AAS #9: Black hole jet of doom from Cen A

By Phil Plait | January 9, 2008 11:00 am

Centaurus A is a nearby galaxy which is bit of mess. Actually, it’s an incredible mess. 12 million light years away, it looks at first like an elliptical galaxy, an unassuming football of billions of stars. A closer look reveals a dark dust lane across the middle, which is our first hint things are amiss: ellipticals tend not to have very much dust, but Cen A has it in bucketloads (as you can see in the picture on the left, courtesy Chandra and AURA/NOAO/NSF).

Cen A has been known to harbor a central supermassive black holes — all big galaxies, including our own, have one. But Cen A’s is active: it is currently feeding off gas and dust, gobbling down huge amounts of matter. This material forms a gigantic flattened disk as it swirls around the hole. This disk is under the influence of a witch’s brew of forces magnetic, gravitational, and even mechanical: friction heats the disk fiercely, and it gets so hot it glows in visible, ultraviolet, and even X-rays.

It’s not clear exactly how these forces interact, but they can combine to focus twin beams of matter and energy, titanic jets that scream outwards from the disk. The jets come from the part of the disk just before the matter takes the Final Plunge, before it falls into the black hole.

The Chandra X-ray Observatory has taken deep images of Cen A, revealing new details about the jets emanating from its central black hole. The jet to the upper left is aimed more or less at us (never fear, it’s way too far away to hurt us), and you can see the counterjet as a shorter stubbier line of light to the lower right. The jets may be equal in size and intensity, but weird effects (due to Einsteinian relativity) make the one aimed away from us appear to be dimmer.

A closer-in view of the jet shows that it is plagued with knots, clumps of emission. These are probably due to shock waves, when faster material is ejected from the vicinity of the black hole and slams into slower moving material. This dumps vast amounts of energy into the material, reheating and re-accelerating it outwards. The magnetic fields of the material keeps it focused, and the shock wave re-energizing keeps it moving up, up, and away from the black hole. These jets can be many thousands of light years long, so the energies involved are fantastic, almost incomprehensible.

But, in fact, there are comprehensible. Jets like these are incredible, but we are beginning to understand them. Observations like this one from Chandra are aiding astronomers’ understanding of how matter behaves when it’s On The Brink.


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