A colossal
black hole has been spotted exiting its home galaxy, kicked out after a huge
cosmic merger took place.
The event,
seen for the first time, was announced today.
When two colliding
galaxies finally merge, it is thought that the 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 100-million-solar mass 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.
Black
holes get a kick
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."
The black
holes may not merge right away though.
"One
possibility is that for a long time they just orbit each other," like
binary stars, Komossa told SPACE.com.
Eventually,
the orbiting
black holes might interact with a star or surrounding gas which could cause
them to lose angular momentum. "That would be a way to push them
ever-closer towards each other," Komossa said.
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 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.
Galactic
evolution
In theory,
these mergers
and escapes would leave several black holes without galaxies and galaxies
without black holes out in space.
Detecting
black holes at the center of galaxies is a difficult process. Because their
gravity is so powerful, light is trapped, which is why they're black. Only by
looking at their effects on surrounding material are they presumed to exist, and
this is typically done only with relatively nearby galaxies, so looking for a
missing black hole in the center of a distant galaxy is a tricky prospect.
The
evolution of black holes and galaxies is very closely linked, so what exactly
the effect would be on the separated partners is uncertain and the subject of
further research.
In
simulations where a black hole receives a slightly weaker kick, it can't escape
the galaxy's gravity, so it falls back and oscillates until it comes to rest
again at the galaxy's core. Recent simulations of this situation showed that
stellar orbits adjust to the yo-yoing black hole, "so it clearly has an
effect on the core of the galaxy," Komossa said.
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