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Starts With A Bang

This Is What The Milky Way’s Magnetic Field Looks Like

The dust in the Milky Way, shown in darker and redder colors, are regions where new star formation is taking place. These dusty regions are correlated with the magnetic fields present in our galaxy, and the background light gets polarized in a measurable way as a result. (ESA/PLANCK COLLABORATION. ACKNOWLEDGMENT: M.-A. MIVILLE-DESCHÊNES, CNRS — INSTITUT D’ASTROPHYSIQUE SPATIALE, UNIVERSITÉ PARIS-XI, ORSAY, FRANCE)

If you thought the Planck satellite just made temperature maps of the cosmic microwave background, this will astound you.


The Milky Way, to human eyes, appears as simply a mix of stars and light-blocking dust.

A map of star density in the Milky Way and surrounding sky, clearly showing the Milky Way, the Large and Small Magellanic Clouds (our two largest satellite galaxies), and if you look more closely, NGC 104 to the left of the SMC, NGC 6205 slightly above and to the left of the galactic core, and NGC 7078 slightly below. In visible light, only starlight and the presence of light-blocking dust is revealed, but other wavelengths have the capacity to reveal fascinating and informative structures far beyond what the optical part of the spectrum can. (ESA/GAIA)

However, a glimpse in additional wavelengths reveals enormously rich, detailed structures.

This ultra-detailed view of the Milky Way spans many different wavelengths of light, and as such it can reveal gas, charged particles, many types of dust, and many other signals that appear in the microwave and millimeter wavelength ranges. The Planck satellite provides us with our best all-sky view of the cosmos in this wavelength range. (ESA/NASA/JPL-CALTECH)

Observations show galactic foreground signals combined with cosmic signals originating way back from the Big Bang.

The Planck satellite constructed all-sky maps of the sky in nine different wavelengths of light, at frequencies spanning from 30 GHz all the way up to 857 GHz: frequencies that can only be observed from space. Although the foreground features in the Milky Way are quite prominent, the main science goal of Planck was to analyze the background light: the cosmic microwave background. (ESA AND THE PLANCK COLLABORATION)

Leveraging observations across many different wavelengths, Planck scientists identified the cause and source of many galactic foregrounds.

The signal of the Milky Way galaxy as revealed by the Planck satellite during its first year of data-taking observations. Planck is now 10 years old, and understanding which components of the Planck signal are galactic versus extragalactic is of paramount importance to extracting correct information about our Universe. (ESA/ LFI & HFI CONSORTIA)

The Milky Way’s gas, dust, stars and more create fascinating, measurable structures.

The fluctuations in the Cosmic Microwave Background, as seen by Planck. There is no evidence for any repeating structures, and although there is some uncertainty in how accurate and comprehensive our foreground subtraction is, the success of the Planck data in matching and superseding other CMB observations like COBE, Boomerang, WMAP, AFI and others tells us that if we’re not on the perfectly correct track, we’re extremely close. (ESA AND THE PLANCK COLLABORATION)

Subtracting out all the foregrounds yields the cosmic background signal, which possesses tiny temperature imperfections.

This map is of the galactic magnetic foreground of the Milky Way. The contour lines show the direction of the magnetic field projected on the plane of the sky, while light/dark regions correspond to fully-unpolarized/fully-polarized regions of emission from the galaxy. (ESA AND THE PLANCK COLLABORATION)

But the galactic foreground isn’t useless; it’s a map unto itself.

The all-sky map of the galactic foreground emissions overlaid with polarization and magnetic field data. This is the first accurate, high-resolution, all-sky map of our galaxy’s magnetic field and foreground structures. (ESA AND THE PLANCK COLLABORATION)

All background light gets polarized by these foregrounds, enabling the reconstruction of our galaxy’s magnetic field.

