Most Distant X-Ray Jet Yet Discovered Provides Clues To Big Bang
Illustration of quasar with the Chandra image inset.|
(Credit: NASA/CXC/A.Siemiginowska et al.; Illustration: CXC/M.Weiss)
The most distant jet ever observed was discovered in an image of a quasar made by NASA's Chandra X-ray Observatory. Extending more than 100,000 light-years from the supermassive black hole powering the quasar, the jet of high-energy particles provides astronomers with information about the intensity of the cosmic microwave background radiation 12 billion years ago.
The discovery of this jet was a surprise to the astronomers, according to team members. Astronomers had previously known the distant quasar GB1508+5714 to be a powerful X-ray source, but there had been no indication in previous images of any complex structure or a jet.
"This jet is especially significant because it allows us to probe the
cosmic background radiation 1.4 billion years after the big bang,"
said Aneta Siemiginowska of the Harvard-Smithsonian Center for
Astrophysics in Cambridge, Mass., lead author of a report on this
research in the November 20, 2004 Astrophysical Journal Letters. Prior to this discovery, the most distant confirmed X-ray jet corresponded to a time about 3 billion years after the big bang.
Quasars are thought to be galaxies that harbor an active central
supermassive black hole fueled by infalling gas and stars. This
accretion process is often observed to be accompanied by the
generation of powerful high-energy jets. One mechanism for emitting
x-rays is from synchroton radiation that is generated as
the electrons move in the magnetic field of the jet.
However, the X-rays may also be generated by another means: As the
electrons in the jet fly away from the quasar at near the speed of
light, they move through the sea of cosmic microwave background radiation
left over from the hot early phase of the universe. When a
fast-moving electron collides with one of these background photons,
it can boost that photon's energy up into the X-ray band. The X-ray brightness of the jet depends on the power in the electron beam and the intensity of the background radiation.
Siemiginowska and her colleagues compared the intensity of X-rays
expected from both mechanisms and concluded that the jet in this
quasar was likely due to interaction between the electrons and the
cosmic microwave backround. This is in part due to the fact that the
density of the cosmic background radiation increases with increasing
distance. This provides more seed photons to be boosted to X-ray energies.
"In fact, if this interpretation is correct, then discovery of this jet is consistent with our previous prediction that X-ray jets can be detected at arbitrarily large distances!" said team member Dan Schwartz, also of the Harvard-Smithsonian Center for Astrophysics.
This discovery also provides a means to study the cosmic microwave
background in the early universe. The Big Bang cosmology predicts an
evolution of this radiation.
"Everyone assumes that the background radiation will change in a predictable way with time, but it is important to have this check on the predictions," said Siemiginowska. "This jet is hopefully just the first in a large sample of these distant objects that can be used to tell us how the intensity of the cosmic microwave background changed over time."
Chandra originally observed GB 1508+5714 with the purpose of studying the X-ray emission from the dust located between the Earth and the far-flung quasar. The jet was found by Siemiginowska and her colleagues when they examined the data once it became available publicly in the Chandra archive.
This led Teddy Cheung of Brandeis University to then carefully look at radio observations of the object. Indeed, archived Very Large Array data confirmed the existence of the jet associated with the quasar GB 1508+5714.
Another group of astronomers led by Weimen Yuan of the University of Cambridge, United Kingdom, independently reported the discovery of the extended emission in GB 1508+5714 in X-rays. In a paper to be published in an upcoming issue of the Monthly Notices of the Royal Astronomical Society, the authors note that significant energy is being deposited in the outer regions of the host galaxy at a very early stage. This energy input could have a profound effect on the evolution of the galaxy by triggering the formation of stars, or inhibiting the growth of the galaxy through accretion of matter from intergalactic space.