A colossalblack hole has been spotted exiting its home galaxy, kicked out after a hugecosmic merger took place.
The event,seen for the first time, was announced today.
When two collidinggalaxies finally merge, it is thought that the black holes at their coresmay fuse together too. Astronomers have theorized that the resulting energyrelease could propel the new black hole from its parent galaxy out into space,but no one has found such an event.
"Wehave observed the pre-merger stages of black holes," said Stefanie Komossaof the Max Planck Institute for extraterrestrial Physics, part of the team thatmade the new discovery. "But we haven't seen the actual mergerevent."
Komossa andher team have now detected the consequences of such a merger: a 100-million-solar mass black hole inthe process of leaving its home galaxy.
"Theconsequence was that the merged black hole, the final product, the new blackhole was expelled from the galaxy," Komossa said. The team's results aredetailed in the May 10 issue of the journal Astrophysical Journal Letters.
Blackholes get a kick
Komossaexplained that the theory behind these mergers follows from the observationsthat many galaxies have very massive black holes at their cores. If twogalaxies with these black holes collide, "then it's sort of inevitablethat these two black holes will come very close to each other."
The blackholes may not merge right away though.
"Onepossibility is that for a long time they just orbit each other," likebinary stars, Komossa told SPACE.com.
Eventually,the orbitingblack holes might interact with a star or surrounding gas which could causethem to lose angular momentum. "That would be a way to push themever-closer towards each other," Komossa said.
Eventually,the black holes would fuse, and "in the final coalescence, or merger, ofthese two black holes, a giant burst of gravitational waves is emitted,"she said. "Since these waves are generally emitted in one preferreddirection, the black hole is then kicked in the other direction."
The"kick" the black hole receives is akin to the recoil of a rifle. Itcan propel the black hole to speeds of up to several thousand kilometers persecond, according theoretical simulations. The escaping black hole Komossa andher team observed was racing along at 5,900,000 mph (2,650 kilometers persecond).
The pull ofthe galaxy's gravity is no match for these incredible speeds, and the blackhole, "will inevitably go to intergalactic space," Komossa said.
Galacticevolution
In theory,these mergersand escapes would leave several black holes without galaxies and galaxieswithout black holes out in space.
Detectingblack holes at the center of galaxies is a difficult process. Because theirgravity is so powerful, light is trapped, which is why they're black. Only bylooking at their effects on surrounding material are they presumed to exist, andthis is typically done only with relatively nearby galaxies, so looking for amissing black hole in the center of a distant galaxy is a tricky prospect.
Theevolution of black holes and galaxies is very closely linked, so what exactlythe effect would be on the separated partners is uncertain and the subject offurther research.
Insimulations where a black hole receives a slightly weaker kick, it can't escapethe galaxy's gravity, so it falls back and oscillates until it comes to restagain at the galaxy's core. Recent simulations of this situation showed thatstellar orbits adjust to the yo-yoing black hole, "so it clearly has aneffect on the core of the galaxy," Komossa said.
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