Astronomers using NASA's Chandra X-ray
Observatory have found evidence for an "awesome upheaval"
in a massive cluster of galaxies. A bright arc of ferociously
hot gas extending more than two million light years requires
one of the most energetic events ever detected.
"The
huge feature we detected in the cluster combined with its
high temperature (170 million oC) points to an
exceptionally dramatic event in the nearby Universe, "
says Ralph Kraft of the Harvard-Smithsonian Center for Astrophysics,
leader of a team of astronomers involved in this research.
"While we're not sure what caused it, we have narrowed
it down to a couple of exciting possibilities."
In this side-by-side comparison, an apparently ordinary star
field in optical light (left) is shown to be dramatically
different when observed in X-rays (right). Chandra's image
of 3C438, the central galaxy within a massive cluster, reveals
evidence for one of the most energetic events in the local
Universe.
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The
favored explanation is that two massive galaxy clusters are
running into each other at about 4 million miles per hour.
When hot clouds of gas in the two clusters meet, shock waves
produce a sharp change in pressure along the boundary where
the collision is taking place, giving rise to the observed
arc, which resembles an titanic weather front.
"Although
this would be an extreme collision, one of the most powerful
ever seen, we think this may be what is going on, " says
team member Martin Hardcastle of the University of Hertfordshire
in the United Kingdom.
One
problem with the collision theory is that only a single peak
in the X-ray emission is seen, whereas two would be expected.
Longer observations with Chandra and the XMM-Newton X-ray
observatories should help determine how serious this problem
is for the collision hypothesis.
Another
possible explanation is that the disturbance was caused by
an outburst generated by matter falling into a supermassive
black hole. In this scenario, the black hole would inhale
most of the matter but expel some of it outward in a pair
of high-speed jets, heating and pushing aside surrounding
gas.
Such
events are known to occur in this cluster. A galaxy named
3C438 near the center of the cluster is a powerful source
of explosive activity--presumably due to a supermassive black
hole. But the energy in these outbursts is not nearly large
enough to explain the Chandra data.
Right:
A radio map of 3C438 reveals jets spewing from the galaxy's
core--a sign of explosive activity. [More]
"If
this event was an outburst from a supermassive black hole,
then it's by far the most powerful one ever seen, " says
team member Bill Forman, also from the Center for Astrophysics.
The
phenomenal amount of energy involved implies a very large
amount of mass swallowed by the black hole, about 30 billion
times the Sun's mass consumed over a period of 200 million
years. The authors consider this rate of black hole growth
implausible.
"These
values have never been seen before and, truthfully, are hard
to believe, " notes Kraft. Until these issues are sorted
out, the awesome upheaval remains a mystery.
These
results were presented at the American Astronomical Society
meeting in Honolulu, HI, and will appear in an upcoming issue
of The Astrophysical Journal. NASA's Marshall Space Flight
Center, Huntsville, Ala., manages the Chandra program for
the agency's Science Mission Directorate. The Smithsonian
Astrophysical Observatory controls science and flight operations
from the Chandra X-ray Center, Cambridge, Mass.
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