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OJ 287

 
 

Map of Polarisation of Starlight at the Galactic Poles

Astronomers Andrei Berdyugin, Vilppu Piirola and Pekka Teerikorpi at Tuorla Observatory, have made the first complete maps of the amount of polarisation of starlight above and below the plane of the Milky Way galaxy.

For many decades, astronomers have known that in addition to the stars and gas scattered through the near emptiness of space, there are also large amounts of tiny particles which they call "dust".

Although the dust particles are very much smaller than the dust we blithely suck up with domestic vacuum cleaners, the analogy is not so far fetched, since for many purposes astronomers would gladly do away with cosmic dust. Looking up into the Milky Way on a good dark night, many "dark" patches can be readily seen delineated by the brighter surrounding regions. One of the most famous of these clouds of interstellar dust is the "Coal Sack" in the southern hemishpere, near to the Southern Cross. Looking along certain directions in the Milky Way, this dust builds up effectively into a screen and blocks the light from stars further away still.

The dust particles are known to be irregularly shaped. Because they are also slightly magnetic, they tend to align themselves along the directions folowed by magnetic fields in space. Aligned like this, the screen of dust is very much like a pair of polaroid sunglasses; they polarise slightly the light coming through layers of dust from stars behind.

Andrei Berdyugin, Vilppu Piirola and Pekka Teerikorpi have been working for several years to measure the polarisation of all the stars at the North and South Galactic poles -- regions of the sky perpendicular to the plane of the Milky Way. This way they can build up a map of how the dust is layered above and below the Milky Way.

The measurements not only reveal how much dust is scattered amongst the stars --- long a thorny issue in astronomy --- but also the direction of the magnetic field of the Milky Way.

The figure shows their measurements at the Southern Galactic pole. For each star, the amount of dust along the line of sight between us and the star is shown as a line. The length of the line indicates how much dust (polarisation) is seen, and its direction indicates the alignment of the Galactic magnetic field.

Apart from measuring the magnetic field of the galaxy, the maps also allow the astronomers to measure how thick the dust layer is in our Galaxy and its relation to the gas and stars in the Galaxy's flattened disk. Although the amount of dust seen in the maps only blocks a few percent of the light coming from behind, it is important to know exactly how much because it plays an important part in the puzzle of figuring out how far away distant galaxies are from the Milky Way.

The paper on high latitude interstellar polarisation is by Andrei Berdyugin, Vilppu Piirola and Pekka Teerikorpi and has been published in the journal Astronomy and Astrophysics.


Updated October 20th 2004.

Polarisation measurements of starlight at the South Galactic Pole. A line indicates for each measured star, the amount of dust (polarisation) along the line of sight, and the alignment of the polarisation. Maps like these help build up a picture of how much dust we have to contend with not only when looking around the Galaxy but also out into the Universe at large. Click here for a more detailed view of the polarisation map

 

The Royal Swedish Academy's 60 cm telescope at La Palma in the Canary Islands, Spain, was used to measure the polarisations of the stars. The telescope has recently been upgraded to allow fully remote observing from Tuorla Observatory, in Turku, Finland.