Hubble spots a high-speed collision in an extragalactic jet

Extragalactic jet
In the central region of galaxy NGC 3862 an extragalactic jet of material can be seen at the 3:00 position, left. Hubble images, right, of knots (outlined in red, green and blue) shows them moving along the jet over 20 years. The “X” is the black hole.
(NASA, ESA and E. Meyer of STScI)

A high-speed, extragalactic jet of high velocity plasma has grown brighter over the last 20 years, and now scientists know why.

Using data collected by the Hubble Space Telescope, astronomers were able to show that over the last two decades one glowing knot of material in the jet slammed into another glowing knot just ahead of it. Now, the two knots are merging into one.

This bumping and merging of the two blobs of plasma is known as a shock-collision and gives energy to the particles involved.

“It basically makes them accelerate, and when they accelerate they tend to radiate at higher energies,” said Eileen Meyer of the Space Telescope Science Institute in Baltimore. 


“We’d never seen a collision before, so to see them merge into one blob and get brighter is very exciting,” said Meyer, who led a study describing the findings published this week in Nature.

The jet Meyer and her colleagues observed is about 1,300-light-years long and very narrow. Meyer said it is less dense than our atmosphere or the sun, but adds, “You wouldn’t want to be in the path of this thing.”

It is traveling at 98% of the speed of light, and is kind of bumpy with a string-of-pearls shape. 

The particles in the jet were shot out of the accretion disk that formed around a super-massive black hole 100-million to 1-billion times the size of our sun. It lies in the center of the galaxy NGC 3862 about 260 million light years from Earth.


“You have all this hot gas sliding into the black hole, but some of these charged particles get caught up in the magnetic field near the event horizon and then fly off,” Meyer said.  

The particles in the jet probably came very close to being swallowed up by the black hole before being flung on a high-speed journey across space, she said.

The research team is not entirely sure how the blob circled in green in the image above caught up with the blob circled in blue. One possibility is that the first one had to clear a path through the universe, which slows it down a bit.

Meyer said the jet will probably brighten over the next 20 to 30 years as the two knots continue to merge.

“Aside from very nearby things we tend not to see a lot of motion in the universe, especially on human timescales,” she said. “We think this collision has 20 to 30 years to finish up, so maybe by the end of my career we’ll have the final figures.”

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