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Blowing Bubbles … in the sky

2 December 2011

At www.physorg.com/print241937908.html … we might ask, is the black hole at the centre of the Milky Way quietly dozing away or is it secretly gobbling all that dares to come alongside? In a paper in Physical Review Letters Subir Sarkar of Oxford University discusses bubbles feeding black holes – or is it vice versa? These bubbles are gigantic structures above and below the centre of the galaxy that were discovered fairly recently and studied by Harvard astrophysicists using material from NASAs Fermi satellite. They exist 50,000 light years above and below the galactic plane – and it is surprising such big structures remained unseen for so long. Subir Sarkar says they are related to an energetic outflow from the elusive supermassive black hole thought to be lurking in the stomach of our galaxy, the one that is not supposed to be feeding at the moment, sort of in limbo after a nosh up dated around a million years ago. The energy outflow is the dregs of that earlier feeding frenzy session (as envisaged by Japanese scientists last year). 

It emerges the bubble is in fact a 'shock front' which produces turbulence in the plasma behind it which in turn accelerates electrons to very high energies through a stochastic process first described by Enrico Fermi. The electrons transfer their energy to low energy photons, the hypothesis continues, from the microwave and infrared backgrounds and producing the 'observed' gamma-rays. The big question though – where does the shock front come from? Well, other galaxies have been observed with powerful jets with an origin in their central parts – where black holes are thought to hide themselves. These jets also create bubble features. Meanwhile, the model created at Oxford has yet to be tested – the forthcoming Cherenkov Telescope Array might just be able to see inside the bubble and look at its structure. We can but hope.

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