Planck unveils the coldest regions of our Galaxy

Giant filaments of cold dust stretching through the coldest regions of our Galaxy are revealed in new images, released today from the Planck satellite. Analysing these structures could help to determine the forces that shape our Galaxy and trigger star formation. The images are a scientific by-product of a mission which will ultimately provide the best picture ever of the early Universe.

Dr David Parker, Director of Space Science and Exploration at the British National Space Centre (BNSC), said, “Less than a year since it was launched, Planck is producing some spectacular results. The Planck spacecraft is just one of a family of cutting edge scientific missions in which the UK is already playing a major role. I'm looking forward to fresh discoveries and continued involvement in such exciting missions with the forthcoming creation of a UK executive space agency.

Planck has now begun the second of four full-sky surveys, which will ultimately provide the most detailed information yet about the size, mass, age, geometry, composition and fate of the Universe. Although the primary goal of Planck is to map the CMB, by surveying the entire sky with an unprecedented combination of frequency coverage, angular resolution, and sensitivity, Planck will also provide valuable data for a broad range of studies in astrophysics. This is clearly demonstrated in the new images which trace the cold dust in our Galaxy and reveal the large-scale structure of the interstellar medium filling the Milky Way.

Image combining Planck and IRAS data to show filamentary dust in the Galaxy.
Planck images a galactic web of cold dust. Image credit: ESA and the HFI Consortium/IRAS

One of the key advantages of Planck is its ability to measure the temperature of the coldest dust particles and locate the coldest dusty clumps in the Galaxy, areas where star formation is about to occur. The  image on the right demonstrates how Planck measures this cold dust: reddish tones correspond to temperatures as cold as 12 degrees above absolute zero, and whitish tones to much warmer ones (a few tens of degrees) in regions where massive stars are currently forming. As the clumps shrink, they become denser and better at shielding their interiors from light and other radiation. This allows them to cool more easily and collapse faster. Planck excels at detecting these dusty clumps across the whole sky and contributes the crucial information required to measure accurately the temperature of dust at these large scales.

“What makes these structures have these particular shapes is not well understood,” says Jan Tauber, ESA Project Scientist for Planck. The denser parts are called molecular clouds while the more diffuse parts are ‘cirrus’. They consist of both dust and gas, although the gas does not show up directly in this image. There are many forces at work in the Galaxy to help shape the molecular clouds and cirrus into these filamentary patterns. For example, on large scales the Galaxy rotates, creating spiral patterns of stars, dust, and gas. Gravity exerts an important influence, pulling on the dust and gas. Radiation and particle jets from stars push the dust and gas around on smaller scales, and magnetic fields also play a role, although to what extent is presently unclear.

The space between stars is not empty but rather is filled with clouds of dust and gas, intimately mixed together and known as the “interstellar medium”. The right hand panel of the image below covers the same region as the previous image: about 55 degrees across and looking in the direction of the centre of our Galaxy. The plane of the Galaxy is seen as the horizontal band across the bottom of the image. Above the plane, the filamentary structure of the interstellar medium in the solar neighbourhood (within a few hundred light years of the Sun) can be seen.

Planck image (right) and Herschel image (left).
Filamentary structures are apparent in the Mily Way both on large-scales (as shown in this Planck image, on the right) and small-scales (as seen on the left, a Herschel image of a region in Aquila). Image credits: ESA/HFI Consortium [right]; and ESA/SPIRE and PACS consortia/P. André (CEA Saclay) for the Gould’s Belt Key Programme Consortium [left].

The image on the left shows a typical “stellar nursery” (about 3 degrees across) in the constellation of Aquila, recently imaged by the Herschel Space Observatory. The filamentary structures seen at the smallest scales by Herschel are strikingly similar in appearance to those seen at the largest scales by Planck.

The richness of the structure that is observed, and the way in which small and large scales are interconnected, provide important clues to the physical mechanisms underpinning the formation of stars and of galaxies. This example illustrates the synergy between Herschel and Planck; together these missions are imaging both the large-scale and the small-scale structure of our Galaxy.  Professor Peter Ade, of Cardiff University and member of the HFI consortium, said "the HFI is living up to our most optimistic pre-flight expectations.  The wealth of the data is seen in these beautiful multicolour images, exposing previously unseen detail in the cold dust components of our galaxy.  There is much to be learned from detailed interpretation of the data which will significantly enhance our understanding of the star formation processes and galactic morphology."

Planck image overlaid on visible light image of the Galaxy
Planck image overlaid on a view of the sky at optical wavelengths. Image credit: ESA/HFI consortium (colour overlay) and Axel Mellinger (background image)

The background of the image on the left shows a view of the sky as seen in visible light, with the plane of our Galaxy stretching across the middle. The image is 180 degrees across, and shows half the sky, looking in the direction of the centre of the Galaxy. The colour overlay is the image from Planck, 55 degrees across. Planck is able to observe very large regions of the Galaxy as is scans around the sky, and as of mid-March 2010 has observed 98% of the sky.

Dr David Clements from Imperial College London, said, "These wonderful new images from Planck clearly show its power for revealing new things about the universe. We always knew there would be a lot of great new science found as we peeled away the layers of the cosmic onion to reach the microwave background, and these results demonstrate that happening in our own galaxy. What I hadn't really grasped was just how beautiful the Planck foreground images were going to be!"

Tom Bradshaw from the Science and Technology Facilities Council’s Rutherford Appleton Laboratory added, “Planck is a satellite designed to measure the temperature of deep space to unprecedented accuracy. The technology to achieve this has been under development for over 15 years. A significant part of this was developed in the UK. After all the hard work that has been put into what is probably one of the most complex satellites ever flown, it is gratifying to see such stunning images that will help us understand our place in the universe.