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Catching the Milky Way's Monsters
A revolution that began with colliding neutron stars is taking place in astronomy. Will Gater looks at how electromagnetic and gravitational wave observations are expanding our view of the cosmos.
A brave trio of astronomers based at Harvard's Center for Astrophysics have been monster hunting in the Milky Way. Their first discovery was 'Nessie'; not a creature from the depths of a Scottish loch, but rather a long, dark filament slashed through our Galaxy's disc. The structure, made up of a long thread of relatively dense gas whose sinuous turns reflect those of the monster 'seen' in the classic photo, is hundreds of lightyears long.
When I first heard about it, I thought the existence of such a structure was just a curiosity, but this 'Nessie' is a complicated beast. Understanding how such a filament could have formed, and how it has resisted being ripped apart by the turbulent structure of the Galaxy's gas clouds, is not easy. A proper survey is needed and others have set out on this quest before. Six separate papers have tried to compile catalogues of giant filaments, using data from infrared and radio surveys within which dense clumps of gas stand out. Some inspected their data by eye while others used algorithms and machine learning to look for long filaments, so the first task for Catherine Zucker — the PhD student leading this monster hunt — was to bring these different datasets together in a useful way, using data from ESA's Herschel observatory to measure their properties.
The results of her and her team's hard work are fascinating. There are, it turns out, several types of monsters lurking in the Milky Way. While all share a habitat — closer to the center of the Galaxy than we are, and close to the middle of the disc — there are distinct differences. The most obvious bear similarities to how we imagine the Loch Ness Monster to look: they're long, thin filaments that, thanks to a significant fraction of dense gas, appear capable of forming massive stars (in some, three quarters of their gas is dense enough to be able to form stars). Such large and thin features are almost certainly the result of gravity working on a grand scale. What's more, these giant filaments may be very important, acting like bones to underpin the whole spiral structure of the Milky Way.
The second type, which have less dense gas and a more rounded appearance, may be squeezed versions of normal molecular clouds, which form the bulk of the Milky Way's star-formation regions. A comparison with recent simulations suggests that this idea is at least plausible, though more work — probably with more powerful computers — is needed. The third and final type sits between the previous two; these are as thin as the 'Nessie' filament but contain relatively little dense gas. They seem to be networks of molecular clouds, sorted into a regular pattern by gas collapsing in a particular way, specifically due to something called a 'sausage instability' (a wonderful technical term).
The three types of filaments seem to tell different stories about how gas collapses locally and how the large-scale structure of the Milky Way is put together. In corralling all of these beasts in the same place, Zucker and her team have done a great service to those who'll follow and continue our exploration of the Milky Way's wild places.
ABOUT THE WRITER
Chris Lintott is an astrophysicist and co-presenter of The Sky at Night on BBC TV. He is also the director of the Zooniverse project.
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