This image may look like a cloudy sunset reflected over the ocean — it's actually a shot of the Milky Way, as seen by the ESA's Planck Satellite. The unique perspective reveals previously unseen characteristics of our galaxy. And it may provide crucial insights into how the universe was born.
We're located inside the Milky Way, so it's not easy for astronomers to visualize what it looks like. Over the past few years, however, astronomers have gained an enhanced sense of its appearance, our location within it, and its place in the cosmos. For example, we know it's a spiral galaxy with a peanut-shaped core and that we're located in a large and prominent spiral branch called the Local Arm. We've also created a map from its infrared signature. Pulling back a bit, astronomers also know that the Milky Way is encircled by the Magellanic Stream and that it's situated among the so-called Council of Giants.
And now we have the most comprehensive map of the galaxy's magnetic field and the immense distribution of dust and gas within in.
Like Polarized Sunglasses
The visualization was compiled from the first all-sky survey observations of polarized light produced by interstellar dust in the Milky Way. The map was acquired using detectors on Planck, that acted as the astronomical equivalent of polarized sunglasses. By analyzing this incoming polarized light, astronomers can study the structure of the galactic magnetic field and the orientation of the field lines projected on the plane of the sky.
A Planck image from 2012: An all-sky image showing the distribution of carbon monoxide (CO), a molecule used by astronomers to trace molecular clouds across the sky. (credit: ESA)
Polarized light describes light in which individual light waves are aligned parallel to one another. Light becomes polarized when it's reflected from a transparent material, such as glass or water. But the degree of polarization depends on the material and the angle at which the light is reflected. The glare you see on a sunny day is composed of partially polarized light in a horizontal plane. To counter this effect, sunglasses made of Polaroid plastic have the axis of polarization in a vertical plane, thus blocking out the polarized glare.
From a cosmological perspective, incoming light can convey a wealth of information about what's happening along its path. In space, the light emitted by stars, gas, and dust can be polarized in a number of ways. Astronomers measure the amount of polarization in this light in order to study the physical processes that caused the polarization.
The Dusty Galaxy
By using a visualization technique called line integral convolution (LIC), the Planck Satellite can scan the most distant light in the universe, but in the microwave part of the spectrum where signals from the most distant reaches of the universe can be detected. But rather than showing stars, the image shows the magnetic orientation of countless microscopic dust particles that permeate the galaxy.
That dark horizontal line you see running across the image corresponds to the galactic plane of the disc-shaped extent of the galaxy where the dust is most plentiful and where its microwave energy is easier to detect; darker regions correspond to stronger polarized emissions, while the striations show the direction of the magnetic field projected on the plane of the sky.
The gaps at top and bottom are where the dust is thinner and where the microwave signal is weaker. The map also shows differences in the polarization direction within nearby clouds of gas and dust — the swirling features above and below the plane where the local magnetic field is quite disorganized.
Dust may seem like a minor component of the galaxy. And indeed, a typical volume of space the size of a domed stadium contains no more than one microbe-sized speck of dust. But that same dust can be concentrated in large quantities during the process of star formation, and the onset of protoplanetary disks.
"Dust is often overlooked but it contains the stuff from which terrestrial planets and life form," writes UBC Astrophysicist Douglas Scott. "So by probing the dust, Planck helps us understand the complex history of the galaxy, as well as the life within it."
A more complete visualization should be coming in October, after astronomers have the opportunity to scan the magnetic signature of distant parts of the universe directly behind the Milky Way. By subtracting our galaxy's influence, the Planck team could either confirm or refute the recent discovery of gravitational waves from the Big Bang.
This visualization and other results will be described in four forthcoming papers in the journal Astronomy & Astrophysics.