Astronomers using the NASA/ESA Hubble Space Telescope have observed a collision between two knots of ejected matter in the extragalactic jet blasting out the core of the elliptical galaxy NGC 3862.
In this image of the central region of the elliptical galaxy NGC 3862, an extragalactic jet of material moving at nearly the speed of light can be seen at the three o’clock position. The jet of ejected plasma is powered by energy from a supermassive black hole at NGC 3862’s center. A sequence of Hubble images of knots (in red, green, and blue) shows them moving along the jet over a 20-year span of observing. Astronomers were surprised to discover that the central knot (green) caught up with and merged with the knot in front of it (blue). Image credit: NASA / ESA / E. Meyer, STScI.
NGC 3862, also known as 3C 264, is located in the constellation Leo, approximately 260 million light-years away.
It is the sixth brightest galaxy in a huge galactic cluster known as Abell 1367 (Leo Cluster).
First spotted in visible light in 1992, NGC 3862’s enormous jet has a string-of-pearls structure of glowing knots of material.
Such extragalactic jets are not well understood. They appear to transport energy in a confined beam out from the active nucleus of the host galaxy. Presumably super massive black holes are the powerhouses behind jets.
Extragalactic jets have been detected in radio wavelengths in many active galaxies, but only a few have been seen in optical light. Astronomers do not yet understand why some jets are seen in visible light and others are not.
Taking advantage of Hubble’s sharp resolution and long-term optical stability, a group of astronomers led by Dr Eileen Meyer of the Space Telescope Science Institute assembled a video from archival data to better understand NGC 3862’s jet motions.
The astronomers were surprised to see a fast knot with an apparent speed of seven times the speed of light catch up with the end of a slower moving, but still superluminal, knot along the string. The resulting ‘shock collision’ caused the merging blobs to brighten significantly.
“Something like this has never been seen before in an extragalactic jet. As the knots continue merging they will brighten further in the coming decades. This will allow us a very rare opportunity to see how the energy of the collision is dissipated into radiation,” said Dr Meyer, lead author of the paper reporting the results in the journal Nature this week.
It’s not uncommon to see knots of material in jets ejected from gravitationally compact objects, but it is rare that motions have been observed in visible light.
In addition to black holes, infant stars eject narrowly collimated streamers of gas that have a knotty structure. One theory is that material falling onto the central object is superheated and ejected along the object’s spin axis. Powerful magnetic fields constrain the material into a narrow jet. If the flow of the infalling material is not smooth, blobs are ejected like a string of cannon balls rather than a steady hose-like flow.
Whatever the mechanism, NGC 3862’s fast-moving knot will burrow its way out into intergalactic space. A knot launched later, behind the first one, may have less drag from the shoveled-out interstellar medium and catch up to the earlier knot, rear-ending it in a shock collision.
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Eileen T. Meyer et al. 2015. A kiloparsec-scale internal shock collision in the jet of a nearby radio galaxy. Nature 521, 495–497; doi: 10.1038/nature14481
