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Skid Marks in the Galaxy - Astronomers localise galactic particle accelerator

October 16, 2002

Radio galaxies are amongst the most luminous celestial objects - however, they mainly emit radio waves, not light. These occur when electrically charged particles travelling at almost the speed of light are slowed down, thereby losing energy. Until recently it was not known exactly where the particles reach such high speeds. A group of scientists - among them an astrophysicist from the University of Bonn - have now for the first time been able to determine more precisely the region in which the particles are accelerated. They have now published their research in the October issue of the prestigious scientific journal Science (Science 2002; 298: 193-195).

They are the behemoths of the universe: radio galaxies are among the largest single objects in the universe. What is more, they are enormous transmitters: they emit radio waves which can be made visible millions of light years away by modern radio telescopes. We are talking here about what is known as synchrotron radiation, which always occurs in the cosmos when relativistic particles - these are particles which move at almost the speed of light - hit a magnetic field and are thus deflected.

Where precisely these particles are thus accelerated has just been established by a team of astrophysicists which includes Dr. Karl-Heinz Mack of the Bonn Institute of Radio Astronomy. In the centres of many radio galaxies there are probably located enormous black holes, as heavy as several billion suns. They produce two jet streams which go in two different directions and which consist of very fast electrons - how exactly this occurs is not yet known. These jet streams move at high speed several 100,000 light years into intergalactic space. Just as a plane compresses the air in front of its nose, so they push the very thinly spread matter in front of them. And just as with a supersonic plane it finally ends up in a big bang: it produces strong shock waves which accelerate the electrically charged particles in the jet streams even further until they are ultimately almost as fast as light. This is then followed by a constant high loss of energy which correspondingly decelerates the particles again.In doing so they leave what can be compared to 'skid marks', which initially start life as visble light, then turn into infrared radiation and eventually, when they have been slowed down a great deal, consist of lower-energy radio waves.

These radio waves occur at very high intensity and are visible in the radio telescope as bright spots, known as 'hot spots'. The radiation produced at the beginning of the 'skid marks' has in the past only been proved in a few cases - and then usually in very poor resolution. The three astronomers Almudena Prieto, Gianfranco Brunetti and Karl-Heinz Mack have now considerably improved upon this: by using long exposure times for the radio galaxy 3C445 with the Very Large Telescope of the European Southern Observatory in Chile, they were able to prove the existence of 'braking radiation' in the infrared and optical ranges and resolve their regions of origin - 'a real surprise', Professor Uli Klein of the University of Bonn's Institute of Radio Astronomy comments. By this means the researchers were able, for the first time, to pinpoint exactly where the shock acceleration and the subsequent loss of energy of the relativistic particles begin in the jet streams.

As the researchers were thus able to localise the 'skid marks' more precisely, they now also know more precisely what area this enormous acceleration takes place in: in an area about 15,000 light years beyond the first 'supersonic bang' the energy of the particles is progressively boosted. 'At this point the result is extreme turbulence, with the aid of which the acceleration takes place,' explains Dr. Mack, who is currently doing research at Bologna. 'This turbulence, in turn, seems to derive from the jet streams themselves, while they are 'gouging' their way into the intergalactic medium.' The three astronomers' discovery therefore has far-reaching consequences for the interpretation of radio galaxies.

Bonn, Universitaet