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Collision on the black hole superhighway

Astronomers have pieced together Hubble data to observe a collision between knots of matter in the jet of a supermassive black hole for the first time.

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The jet seen shooting from galaxy NGC 3862 at the three o'clock position (left); and a sequence of Hubble images shows knots moving along the stream over a 20-year period. NASA, ESA, and E. Meyer (STScI)

There are a whole lot of giant kabooms in space, but we've never seen anything quite like this before: for the first time, astronomers have captured on video a collision between knots of matter inside a super-speed jet of plasma shooting from a black hole -- demonstrating a hypothesis that may explain how these jets can travel at such incredible speeds.

The discovery is only the second time matter that seems to be travelling faster than the speed of light has been observed at distances of hundreds of thousands of light-years away from the source of a black hole. This indicates that jets continue to travel at near light-speed at distances that rival the size of the host gallery, and could provide clues to understanding galactic evolution.

As a black hole spins on its polar axis, it shoots massive jets of plasma into space from its poles. These are known as relativistic jets because they travel at what is known as relativistic speed, or speed that is a very close to the speed of light. The speed of relativistic jet energy is beyond what could be produced by the spin of the black hole's accretion disc, and it's something that astrophysicists are still trying to figure out.

One hypothesis for how the plasma is energised to the point where it can travel so fast is called the "internal shock model". It is not really known how the jets are created, but one hypothesis suggests that, as material falls towards the black hole, it is superheated and ejected along the black hole's spin axis, confined to a narrow jet by strong magnetic fields. If the fall of the material towards the hole is uneven, it's ejection would not be even either, creating knots of matter within the jet.

The internal shock model proposes that some of these knots of matter are faster than others. Perhaps the slower knots carve out a path in the jet, creating a path of less resistance, which would allow a knot coming behind to travel faster. However it happens, these faster knots of matter within the jet catch up and collide with slower ones, creating internal shocks that accelerate particles and generate magnetic fields, which in turn also accelerates particles.

It is this process that astronomers have captured on video, a rear-end collision and fusion between two knots of matter in the jet stream of a black hole from the galaxy NGC 3862. The research is described in a paper published Friday in the journal Nature.

The video is actually a time lapse, created using two decades' worth of Hubble images of the galaxy -- the sixth brightest we've seen, and one of the very few with jets that can be seen in visible light.

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Relativistic jet streaming from galaxy M87, captured by the Hubble telescope in 1998.NASA and The Hubble Heritage Team (STScI/AURA)

The jet in question, originally discovered in optical light in 1992, has a structure that looks something like a string of pearls, with many bright knots of material in the stream. In order to try to understand jet motions better, Eileen Meyer of the Space Telescope Science Institute in Baltimore was matching archival Hubble images of the jet with a new Hubble image taken in 2014.

To her surprise, she came across a knot that seemed to be travelling at seven times the speed of light (an illusion caused by the real speed of the plasma, which is almost travelling at light speed, combined with the angle at which the jet is pointed towards the observer; this phenomenon is called "superluminal motion") catching up to a slower superluminal knot.

The collision between the two caused them to glow even more brightly; and, as they continue to merge in the coming decades, they will brighten even more.

"Something like this has never been seen before in an extragalactic jet," said Meyer. "This will allow us a very rare opportunity to see how the kinetic energy of the collision is dissipated into radiation."

Meyer's next step in the research is to look for similar events in other black hole jets. She is compiling time lapse videos of two more jets in nearby galaxies to try and find them.

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