The ESA's Planck mission has released one of the most detailed maps of the Milky Way's magnetic field.

Milky Way's magnetic field
The magnetic field of our Milky Way Galaxy as seen by ESA’s Planck satellite. This image was compiled from the first all-sky observations of polarized light emitted by interstellar dust in the Milky Way. Darker regions correspond to stronger polarized emission, and the striations indicate the direction of the magnetic field projected on the plane of the sky.
Credit: ESA / Planck Collaboration

This neat map from the Planck mission reveals the structure of the Milky Way’s magnetic field. As explained in the team’s press release, the map is based on polarized light emitted by interstellar dust grains. Those measurements show places where the magnetic field lines up in an orderly fashion or swirls in turbulent loops. For example, the arch of strong polarized emission that appears to reach up from the galactic center is at least partly due to a nearby supernova remnant.

What interests me about this map is not what it shows (although that’s cool, too), but what it might herald. First, the measurements are based on microwaves, which immediately brings to mind the “haze” discovered in 2003 that turned out to be two gigantic lobes protruding from our galaxy’s center. These lobes, called the Fermi bubbles, are each roughly 25,000 light-years long. (That should blow your mind: the Milky Way’s stellar disk is 100,000 light-years across, so the dumbbell the two bubbles create is about half the size of our galaxy.)

Astronomers still debate what created the Fermi bubbles. As I explained in my feature article in the April 2014 issue, the two main contenders are supernovae and our galaxy’s central supermassive black hole. Polarization is a key component in solving this mystery, because the two processes would order the magnetic fields inside the bubbles in different ways. Although this polarized sky map used a higher frequency than that emitted inside the Fermi bubbles, the polarization data from Planck might give astronomers new insight on the bubbles’ origin.

These data also pave the way for Planck’s polarization measurements of the cosmic microwave background (CMB), the leftover radiation from the universe’s birth. Planck’s primary mission was to map the CMB to high precision. Analysis of its temperature map (a.k.a. the blotchy ellipse you see on various cosmology webpages) has agreed for the most part with our standard cosmology — with a couple of hiccups, but that’s the fun part.

The team didn’t release Planck’s polarization data at the same time as the first 15 months of temperature data, because the spacecraft’s data are a beast to analyze. The team first has to subtract out all the signals from other sources — the most prominent of which is the big galaxy we’re sitting in. That the Planck team is releasing data for our galaxy’s polarized emission suggests they’re making good progress on weeding out non-CMB signals.

Last I heard, the plan was to release Planck’s large-scale polarization data later this year — that is, polarization created by cosmic structure — and then release the smaller-scale data that could include the primordial signals from inflation (the so-called B-modes) in 2015. The recent BICEP2 announcement of the detection of inflation’s fingerprints lends anticipation to that effort. Given the fabulous results from the Planck team in the past, the CMB polarization data are sure to be interesting.

In the meantime, you can check out the "intermediate results" from the Planck team (I've linked to the overview, but the press release has links at the bottom to four separate papers).


Image of Anthony Barreiro

Anthony Barreiro

May 14, 2014 at 6:03 pm

Adrian Cho's post on the Science blog yesterday -- -- said there is "an internet rumor" that the BICEP 2 team used incorrect data for the microwave polarization arising from the Milky Way, and that this this error may account for their positive findings of b-mode polarization. The BICEP 2 team stand by their findings. When the Planck polarization data are released, hopefully this will resolve the question one way or the other.

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