The alignment of neutral hydrogen (white lines) with the polarization data from the CMB (gradients) is an inexplicable surprise, unless there’s an additional galactic foreground. In theory, only ionized hydrogen should align with the polarization data. This surprise is one of the very few observations that the Planck science team exhibits tension with other measurements, such as radio pencil-beam data taken from Arecibo. (CLARK ET AL., PHYSICAL REVIEW LETTERS, VOLUME 115, ISSUE 24, ID.241302 (2015))

Quite surprisingly, neutral hydrogen appears to be aligned with the CMB’s polarization.

As seen in yellow, a bridge of hot gas (detected by Planck) connects the galaxy clusters Abell 399 and Abell 401. The Planck data, when combined with X-ray data (in red) and LOFAR radio data (in blue) reveals a bridge of relativistic electrons connecting these two clusters across a distance of 10 million light-years. This is the largest-scale magnetic field ever detected in our Universe, and shows how successful Planck can be for reconstructing magnetic fields. (ESA/PLANCK COLLABORATION / STSCI/DSS (L); M. MURGIA / INAF, BASED ON F. GOVONI ET AL., 2019, SCIENCE (R))

However, Planck data of distant galaxies matches well with reconstructed magnetic fields.

The current models of galactic (and other) foregrounds along with the cosmic microwave background. There is some evidence that indicates the possibility that free-free scattering (from free electrons) has been modeled insufficiently, but other observations indicate that we may be spot on. This is a minor issue, but one that has not been conclusively resolved. (ESA AND THE PLANCK COLLABORATION)

Scientists continue to evaluate the successes of our best foreground modeling.

A close-up view of one of many regions of our galaxy, with the dustiest regions shown in red. The dark red regions are locations where new stars are forming, and the contour lines that show the reconstructed magnetic fields from our galaxy illustrate the interplay of star-forming regions with these fields. (ESA/PLANCK COLLABORATION. ACKNOWLEDGMENT: M.-A. MIVILLE-DESCHÊNES, CNRS — INSTITUT D’ASTROPHYSIQUE SPATIALE, UNIVERSITÉ PARIS-XI, ORSAY, FRANCE)

What’s certain is that dust grains correlate with these giant magnetic structures.

A quick look at any zoomed-in region of the galaxy shows that magnetic fields are not coherent and unidirectional on scales of the Milky Way, but rather only on the scales of individual star clusters. Beyond distance scales of a few dozen light-years, magnetic fields flip and switch directions, dominated by local, rather than galaxy-scale, dynamics. (ESA/PLANCK COLLABORATION. ACKNOWLEDGMENT: M.-A. MIVILLE-DESCHÊNES, CNRS — INSTITUT D’ASTROPHYSIQUE SPATIALE, UNIVERSITÉ PARIS-XI, ORSAY, FRANCE)

The link is through star-formation, which occurs inside these obscured regions.

Although an image like this might remind you of Van Gogh’s famous ‘Starry Night’ painting, this doesn’t illustrate atmospheric turbulence at all, since 100% of the data used in creating this image was taken from space. These lines represent magnetic fields and polarization instead, which illuminate the Universe in an entirely different way. (ESA/PLANCK COLLABORATION. ACKNOWLEDGMENT: M.-A. MIVILLE-DESCHÊNES, CNRS — INSTITUT D’ASTROPHYSIQUE SPATIALE, UNIVERSITÉ PARIS-XI, ORSAY, FRANCE)

Extragalactic light is unavoidably affected by our galactic magnetic fields, enabling the construction of these beautiful maps.

Even in the direction that points directly away from the galactic center, the plane of our Milky Way still contains dusty, star-forming regions, still generates its own magnetic field, and still polarizes any background light that passes through this region of space. In order to understand the Universe, we have to model and account for every single component successfully. (ESA/PLANCK COLLABORATION. ACKNOWLEDGMENT: M.-A. MIVILLE-DESCHÊNES, CNRS — INSTITUT D’ASTROPHYSIQUE SPATIALE, UNIVERSITÉ PARIS-XI, ORSAY, FRANCE)

Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words. Talk less; smile more.

Ethan Siegel is the author of Beyond the Galaxy and Treknology. You can pre-order his third book, currently in development: the Encyclopaedia Cosmologica.

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