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Radio and X-rays point to a jet from our galaxy's biggest black hole

Image: Black hole jet candidate
Image: Black hole jet candidate
A composite image in X-ray and radio shows a likely candidate for a jet emanating from the supermassive black hole at the center of the Milky Way.X-ray: NASA/CXC/UCLA/Z.Li et al; Radio: NRAO/VLA

Jets of high-energy particles emanating from a black hole have been detected plenty of times before, but in other galaxies — not from the supermassive black hole at the center of the Milky Way. Previous studies and other evidence suggested that perhaps there were jets — or the ghosts of past jets — but the findings and studies often contradicted each other, and none was considered definitive.

Now, astronomers using Chandra X-ray Observatory and the Very Large Array radio telescope have found strong evidence that our own galaxy's supermassive black hole, known as Sagittarius A* or Sgr A*, is indeed producing a jet of high-energy particles.

“For decades astronomers have looked for a jet associated with the Milky Way’s black hole. Our new observations make the strongest case yet for such a jet,” said Zhiyuan Li of Nanjing University in China, lead author of a study in The Astrophysical Journal.

The supermassive black hole at the center of the Milky Way is about 4 million times more massive than our sun and lies about 26,000 light-years from Earth.

Not everything is eaten
While the common notion is that black holes inhale and ingest everything that comes their way, that’s not always true. Sometimes they reject small portions of incoming mass, pushing it away in the form of a powerful jet, and many times a pair of jets. These jets also feed the surroundings, releasing both mass and energy. They are likely to play important roles in regulating the rate of formation of new stars.

Sgr A* is presently known to be consuming very little material, and so the jet is weak, making it difficult to detect. Astronomers don’t see another jet “shooting” in the opposite direction, but that may be because gas or dust is blocking the line of sight from Earth. Or perhaps there's a lack of material to fuel the jet. Or there may be just a single jet.

“We were very eager to find a jet from Sgr A*, because it tells us the direction of the black hole’s spin axis. This gives us important clues about the growth history of the black hole,” said Mark Morris of the University of California at Los Angeles, a co-author of the study.

What's the spin?
The study shows that the spin axis of Sgr A* is pointing in one direction, parallel to the rotation axis of the Milky Way, which indicates to astronomers that gas and dust have migrated steadily into Sgr A* over the past 10 billion years. If the Milky Way had collided with large galaxies in the recent past and their central black holes had merged with Sgr A*, the jet could point in any direction.

The jet appears to be running into gas near Sgr A*, producing the X-rays detected by Chandra and the radio emissions observed by the VLA.

The two key pieces of evidence for the jet are a straight line of X-ray-emitting gas that points toward Sgr A* and a shock front — similar to a sonic boom — seen in radio data, where the jet appears to be striking the gas. Additionally, the energy signature, or spectrum, in X-rays of Sgr A* resembles that of jets coming from supermassive black holes in other galaxies.

The Chandra observations behind this study were made between September 1999 and March 2011, with a total exposure of about 17 days.

This story was originally published by Universe Today as "Best Evidence Yet for a High-Energy Jet Emanating From the Milky Way's Black Hole." Source: Chandra