ESA's Planck Satellite: The magnetic field along the galactic plane

Credit: ESA/Planck Collaboration. 

Acknowledgment: M.-A. Miville-Deschênes, CNRS – Institut d’Astrophysique Spatiale, Université Paris-XI, Orsay, France

While the pastel tones and fine texture of this image may bring to mind brush strokes on an artist's canvas, they are in fact a visualisation of data from ESA's Planck satellite.

The image portrays 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 most ancient light in the history of the Universe, the cosmic microwave background.

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.

Among the foreground sources at the wavelengths probed by Planck is cosmic dust, a minor but crucial component of the interstellar medium that pervades the Galaxy. Mainly gas, it is the raw material for stars to form.

Interstellar clouds of gas and dust are also threaded by the Galaxy's magnetic field, and 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 'polarised', it vibrates in a preferred direction, and, as such, could be caught by the polarisation-sensitive detectors on Planck.

Scientists in the Planck collaboration are using the polarised emission of interstellar dust to reconstruct the Galaxy's magnetic field and study its role in the build-up of structure in the Milky Way, leading to star formation.

In this image, the colour scale 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.

This image shows the intricate link between the magnetic field and the structure of the interstellar medium along the plane of the Milky Way.

In particular, the arrangement of the magnetic field is more ordered along the Galactic plane, where it follows the spiral structure of the Milky Way. Small clouds are seen just above and below the plane, where the magnetic field structure becomes less regular.

From these and other similar observations, Planck scientists found that filamentary interstellar clouds are preferentially aligned with the direction of the ambient magnetic field, highlighting the strong role played by magnetism in galaxy evolution.

The emission from dust is computed from a combination of Planck observations at 353, 545 and 857 GHz, whereas the direction of the magnetic field is based on Planck polarisation data at 353 GHz.

Seven Specks of Interstellar Dust returned by the Stardust probe

Looking toward the constellation Ophiuchus, a cloud of interstellar dust reflects the light of a bright star, painting the sky blue. 

Washington University in St. Louis students analyzed dust captured from a stream flowing into the solar system from this direction. 

Credit: Rolf Wahl Olsen

This August, a consortium of 65 scientists announced in the journal Science that they have so far found seven probable but not confirmed ("level 2" ) interstellar dust specks in a collector returned to Earth by the Stardust spacecraft in 2006.

The September 2014 issue of the journal Meteoritics & Planetary Science was also devoted to the hunt for interstellar dust.

It is a mistake to think of the interstellar dust grains simply as dust, said Christine Floss, PhD, research professor of physics in Arts & Sciences and a member of the consortium.

Floss holding a piece of the Stardust foil. Behind her is the Auger microscope used to identify the elements in crater residues. 

Credit: Joe Angeles /WUSTL Photos

"They are valuable not in and of themselves, but rather for the information they carry."

"To have these grains in your laboratory is like being handed a sealed envelope with 'Secrets of the Universe' scrawled on the outside. Who could resist opening it?"

The seven specks were found in the second of Stardust's two sample trays.

The first tray, which had been exposed to the spray of a comet's tail, was processed in the year following the sample pod's return.

Scientists then turned to the second tray, which was exposed to a stream of interstellar dust flowing through the solar system.

One of the many ironies of the Stardust mission is that the gridwork of aluminum foil, originally intended only to support tiles of a light foam called aerogel that was to collect the dust, proved to be dust collectors as well. Incoming dust pocked them with tiny craters lined with melted residue.

So the searchers were divided into two teams: those scrutinizing the aerogel and those inspecting the foils.

A Stardust sample tray. The aluminum foils accounted for only 15 percent of the surface area but snagged as many interstellar dust candidates as the aerogel. 

Credit: NASA/ JPL/CALTECH

To find the motes in the aerogel, the University of California, Berkeley, organized a citizen-science project, Stardust@home that recruited 30,000 volunteers.

The volunteers, who called themselves 'dusters,' found three of the seven dust particles, which they named Orion, Haylabook and Sorok.

The task of examining the foils was divided among six labs, including the Laboratory for Space Sciences at Washington University in St. Louis.

Floss' husband and colleague, Frank Stadermann, was an expert in the analysis of very small samples of extraterrestrial materials and had devoted his career to developing new techniques for this purpose.

As sample advisor for the Stardust mission he helped define the protocols that would govern the search for interstellar dust.

In creating the protocols he and his colleagues were venturing into unexplored territory.

Since nobody had ever worked with interstellar dust, no one could be certain what it was or how best to identify it.

Undergraduate students hired by Floss and Stadermann, who died unexpectedly from a cerebral hemorrhage in 2010, found three of the four craters in the foil probably made by interstellar particles. The fourth was found by another lab.

"We didn't think to name our particles," Floss said. " We missed the boat on that one."

More information: "Evidence for interstellar origin of seven dust particles collected by the Stardust spacecraft." Science 15 August 2014: Vol. 345 no. 6198 pp. 786-791 DOI: 10.1126/science.1252496