Astronomers say they may finally understand the behavior of an oddball galaxy: It was flipped around twice by a collision with a smaller galaxy.
The theory, if it proves correct, could help scientists identify more such collisions throughout the universe — and those collisions might, in turn, help test the validity of Albert Einstein's general theory of relativity.
The astronomers were studying a black hole at the center of an X-shaped radio galaxy (one that shines bright at radio wavelengths) called 4C +00.58 about 780 million light-years from Earth. This galaxy had strange features, said University of Maryland astronomer Edmund Hodges-Kluck, lead author of the study published in the Astrophysical Journal Letters.
Like planets, black holes rotate on an axis. As surrounding matter gets sucked into the black hole, gas "jets" will sometimes be produced along the axis.
But in this case, the jets seemed to be off-kilter. Images taken by the Chandra X-ray Observatory revealed darker regions within the galaxy where the gas had presumably been blown out by the gas jets — but these cavities were far from the jets' current line of fire.
The astronomers theorize that a smaller galaxy had collided with a larger one, causing the jets to ignite. Next, all the new gas from the galaxy falling into the black hole pushed the black hole's axis into a new position.
Finally, one of two things happened to make the black hole's axis flip a second time, the scientists suggest. Either the two galaxies' black holes merged, or more gas fell into the main black hole.
The evidence is far from conclusive, however, said University of Maryland astronomer Christopher Reynolds, one of the study authors.
"I wouldn't bet my car on this theory. I might bet a good meal," Reynolds said. "We can't think of another theory that works as well."
Joan Centrella, associate deputy director of the astrophysics science division at the NASA Goddard Space Flight Center in Greenbelt, Md., said if the theory is correct, and if more examples of similar galaxies are found, it may give scientists trying to detect gravitational waves a hint of what to look for in the vast reaches of space.
A gravitational wave is a ripple in space-time whose existence was predicted by Albert Einstein. Although these waves have been shown only indirectly to exist, systems that involve two black holes merging could be a good place to look, Centrella said.