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Controversy over mystery object orbiting supermassive black hole

Jenny Winder, News Writer
Nov 11, 2014, 7:57 UTC

Sen—Astronomers using the W. M. Keck Observatory in Hawaii believe that they have solved the riddle of a bizarre object spotted in the centre of our Milky Way galaxy. But their claim has sparked fresh controversy, and is disputed by the object's discoverer.

The object, widely known as G2, was earlier declared to be a hydrogen gas cloud headed toward our galaxy’s enormous black hole, and becoming stretched and distorted by the powerful gravitational forces.

However, having studied it during its closest approach to the supermassive black hole this summer the team led by Andrea Ghez, professor of physics and astronomy at UCLA, now suggest that it is an extremely large star, the product of a pair of binary stars having merged.

They say the binary system had been orbiting the black hole in tandem and merged together to form the star, cloaked in gas and dust, its behaviour controlled by the black hole’s powerful gravitational field.

If G2 had been a hydrogen cloud, it could have been torn apart by the black hole, and the resulting celestial fireworks would have dramatically changed the state of the black hole, they say.

“G2 survived and continues happily on its orbit; a gas cloud would not do that,” said Andrea Ghez. “G2 was completely unaffected by the black hole; no fireworks.”

The new result is contested by Stefan Gillessen, of the Max Planck Institute for Extraterrestrial Physics in Germany, who led the discovery team that found it in 2011 after studying images obtained by the Very Large Telescope (VLT) in Chile.


An image from W. M. Keck Observatory near infrared data shows that G2 survived its closest approach to the black hole and continues happily on its orbit. The green circle just to its right depicts the location of the invisible supermassive black hole. Image credit: Andrea Ghez, Gunther Witzel/UCLA Galactic Center Group/W. M. Keck Observatory

Gillessen says though the new data shows the object to be more compact, he believes it still to be a clump of gas rather than a star.

Ghez, who used Keck Observatory to study thousands of stars around the supermassive black hole, said: “G2 is not alone. We’re seeing a new class of stars near the black hole, and as a consequence of the black hole.”

Black holes form out of the collapse of matter and have such high density that nothing can escape their gravitational pull, not even light. They cannot be seen directly, but their influence on nearby stars is visible and provides a signature.

Keck Observatory employs a powerful technology called adaptive optics, which Ghez helped to pioneer, to correct the distorting effects of the Earth’s atmosphere in real time, and to reveal the region of space around the black hole. Laser beams are projected into the sky to resemble artificial stars and show the turbulence, allowing it to be cancelled out.

In our galaxy, massive stars primarily come in pairs. According to Ghez’s team, G2 appears to be just one of an emerging class of stars near the black hole that are created because the black hole’s powerful gravity drives binary stars to merge into one. When this happens, the star expands for more than 1 million years before it settles back down.

Ghez said: “This may be happening more than we thought. The stars at the centre of the galaxy are massive and mostly binaries. It’s possible that many of the stars we’ve been watching and not understanding may be the end product of mergers that are calm now.”

He added: “We are seeing phenomena about black holes that you can’t watch anywhere else in the universe. We are starting to understand the physics of black holes in a way that has never been possible before, and is possible only at the centre of the galaxy.”

G2, in that explosive stage now, makes an unusual, 300-year elliptical orbit around the black hole. Ghez said that G2 now is undergoing “spaghetti-fication”, a common phenomenon near black holes in which large objects become elongated. At the same time, the gas at G2’s surface is being heated by stars around it, creating an enormous cloud of gas and dust that has shrouded most of the massive star.

But discoverer Gillessen told Sen in an exclusive interview: “I don’t think it conclusively shows what they claim. The observational fact they report is: The object is compact, i.e. unresolved at their wavelength. And the model they propose is: It is a recently merged binary star.

“And yes, while a merged star could look like that, it is by no means clear. I am really taken by surprise, how the team can make such a bold claim. Or actually, the press release is having the bold claim, while the actual paper is more modest in its statements.

“In particular, it is by no means excluded that the object is simply a gas cloud. The observational facts are clear, I guess. There is gas, which shows a beautiful tidal evolution, as witnessed in the radial velocities. And there is dust emission, which appears to be compact. What model we can use to explain that, is a different story.

“I personally am skeptical for the new proposed model. While I trust the observations, I don’t think it is very likely that we can catch an object in the transitional stage of merging. My guess is still that it is an unlucky clump of gas on an almost plunge orbit. The new observations show that it might be more compact by some moderate factor than what we thought.”

Gillessen added: “The press release also mentions the absence of fireworks. This is factually correct, but no models actually predicted increased accretion (material falling onto the black hole) in 2014—that would come later only. So the absence now is not telling much, I fear.

“On the other hand, it was estimated that the shock front of G2 rushing through the ambient gas might observable. That has not been observed, which in turn means that G2 is more compact or the ambient gas is thinner than assumed in these calculations. But G2 does not need to be as small as a star for that.”

Additional reporting by Paul Sutherland, News Editor.