Black hole expelled from its parent galaxy (Forwarded)

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Press and Public Relations Department
Max Planck Society for the Advancement of Science
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D-80539 Munich
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Contact:

Dr. Mona Clerico, Press Officer
Max Planck Institute for Astrophysics
and
Max Planck Institute for Extraterrestrial Physics, Garching
Tel.: +49 89 30000-3980
Fax: +49 89 30000-3569

Dr. Stefanie Komossa
Max-Planck-Institute for Extraterrestrial Physics, Garching
Tel.: +49 89 30000-3577
Fax: +49 89 30000-3569

April 30th, 2008

News SP / 2008 (98)

Black hole expelled from its parent galaxy

Max Planck Astronomers discover a gravitationally propelled mass monster

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)
have 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.

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, it 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-ray wavelengths. 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, the position corresponding with the
distant galaxy.

Large kicks for mergers

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 in 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, are set to track
gravitational waves. The MPE team's discovery 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.

Original work:

Komossa, S., Zhou, H., Lu, H.
A recoiling supermassive black hole in the quasar SDSSJ092712.65+294344.0?
Astrophysical Journal Letters, Vol. 678, L81, 2008 (May 10, 2008)

IMAGE CAPTION:
[http://www.mpg.de/bilderBerichteDokumente/multimedial/bilderWissensch...
(737KB)]
Ejection from the nucleus: for the first time in nature, astronomers have
observed a supermassive black, which -- propelled by gravitational waves -
leaves its parent galaxy. The illustration depicts this scenario. Image:
MPE/HST-Archive