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Andrea Thompson
Senior Staff Writer
SPACE.comTue Apr 29, 4:45 PM ET
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
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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