Mystery over monster cosmic cloud
Observations of a cosmic confrontation between a huge gas cloud and the black hole at the centre of our galaxy have sparked debate among astronomers.
Celestial fireworks were thought to be a possibility as the gas was torn apart by the black hole.
But their absence so far has rekindled suggestions that it may not be a pure gas cloud after all.
One study suggests the cloud, named G2, is in fact a pair of stars that have merged into a much bigger one.
This conclusion has been prompted by observations that G2 survived its closest approach to the Milky Way's black hole, known as Sagittarius A* (Sgr A*).
"A gas cloud would not do that," said Prof Andrea Ghez, from the University of California, Los Angeles (UCLA), who co-authored the research in Astrophysical Journal Letters.
But other astronomers are not as sure, and suggest that a more compact gas cloud should still fit the data.
Beyond a certain threshold point - the event horizon - nothing can escape the pull of a black hole, not even light itself. But outside that is a swirling mass of material, not unlike water circling a drain.
Astronomers had already seen G2 being stretched out like a string of spaghetti by the black hole's extreme gravitational field.
Over an extended period of time, it was expected that about half of the cloud would be swallowed up, with the remainder flung out into space by Sgr A*.
The acceleration of the matter in the cloud would set off a shower of X-rays - the much-awaited celestial fireworks - that would help astronomers learn more about our local supermassive black hole.
But Prof Ghez explained that the cloud was "completely unaffected by the black hole; no fireworks".
The team has proposed an alternative explanation for G2 based on their detailed study of the cloud with the huge optical and infrared telescopes at the Keck Observatory in Hawaii.
In their view, the object is in fact best explained by a pair of stars - a binary system - that had been orbiting the black hole in tandem. The stars then merged to form an extremely large star cloaked in gas and dust.
But Dr Stefan Gillessen, who was not involved in the recent study, maintains that the original interpretation of G2 stands. Dr Gillessen led the team that detected the red cloud approaching Sgr A*, reporting their discovery in Nature journal in 2012.
"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," Dr Gillessen, from the Max Planck Institute for Extraterrestrial Physics in Garching, Germany, told BBC News.
"I personally am sceptical about 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."
Prof Ghez and colleagues suggest that the newly formed object will eventually look like the massive young stars that are tightly clustered around the Milky Way's black hole - the so-called S-stars.
They also propose that the gravitational influence of the black hole could make mergers of binary stars more likely by increasing the eccentricity of their orbits.
Luck of the draw
But Dr Gillessen explained: "I think we are far from excluding any model."
He added: "My guess is still: 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."
Dr Gillessen suggested there was a very low chance - perhaps lower than 10% - that the observations could be explained by a recently merged binary star. And he added that mathematical models of the object as a gas cloud could be tweaked to enable them to match the new data.
He added that the UCLA team did not detect any emission from the surface of the star.
Of the absence of fireworks, he said: "This is factually correct, but no models actually predicted increased accretion 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 be 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."
The debate is likely to continue, but in the meantime, astronomers will be keeping their eyes glued on the centre of our galaxy.
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