|
A few days ago we referred to the similarity between our own galaxy and NGC 6744, a beautiful spiral visible in the southern hemisphere. Now we receive the surprising news of the discovery of a new spiral arm in the Milky Way (the name given to our own galaxy), that fits perfectly into the symmetric pattern made by the other arm segments already mapped. The resulting picture of the place that our solar system calls home, is a very symmetrical spiral pattern.
The story of the discovery is reminiscent of how discoveries were made decades ago, because it was made using a small radio telescope only 1.2 metres in diameter, from the roof of a scientific institute (the Harvard-Smithsonian Center for Astrophysics, CfA), and not with a gigantic instrument far from any built-up area.
The work was done by Tom Dame and Pat Thaddeus, two CfA astronomers, who made use of the fact that radio waves can penetrate the dense clouds of dust that occupy the galactic plane (and obscure what lies beyond at other wavelengths), to look for traces of the carbon monoxide molecule, which tends to be found in spiral arms and so is an excellent tracer of galactic structure. Spectral line emission from the carbon monoxide molecule is used to follow the distribution of molecular hydrogen, which does not emit radiation itself under normal circumstances.
Dame and Thaddeus pointed their telescope towards the most external parts of our galaxy, discovering the new arm, which is likely to be the end of the Scutum-Centaurus arm, although the segment which connects the two is purely an extrapolation, as shown above, where all the minor arm segments (including the one in which we live) have been omitted.
Considering also the closer Perseus arm, the Milky Way then has two main spiral arms, almost perfectly symmetrical, which spiral outwards from the ends of the small central bar.
|
