This time-lapse movie of an extragalactic jet was assembled from 20 years of Hubble Space Telescope observations of the core of the elliptical galaxy NGC 3862.
Credits: NASA, ESA, and E. Meyer STScI
In the central region of galaxy NGC 3862 an
extragalactic jet of material can be seen at the 3 o'clock 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. Credit: NASA, ESA, and E. Meyer (STScI). Hi-res Image
When you're blasting though space at more than 98 percent of the speed of light, you may need driver's insurance. Astronomers have discovered for the first time a rear-end collision between two high-speed knots of ejected matter from a super-massive black hole. This discovery was made while piecing together a time-lapse movie of a plasma jet blasted from a supermassive black hole inside a galaxy located 260 million light-years from Earth.
The finding offers new insights into the behavior of
"light-saber-like" jets that are so energized that they appear to zoom
out of black holes at speeds several times the speed of light. This
"superluminal" motion is an optical illusion due to the very fast real
speed of the plasma, which is close to the universal maximum of the
speed of light.
Such extragalactic jets are not well understood. They appear to
transport energetic plasma in a confined beam from the central black
hole of the host galaxy. The new analysis suggests that shocks produced
by collisions within the jet further accelerate particles and brighten
the regions of colliding material.
The video of the jet was assembled with two decades' worth of NASA
Hubble Space Telescope images of the elliptical galaxy NGC 3862, the
sixth brightest galaxy and one of only a few active galaxies with jets
seen in visible light. The jet was discovered in optical light by Hubble
in 1992. NGC 3862 is in a rich cluster of galaxies known as Abell 1367.
The jet from NGC 3862 has a string-of-pearls structure of glowing
knots of material. Taking advantage of Hubble's sharp resolution and
long-term optical stability, Eileen Meyer of the Space Telescope Science
Institute (STScI) in Baltimore, Maryland, matched archival Hubble
images with a new, deep image taken in 2014 to better understand jet
motions. Meyer was 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," said Meyer. 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 kinetic energy of the collision is dissipated
into radiation."
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 with optical telescopes, and so far out from the black hole,
thousands of light-years away. In addition to black holes, newly forming
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, the 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.
Beyond the collision, which will play out over the next few decades,
this discovery marks only the second case of superluminal motion
measured at hundreds to thousands of light-years from the black hole
where the jet was launched. This indicates that the jets are still very,
very close to the speed of light even on distances that start to rival
the scale of the host galaxy. These measurements can give insights into
how much energy jets carry out into their host galaxy and beyond, which
is important for understanding how galaxies evolve as the universe ages.
Meyer is currently making a Hubble-image video of two more jets in
the nearby universe, to look for similar fast motions. She notes that
these kinds of studies are only possible because of the long operating
lifetime of Hubble, which has now been looking at some of these jets for
over 20 years.
Extragalactic jets have been detected at X-ray and radio wavelengths
in many active galaxies powered by central black holes, 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.
Meyer's results are being reported in the May 28 issue of the journal Nature.
Contact
Felicia Chou
NASA Headquarters, Washington
202-358-0257
felicia.chou@nasa.gov
Ray Villard
Space Telescope Science Institute, Baltimore
410-338-4514
villard@stsci.edu
For images and more information about the Hubble Space Telescope, visit: http://www.nasa.gov/hubble and http://hubblesite.org/news/2015/05
Contact
Felicia Chou
NASA Headquarters, Washington
202-358-0257
felicia.chou@nasa.gov
Ray Villard
Space Telescope Science Institute, Baltimore
410-338-4514
villard@stsci.edu
Source: NASA/Mission Pages/Hubble
