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'X' marks the spot where black holes meet

Jets from the core of radio galaxy NGC326, seen closer in the inset, seem to have abruptly switched direction, a possible sign of a black hole merger.
Jets from the core of radio galaxy NGC326, seen closer in the inset, seem to have abruptly switched direction, a possible sign of a black hole merger.  

By Richard Stenger

(CNN) -- When two galaxies collide, massive black holes in their respective centers fuse in a dramatic flourish that creates a telltale "X" mark, according to astronomers.

The conclusion offers strong support to the theory that the gravitationally powerful black holes merge when galaxies crash into one another.

A joint U.S.-Australian team of scientists calculated that the fusion of two black holes would knock the larger one out of alignment and flip its spin axis.

Such a jolt would change the direction of radio-emitting jets, which stream in perpendicular fashion from a swirling disk of gas around central black holes.

The "smoking gun" evidence shows up often in radio images of galactic cores, revealing both the old and new jet paths.

"Flipped jets suggest that the black hole has suddenly been realigned," said co-investigator Ron Ekers of the Australia Telescope National Facility in Australia.

Massive black holes have been spotted in the center of most observed galaxies. Astronomers theorize that they formed from gas clouds or the collapse of big stars.

"We have known about X-shaped galaxies for a long time, but until now we have never had a convincing explanation for them," said Ekers' colleague David Merritt, a black hole expert at Rutgers University in New Jersey.

Anatomy of a black hole 

Ekers is an expert in radio astronomy, the study of a band of electromagnetic waves with greater wavelengths than those of light. Many cosmic bodies, including galaxies, emit energy strongly at radio wavelengths.

Merritt and Ekers, who published their report in an online edition of Science Express this month, think that even a small black hole, perhaps five times smaller than its partner, can knock the larger one out of kilter.

The pair plan to study more of the X-shape galactic shapes to gleam more about black hole pairings. They should have plenty of places to look.

About seven percent of galaxies known to give off radio emissions display the characteristic X-shape, said the scientists, who estimate that such mergers take place about once every year.

Space scientists were mixed in their reaction to the study.

"I find the interpretation plausible, and they make a reasonable attempt to discuss the uncertainties [but] I think many of the details will be argued over," said Vivek Dhawan of the National Radio Astronomy Observatory in Socorro, New Mexico.

Qingjuan Yu, an astrophysicist at Princeton University in New Jersey, said there could be other reasons besides the merger of black holes of different masses to account for the radio readings.

"It is worthy to point out that there exist alternative explanations for the X-type radio sources," Yu said.

The strange shape could be mostly a function of the shape of the host galaxy. For example, such X-shaped jets appear only in highly elliptical galaxies.

Nonetheless, Yu welcomed the report and looked forward to more research on the matter.

"It is possible that X-shaped radio sources is the result of coalescence of binary black holes. It is an important finding if the explanation ... is confirmed" Yu said.




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