The Milk Way’s Magnetic Field

The_magnetic_field_along_the_Galactic_plane_node_full_image_2This image looks like it was lifted from something by Van Gogh. The pastel tones and fine texture remind me of the brush strokes on one of the artist’s canvases. In fact, the picture is a visualization of data from ESA’s Planck satellite detailing the interaction between interstellar dust in the Milky Way and the structure of our Galaxy’s magnetic field.

Between 2009 and 2013, Planck scanned the sky to detect the Cosmic Microwave Background, the oldest light in the history of the Universe. It also detected significant foreground emission from diffuse material in our Galaxy which, although a nuisance for cosmological studies, is extremely important for studying the birth of stars and other phenomena in the Milky Way. One of the foreground sources at the wavelengths scanned is cosmic dust, a minor but crucial component of the interstellar medium that pervades the Galaxy. It’s mostly gas, and it is the raw material for stars to form.

These interstellar clouds of gas and dust are shepherded by the Galaxy’s magnetic field. The dust grains tend to align their longest axis at right angles to the direction of the field. As a result, the light emitted by dust grains is partly polarized. It vibrates in a preferred direction. From these and other similar observations, scientists found that filamentary interstellar clouds are preferentially aligned with the direction of the ambient magnetic field, suggesting a strong role played by magnetism in galaxy evolution.

The color scale of the image represents the total intensity of dust emission, revealing the structure of interstellar clouds in the Milky Way. The texture is based on measurements of the direction of the polarised light emitted by the dust, which in turn indicates the orientation of the magnetic field. The arrangement of the magnetic field is more orderly along the Galactic plane, where it follows the Galaxy’s spiral structure. Small clouds are seen just above and below the plane, where the magnetic field structure becomes less regular.

Image Credit: ESA / Planck Collaboration.
Acknowledgment: M.-A. Miville-Deschênes, CNRS – Institut d’Astrophysique Spatiale, Université Paris-XI, Orsay, France