“The X Galaxy”: A Cosmic Radio Mystery Solved

1-blackholejer The radio image of this galaxy shows a bright pair of jets pointing from left to right and a fainter, more distant line of radio emission running in a different direction. More specifically, 4C +00.58 belongs to a class of "X-shaped" galaxies, so called because of the outline of their radio emission.

The new Chandra data have allowed astronomers to determine what may be happening in this system, and perhaps in others like it. The X-ray image reveals four different cavities around the black hole. These cavities come in pairs: one in the top-right and bottom-left, and another in the top-left and bottom-right.

When combined with the orientation of the radio jets, the complicated geometry revealed in the Chandra image may tell the story of what happened to this supermassive black hole and the galaxy it inhabits.

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

According to the scenario presented by Hodges-Kluck and his colleagues, the spin axis of the black hole ran along a diagonal line from top-right to bottom-left. After a collision with a smaller galaxy, a jet powered by the black hole ignited, blowing away gas to form cavities in the hot gas to the top-right and bottom-left. Since the gas falling onto the black hole was not aligned with the spin of the black hole, the spin axis of the black hole rapidly changed direction, and the jets then pointed in a roughly top-left to bottom-right direction, creating cavities in the hot gas and radio emission in this direction.

Then, either a merging of the two central black holes from the colliding galaxies, or more gas falling onto the black hole, caused the spin axis to jerk around to its present direction, roughly left to right. These types of changes in the angle of the spin of a supermassive black hole have previously been suggested to explain X-shaped radio galaxies, but no convincing case has been made for any individual object.

"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."

Source: Chandra X-ray Center

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