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Black hole flung around twice
Posted: 22 July 2010

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A giant black hole has been spun around twice, forcing a change in the orientation of its spin axis, according to new evidence gleaned from Chandra X-ray Observatory data.

Black holes can be ejected from galaxies by collisions or interactions between two galaxies, but unlike these so-called 'recoiling' black holes, Chandra's latest target has remained stationary – only its spin axis has changed orientation.

Four cavities – regions of lower than average X-ray emission – are located in pairs around the black hole. In the diagram, one pair is labelled 1 and 2, the second pair 3 and 4. Image: X-ray (NASA/CXC/UMD/Hodges-Kluck et al): Radio (NSF/NRAO/VLA/UMD/Hodges-Kluck et al).

“We think this is the best evidence ever seen for a black hole having been jerked around like this,” says Edmund Hodges-Kluck of the University of Maryland. “We’re not exactly sure what caused this behaviour, but it was probably triggered by a collision between two galaxies.”

The observation came thanks to Chandra's long, thirty hour exposure of the 780 million light year distant galaxy 4C +00.58. Gas swirls around the mouth of the supermassive black hole in a vast disc, and twisted magnetic field lines force some of the material away in high speed jets. Radio images of the galaxy reveal a bright pair of jets – one jet points from left to right while a fainter jet lies almost perpendicular, earning the galaxy a place in the category of so-called X-shaped galaxies. Four cavities – regions of lower than average X-ray emission – lie around the black hole in two pairs. One pair lies to the top right and bottom left, while the other pair lie top left and bottom right.

This bizarre geometry tells the story of the galaxy's complicated history. “Not once, but twice, something has caused this black hole to change its spin axis,” says Christopher Reynolds, also of the University of Maryland.

A large optical image from the Sloan Digital Sky Survey (left) is centred on the radio galaxy 4C +00.58, shown on the right in X-rays (gold) from the Chandra X-ray Observatory and radio waves (blue) from the Very Large Array. Image: X-ray (NASA/CXC/UMD/Hodges-Kluck et al): Radio (NSF/NRAO/VLA/UMD/Hodges-Kluck et al); Optical (SDSS).

The team think that originally, the black hole's spin axis lay along a diagonal line from top right to bottom left. A collision with a smaller galaxy ignited a jet from the black hole, sweeping away nearby gas to create the cavities visible in the top right and bottom left. Gas falling onto the black hole was then not aligned with its spin, creating an instability that was compensated by the black hole rapidly changing its spin axis. With the jets then pointing top left to bottom right, two more cavities developed in the gas in the same direction. As if that wasn't enough the spin axis then jerked around to its current, approximately left-to-right direction, either as a consequence of the two galaxy's black holes merging, or by a further influx of gas into the black hole.

An evolution like this has been suggested previously as the likely chain of events behind the characteristics of the X-shaped class of radio galaxy, but this study provides the first strong evidence for such a scenario.

“If we’re right, our work shows that jets and cavities are like cosmic fossils that help trace the merger history of an active supermassive black hole and the galaxy it lives in,” says Hodges-Kluck. “If even a fraction of X-shaped radio galaxies are produced by such “spin-flips”, then their frequency may be important for estimating the detection rates with gravitational radiation missions.”