Black Holes get a kick

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 on Tuesday April 29.

When two colliding galaxies finally merge, it is thought that the black holes at their cores may fuse together too. Astronomers theorize 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."

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, said Komossa.

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.

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. 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.

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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