Astronomers have uncovered a monster black hole that has been propelled out of the center of a distant galaxy, called 3C 186, by what could be the power of gravitational waves. A paper describing the discovery will appear in the March 30, 2017 issue of the journal Astronomy & Astrophysics (arXiv.org preprint).
The elliptical galaxy 3C186, located about 8 billion years from Earth, is most likely the result of a merger of two galaxies. This is supported by arc-shaped tidal tails, usually produced by a gravitational tug between two colliding galaxies, identified by M. Chiaberge et al. The merger of the galaxies also led to a merger of the two supermassive black holes in their centers, and the resultant black hole was then kicked out of its parent galaxy by the gravitational waves created by the merger. The bright, star-like looking quasar can be seen in the center of the image. Its former host galaxy is the faint, extended object behind it. Image credit: NASA / ESA / M. Chiaberge, STScI.
Though several other suspected runaway black holes have been seen elsewhere, none has so far been confirmed.
Now an international team of researchers has detected a supermassive black hole — with a mass of one billion times the Sun’s — being kicked out of its host galaxy.
“We estimate that it took the equivalent energy of 100 million supernovae exploding simultaneously to jettison the black hole,” said co-author Dr. Stefano Bianchi, from the Roma Tre University, Italy.
The images taken by the NASA/ESA Hubble Space Telescope provided the first clue that 3C 186, located 8 billion light-years away, was unusual.
“When I first saw this, I thought we were seeing something very peculiar,” said lead author Dr. Marco Chiaberge, from the Space Telescope Science Institute and Johns Hopkins University.
“When we combined observations from Hubble, Chandra X-ray Observatory, and the Sloan Digital Sky Survey, it all pointed towards the same scenario. The amount of data we collected, from X-rays to ultraviolet to near-infrared light, is definitely larger than for any of the other candidate rogue black holes.”
Hubble images of 3C 186 revealed a bright quasar, the energetic signature of an active black hole, located far from the galactic core.
“Black holes reside in the centers of galaxies, so it’s unusual to see a quasar not in the center,” Dr. Chiaberge said.
The astronomers calculated that the black hole has already traveled about 35,000 light-years from 3C 186’s center, which is more than the distance between the Sun and the center of the Milky Way.
And it continues its flight at a speed of 4.7 million mph (7.5 million km per hour). At this speed the black hole could travel from Earth to the Moon in 3 min.
Although other scenarios to explain the observations cannot be excluded, the most plausible source of the propulsive energy is that this supermassive black hole was given a kick by gravitational waves unleashed by the merger of two massive black holes at the centre of its host galaxy.
This theory is supported by arc-shaped tidal tails identified by the team, produced by a gravitational tug between two colliding galaxies.
According to the team’s theory, 1-2 billion years ago two galaxies — each with central, supermassive black holes – merged.
The black holes whirled around each other at the center of the newly-formed elliptical galaxy, creating gravitational waves that were flung out like water from a lawn sprinkler.
As the two black holes did not have the same mass and rotation rate, they emitted gravitational waves more strongly along one direction.
When the two black holes finally merged, the anisotropic emission of gravitational waves generated a kick that shot the resulting black hole out of the galactic center.
“If our theory is correct, the observations provide strong evidence that supermassive black holes can actually merge,” Dr. Bianchi said.
“There is already evidence of black hole collisions for stellar-mass black holes, but the process regulating supermassive black holes is more complex and not yet completely understood.”
The astronomers now want to secure further observation time with Hubble, in combination with the Atacama Large Millimeter/submillimeter Array and other facilities, to more accurately measure the speed of the black hole and its surrounding gas disc, which may yield further insights into the nature of this rare object.
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M. Chiaberge et al. 2017. The puzzling case of the radio-loud QSO 3C 186: a gravitational wave recoiling black hole in a young radio source? A&A, in press; doi: 10.1051/0004-6361/201629522
