaxy. But that’s only the visible light that our eyes can detect — the Milky Way shines brightly in many other parts of the electromagnetic spectrum, as does the entire universe. And it’s in these hidden wavelengths that European Space Agency’s Planck spacecraft observes our galaxy — and beyond — mapping the leftover light from the birth of the cosmos, before even the very first stars formed.
In order to best determine which light originates from the Big Bang and which is from much closer and younger sources, like the stars and dust in our own home galaxy, scientists have to know how to filter out one from the other. Using Planck, the Milky Way is observed across a wide range of the electromagnetic spectrum and its specific pattern emerges, revealing not just light but also its magnetic “fingerprint,” created by polarized light from tiny interstellar dust grains aligned with magnetic fields.
The dust grains, part of the interstellar medium that pervades the entire galaxy, are very, very cold but still emit light in the infrared and microwave portion of the spectrum, light Planck is designed to detect. As these tiny grains spin they tend to emit more radiation along their longest axis, creating a preferential direction to the light — an effect known as polarization.
If you have polarized sunglasses you can easily appreciate this effect, as film inside the lenses specifically blocks reflected horizontally-aligned light and remove distracting glare.
In the new Planck visualization above, made with a technique called line integral convolution (LIC), polarized light emitted by dust particles traces out swirling linear patterns, not unlike the unique pattern of loops and whorls in a human fingerprint. The wavy lines are created by complex magnetic fields weaving throughout the Milky Way, which align the rotations of the interstellar dust particles and thus shape the net polarization of their radiation.
Darker regions correspond to stronger polarized emissions, and where there’s just a dark line running through the center is the densest part of the plane of the Milky Way, and strong parallel patterns have overlapped in three dimensions.
Even more than just revealing the magnetic field lines within our galaxy, this Planck data will be used to help better determine the validity of recent findings announced by the BICEP2 experiment, which in March announced detection of the first evidence of polarization in the leftover light from the Big Bang. But since BICEP2 was observing a small area of the sky in a single wavelength with a ground-based telescope located at the South Pole, scientists will be scrutinizing these Planck observations to see if any foreground patterns may have gotten in the way.
Launched on May 14, 2009 from ESA’s Spaceport in French Guiana, Planck observes the universe in nine wavelengths of light from its position around the L2 Lagrangian point, 1.5 million km away from Earth opposite the sun.