Mysterious G2 Cloud Near Black Hole Identified

©Nathan Tweedie/Keck Observatory

Looking at G2

The mystery about a thin, bizarre object in the center of the Milky Way headed toward our galaxy's enormous black hole has been solved by UCLA astronomers using the W. M. Keck Observatory, home of the two largest telescopes on Earth.

The scientists studied the object, known as G2, during its closest approach to the black hole this summer, and found the black hole did not dine on it. The research is published today in the journal Astrophysical Journal Letters.

While some scientists believed the object was a cloud of hydrogen gas that would be torn apart in a fiery show, Ghez and her team proved it was much more interesting.

"G2 survived and continues happily on its orbit; a gas cloud would not do that," said Andrea Ghez, UCLA professor of physics and astronomy who holds the Lauren B. Leichtman and Arthur E. Levine Chair in Astrophysics, and directs the UCLA Galactic Center Group. "G2 was completely unaffected by the black hole; no fireworks."

Instead, the team has demonstrated it is a pair of binary stars that had been orbiting the black hole in tandem and merged together into an extremely large star, cloaked in gas and dust, and choreographed by the black hole's powerful gravitational field.

"G2 is not alone," said Ghez, who uses Keck Observatory to study thousands of stars in the neighborhood of the supermassive black hole. "We're seeing a new class of stars near the black hole, and as a consequence of the black hole."

Ghez and her colleagues -- who include lead author Gunther Witzel, a UCLA postdoctoral scholar in Ghez's research group, and Mark Morris, a UCLA professor of physics and astronomy -- studied the event with both of the 10-meter telescopes at Keck Observatory.

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. With adaptive optics, Ghez and her colleagues have revealed many surprises about the environments surrounding supermassive black holes, discovering, for example, young stars where none were expected and seeing a lack of old stars where many were anticipated.

"The Keck Observatory has been the leader in adaptive optics for more than a decade and has enabled us to achieve tremendous progress in correcting the distorting effects of the Earth's atmosphere using high-angular resolution imaging techniques," Ghez said.

The researchers wouldn't have been able to arrive at their conclusions without the Keck's advanced technology. "It is a result that in its precision was possible only with these incredible tools, the Keck Observatory's 10-meter telescopes," Witzel said.

"We are seeing phenomena about black holes that you can't watch anywhere else in the universe," Ghez added. "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 center of the galaxy."

Massive stars in our galaxy, she noted, primarily come in pairs. When the two stars merge into one, the star expands for more than 1,000,000 years "before it settles back down," Ghez said. "This may be happening more than we thought; the stars at the center 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 a merger that are calm now."

G2, in that explosive stage now, has been an object of fascination. "Its closest approach to the black hole was one of the most watched events in astronomy in my career," Ghez said.

G2 makes an unusual, 300-year elliptical orbit around the black hole, and Ghez's group calculated its closest approach occurred this summer -- later than other astronomers believed -- and they were in place at Keck Observatory to gather the data.

Black holes, which form out of the collapse of matter, 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, said Ghez, a 2008 MacArthur Fellow.

Reference: "Detection of Galactic Center Source G2 at 3.8 microns During Periapse Passage," G. Witzel et al., 2014, Astrophysical Journal Letters, Vol. 796, No. 1, Let. 8 [, preprint:].

The W. M. Keck Observatory operates the largest, most scientifically productive telescopes on Earth. The two, 10-meter optical/infrared telescopes near the summit of Mauna Kea on the Island of Hawaii feature a suite of advanced instruments including imagers, multi-object spectrographs, high-resolution spectrographs, integral-field spectrographs and world-leading laser guide star adaptive optics systems.

NIRC2 (the Near-Infrared Camera, second generation) works in combination with the Keck II adaptive optics system to obtain very sharp images at near-infrared wavelengths, achieving spatial resolutions comparable to or better than those achieved by the Hubble Space Telescope at optical wavelengths. NIRC2 is probably best known for helping to provide definitive proof of a central massive black hole at the center of our galaxy. Astronomers also use NIRC2 to map surface features of solar system bodies, detect planets orbiting other stars, and study detailed morphology of distant galaxies.

Keck Observatory is a private 501(c) 3 non-profit organization and a scientific partnership of the California Institute of Technology, the University of California and NASA.

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