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Superkick: Black hole expelled from its parent galaxy
Gravitational rocket propelled the monster at a speed of thousands of
kilometres per second
By an enormous burst of gravitational waves that accompanies the merger
of two black holes the newly formed black hole was ejected from its galaxy.
This extreme ejection event, which had been predicted by theorists, has
now been observed in nature for the first time. The team led by Stefanie
Komossa from the Max Planck Institute for extraterrestrial Physics (MPE)
thereby opened a new window into observational astrophysics. The discovery
will have far-reaching consequences for our understanding of galaxy
formation and evolution in the early Universe, and also provides observational
confirmation of a key prediction from the General Theory of Relativity
(Astrophysical Journal Letters, May 10, 2008).
When two black holes merge, waves of gravitational radiation ripple outward through
the galaxy at the speed of light. Because the waves are emitted mainly in one
direction, the black hole itself is pushed in the opposite direction, much like the
recoil that accompanies the firing of a rifle or the launching of a rocket. The
black hole is booted from its normal location in the nucleus of the galaxy. If the
kick velocity is high enough, the black hole can escape the galaxy completely.
Image Credit: Illustration: MPE, optical image: HST
Artistís conception of a black hole ejected from a galaxy
The MPE team's discovery verifies, for the first time, that these extreme events
actually occur; up to now they had only been simulated in supercomputers. The
recoiling black hole caught the astrophysicists' attention by its high speed -
2650 km/s - which was measured via the broad emission lines of gas around the
black hole. At this speed, one could travel from New York to Los Angeles in just
under two seconds. Because of the tremendous power of the recoil the black hole,
which has a mass of several 100 millions solar masses, was catapulted from the
core of its parent galaxy.
In addition to the emission lines from gas bound to the recoiling black hole,
the astronomers were also struck by a remarkably narrow set of emission lines
originating from gas left behind in the galaxy. This gas has been excited by
radiation from the recoiling black hole.
Gas that moves with the black hole - the so-called accretion disk gas - continues
to "feed" the recoiling black hole for millions of years. In the process of being
accreted, this gas shines in X-rays. In fact the team around Komossa also detected
this X-ray emission from the disk around the black hole at a distance of 10 billion
light years: by chance the region was scanned by the satellite ROSAT, and at the
extreme end of the visual field an X-ray source was discovered the position of
which corresponds with the distant galaxy.
The new discovery is also important because it indirectly proves that black holes
do in fact merge and that the mergers are sometimes accompanied by large kicks.
This process had been postulated by theory, but never before confirmed via direct
observation. Another implication of the discovery is that there must be galaxies
without black holes in their nuclei - as well as black holes which float forever
in space between the galaxies. This raises new questions for the scientists: Did
galaxies and black holes form and evolve jointly in the early Universe? Or was there
a population of galaxies which had been deprived of their central black holes? And
if so, how was the evolution of these galaxies different from that of galaxies that
retained their black holes?
In a close interplay between theory and observation, the astrophysicists prepare to
answer these questions. Various detectors on earth and in space, for example the
space interferometer LISA, will be set on the track of gravitational waves. The
discovery of the MPE team will provide new impetus for theorists to develop more
detailed models of the superkicks and their consequences for the evolution of
black holes and galaxies.
ApJ Letters, 678, L81, 2008 (May 10, 2008)
preprint astro-ph: 0804.4585
Dr. Mona Clerico
Max Planck Institute for Astrophysics
and Max Planck Institute for extraterrestrial Physics
Phone +49 89 30000-3980
Dr. Stefanie Komossa
Max Planck Institute for extraterrestrial Physics
Phone +49 89 30000-3577
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