Black hole jet collision captured

For the first time ever, astronomers have captured a rear-end collision between two knots travelling along an extragalactic jet, blasting out from a supermassive black hole at the centre of a galaxy.

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Left: a jet of material emanating from the black hole at the centre of galaxy NGC 3862
Right: A sequence of Hubble images of knots moving along the jet, captured over 20 years of observations

Astronomers have managed to capture a collision between two knots of ejected matter travelling at high speeds along an extragalactic jet emanating from a black hole.

These jets shoot out from a supermassive black hole at the centre of a galaxy and travel hundreds of light years at 98% the speed of light.

The discovery was made by observing a video created with two decades of NASA Hubble Space Telescope images of elliptical galaxy NGC 3862, 260 million lightyears from Earth. Its jet contains glowing knots of material much like a pearls on a string, one of which has smashed into a slower-moving knot in a rear-end collision.

"Something like this has never been seen before in an extragalactic jet," says Eileen Meyer of the Space Telescope Science Institute in Baltimore, Maryland, who observed the collision.
 
Extragalactic jets are not much understood; however they seem to transport energetic plasma in a beam from the central black hole of the galaxy. This motion, called 'superluminal', is an optical illusion caused by the speed of the plasma, which is close to the universal maximum of the speed of light.
 
The discovery suggests that shocks produced by collisions within the jet further accelerate particles and brighten the regions of colliding material, as this particular shock collision caused the knots to become more illuminated. The observation is significant because it is rare for such motions to be captured by optical telescopes so far from the black hole source.
 
Newly-forming stars also eject jets, albeit of gas, that too have a knotty structure. One theory to explain these phenomena is material falling onto the central object, which is then superheated and ejected along the object's spin axis. The material is restricted into a narrow jet by magnetic fields, while irregular drops of the material cause the jet to become knotty, as opposed to a smooth, steady stream.
 
It is thought that some knots experience less drag from the interstellar body and catch up with the one in front, causing a collision.
 
This latest collision is set to play out over the coming decades, and is the second case of superluminal motion to be caught hundreds to thousands of lightyears from its black hole source. As the knots continue to merge they will get brighter. "This will allow us a very rare opportunity to see how the kinetic energy of the collision is dissipated into radiation," Meyer says.
 
Meyer is now putting together a Hubble-image video of two more jets in the nearby Universe to try and capture similar collisions.

Front image: jets and radio-emitting lobes emanating from galaxy Centaurus A's central black hole
Credit: ESO/WFI (visible); MPIfR/ESO/APEX/A.Weiss et al. (microwave); NASA/CXC/CfA/R.Kraft et al. (X-ray) 
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