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Black hole recoils from cosmic mystery slingshot

A black hole has been spotted recoiling from a mysterious slingshot effect, possibly from encounters with several other black holes, a new study has found.
Image: Black holes and stars in space
Evidence for a recoiling black hole has been found using data from Chandra (colored blue in this composite image), Hubble (gold), XMM-Newton and several ground-based telescopes. CID-42 is a source in the Cosmic Evolution Survey field thought to be at a distance of almost 4 billion light-years away.
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A black hole has been spotted recoiling from a mysterious slingshot effect, possibly from encounters with several other black holes, a new study has found.

The exact cause of the black hole's kickback is still unknown, but scientists have come up with two possible theories to explain its odd cosmic behavior.

One theory suggests the slingshot effect was produced by interactions within a triple black hole system. The other attributes the recoiling action to gravitational waves produced by two supermassive black holes as they merged together a few million years earlier.

Both theories hinge on two objects near the black hole that were observed in the visible range of the light spectrum. But these optical sources represent different things in each theory.

Astronomers discovered the recoiling black hole, called CID-42, while conducting a large, multi-wavelength survey, called the Cosmic Evolution Survey (COSMOS). It sits in a galaxy about 3.9 million light-years away, which astronomers were able to observe by combining observations from different space observatories and telescopes. [Photo of the recoiling black hole.]

In the black hole image, the X-ray source detected by Chandra is colored blue, while the Hubble observations are shown in gold. While the two optical sources are both visible in the Hubble data, they are too close for Chandra to resolve separately.

The COSMOS survey combines observations from NASA's Chandra X-ray Observatory, the Hubble Space Telescope, the European XMM-Newton, as well as ground-based observatories. To date, it has identified about 2,600 X-ray sources in deep space, but only one the CID-42 black hole corresponds with two optical sources.

A "tail" of merging galaxies
One big clue for astronomers studying the CID-42 black hole is the long tail trailing its host galaxy.

"The galaxy's long tail suggests that a merger between galaxies has occurred relatively recently, only a few million years earlier," Cassini officials said in a statement.

In addition, observations from the Very Large Telescope and the Magellan telescope suggest that the difference in speed of the two optical sources is at least 3 million mph.

Astronomers also studied X-ray observations from Chandra and XMM-Newton to gather further information about CID-42. Absorption from iron-rich gas indicates that the gas is moving rapidly away.

This could be gas in the galaxy between our Milky Way galaxy and one of the black holes that is falling into another black hole, or it could be gas on the far side of the black hole that is blowing away, researchers said in a statement.

"Taken together, these pieces of information allow for two different scenarios for what is happening in this system and the nature of the two optical sources in the center of the image," Cassini officials said.

Astronomers Francesca Civano and Martin Elvis of the Harvard-Smithsonian Center for Astrophysics detailed their research on CID-42 in the July 1 edition of The Astrophysical Journal.

Cosmic bullying?
In the first scenario, researchers think that a triple black hole encounter could have been produced through a two-step process.

First, a collision between two galaxies created a galaxy with a pair of black holes in a close orbit. But, before these black holes could merge, yet another galaxy collision occurred and a third supermassive black hole spiraled toward the existing pair.

The interaction between the three black holes in such a system could result in the lightest one being ejected the recoiling black hole.

For this theory, the two optical sources spotted by the COSMOS survey would be active galactic nuclei, with one thriving off material pulled along by — and falling onto the recoiling supermassive black hole. The second source would then be the result of the two other black holes merging into a single object.

As an added caveat theory, the fast absorption of X-ray emissions seen around CID-42 would be high-speed interstellar winds blowing between the active galactic nuclei.

What about gravity waves?
The scenario that could explain the recoiling black hole's appearance are gravitational waves, though it would require a huge cosmic collision to set them off, researchers said.

According to this theory, a merger between two supermassive black holes at the center of the galaxy would emit gravitational waves from the collision process. These waves, then, could cause the resulting black hole to be flung from the center of the galaxy.

In this case, the ejected black hole is the point source in the lower left, and a cluster of stars that are left behind in the center of the galaxy is in the upper right. The X-ray absorption that the researchers observed would be caused by gas that is falling onto the recoiling black hole.

This scenario was recently proposed by Peter Jonker from the Netherlands Institute for Space Research in Utecht as a possible explanation for a different source in a separate galaxy.

Jonker and his team of researchers discovered a Chandra X-ray source about ten thousand light-years (in projection) away from the center of a galaxy. The study presents three possible explanations for this object: it is an unusual type of supernova, it is an ultraluminous X-ray source with a very bright optical counterpart, or it is a recoiling supermassive black hole that resulted from a gravitational wave kick.

Further observations will likely help to eliminate or support one of the scenarios for CID-42, researchers said.