Black Hole Knocked Off Axis By Galaxy Collision
This X-ray image of hot gas in and around 4C +00.58 reveals four different cavities -- regions of lower than average X-ray emission -- around the black hole. These cavities come in pairs: one in the top-right and bottom-left (labeled cavities #1 and #2 respectively), and another in the top-left and bottom-right (labeled cavities #3 and #4 respectively). Special processing was applied to this image to make the cavities more obvious.
Credit: X-ray (NASA/CXC/UMD/Hodges-Kluck et al): Radio (NSF/NRAO/VLA/UMD/Hodges-Kluck et al); Optical (SDSS) Full Story

The discovery of a giant, spinning black hole that has been knocked off its axis twice has led astronomers to suggest that a violent galaxy collision caused the strange cosmic behavior.

"We think that this black hole has quite a history," said astronomer Christopher Reynolds of the University of Maryland, a co-author in the black hole study. "Not once, but twice, something has caused this black hole to change its spin axis."

In this new study, scientists detected the change in the black hole's axis using the latest data from NASA's Chandra X-ray Observatory. The likely cuprit: A catastrophic collision between two galaxies. [Photo of the askew black hole].

The galactic smash-up is thought to have altered the axis of the spinning black hole. But unlike another recent study, in which colliding galaxies may have created recoiling black holes, this particular black hole was not moved by the crash ? only its spin changed.

Askew black hole

The black hole find emerged from extensive observations by astronomers of a galaxy known as 4C +00.58, which is located approximately 780 million light-years from Earth. Like many galaxies, a supermassive black hole resides at the heart of 4C +00.58.

This particular black hole is actively pulling in large amounts of gas, which is swirling around and forming a disk around the object. The jumbled magnetic fields within the disk generate strong electromagnetic forces that propel some of the gas away from the disk at high speed, producing bursts of radio jets.

But here's where it gets strange.

A radio image of 4C +00.58 shows a bright pair of jets that point from left to right, and a fainter, more distant line of radio emission that runs in a completely different direction. Scientists classify this type of galaxy as "X-shaped," because of the outline of their radio emission.

"We think this is the best evidence ever seen for a black hole having been jerked around like this," said the study's lead author Edmund Hodges-Kluck of the University of Maryland in College Park, Md. "We're not exactly sure what caused this behavior, but it was probably triggered by a collision between two galaxies."

Black hole weirdness

The new observations allowed astronomers to infer the possible mechanisms behind the 4C +00.58 system, and possibly others that may be similar.

Chandra's X-ray image revealed four separate cavities around the supermassive black hole. These cavities come in pairs: one in the top-right and bottom-left, and another in the top-left and bottom-right.

Based on the orientation of the radio jets, the complicated geometry that was revealed by the Chandra image could explain what happened to the system's black hole, as well as the galaxy itself.

The research is detailed in a recent edition of the Astrophysical Journal Letters.

So how did it happen?

Hodges-Kluck and his colleagues think that the original spin axis of the black hole ran along a diagonal line from top-right to bottom-left. Following a collision with a smaller galaxy, a jet powered by the black hole then ignited, blowing away surrounding gas to form cavities in the hot gas to the top-right and bottom-left.

In this scenario, since the gas falling onto the black hole was not aligned with the black hole's spin, the axis of the black hole rapidly changed direction, causing the jets to point in a roughly top-left to bottom-right orientation. This, in turn, created other cavities in the hot gas, and radio emissions in this new direction.

Finally, either a merger of the two central black holes from the colliding galaxies, or more gas falling onto the black hole, caused the spin axis to get moved to its present direction, which is roughly left to right.

These types of changes to the angle of the spin of supermassive black holes have previously been suggested as a way of explaining X-shaped radio galaxies. But so far, no convincing case has been made for any individual object ? perhaps until now.

"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," said 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."