Colliding galaxies send black holes packing

BEIJING, April 30 (Xinhuanet) -- A huge black hole
has been seen leaving its home galaxy after a colossal cosmic merger occurred.
The event, seen for the first time, was announced Tuesday.

When two colliding galaxies finally merge, it is
thoughtthe black holes at their cores may fuse together too. Astronomers
have theorized that the resulting energy release could propel the new black hole
from its parent galaxy out into space, but no one has found such an event.

"We have observed the pre-merger stages of black
holes," said Stefanie Komossa of the Max Planck Institute for extraterrestrial
Physics, part of the team that made the new discovery. "But we haven't seen the
actual merger event."

Komossa and her team have now detected the
consequences of such a merger: a black hole in the process of leaving its home
galaxy.

"The consequence was that the merged black hole, the
final product, the new black hole was expelled from the galaxy," Komossa said.
The team's results are detailed in the May 10 issue of the journal Astrophysical
Journal Letters.

Komossa explained that the theory behind these
mergers follows from the observations that many galaxies have very massive black
holes at their cores. If two galaxies with these black holes collide, "then it's
sort of inevitable that these two black holes will come very close to each
other."

Eventually, the black holes would fuse, and "in the
final coalescence, or merger, of these two black holes, a giant burst of
gravitational waves is emitted," she said. "Since these waves are generally
emitted in one preferred direction, the black hole is then kicked in the other
direction."

The "kick" the black hole receives is akin to the
recoil of a rifle. It can propel the black hole to speeds of up to several
thousand kilometers per second, according to theoretical simulations. The
escaping black hole Komossa and her team observed was racing along at 5,900,000
mph (2,650 kilometers per second).

The pull of the galaxy's gravity is no match for
these incredible speeds, and the black hole, "will inevitably go to
intergalactic space," Komossa said.

In theory, these mergers and escapes would leave
several black holes without galaxies and galaxies without black holes out in
space.

(Agencies)