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Astronomers lock horns over mystery object G2

Jenny Winder, News Writer
Apr 7, 2015, 0:55 UTC

Sen—Astronomers are challenging each other’s findings about a mysterious object making a close approach to the giant black hole at the centre of our Galaxy.

The object, known as G2, was discovered in 2011 by a team of astronomers at the Max Planck Institute for Extraterrestrial Physics in Germany, led by Stefan Gillessen. They used the European Southern Observatory’s Very Large Telescope (VLT) in Chile to observe in the infrared, and so view through intervening dust.

Gillessen believes it to be a dense stream of hydrogen gas. But now, a new study of G2, led by Andreas Eckart of the University of Cologne in Germany, has come to a different conclusion—that it is a single object with a dense core.

Eckart’s team also used ESO’s VLT to study the object in infrared light, enabling them to track it through thick clouds of galactic dust, as it headed towards the supermassive black hole at the centre of the Milky Way. It is believed to have made its closest approach, known as peribothron, in May 2014.

It had been suggested that the black hole, with a mass four million times that of our Sun, might rip the cloud apart, with some of the material feeding the black hole, leading to increased activity and possible flaring.

Eckart’s observations were made during the critical period before and after peribothron, from February to September 2014 and he says they suggest the object is very compact and has not been significantly dispersed under the extreme gravitation so close to the black hole.


Here the blobs have been coloured to show the motion of the cloud, red indicates the object is receding and blue approaching. The cross marks the position of the supermassive black hole. Image credit: ESO/A. Eckart

The team say they believe that it is most likely to be a young star that is still accreting material. They say their view is also supported by the lack of any increase in the activity of black hole itself as it fed on the cloud.

Eckhart told Sen, “Our model calculations tell us that the star has a mass of about two solar masses. It is a young star that accretes matter from the inner edge of a small accretion disk or a dust shell around it.”

Eckart’s team measured the polarisation of the light coming from the region which they say revealed that the material being accreted onto the black hole remains stable and has not been disrupted by the arrival of material from G2 as would have been expected if it were a free-floating cloud.

The scientists also estimated the velocity of the cloud. Prior to its closest approach the cloud was moving away from the Earth at about ten million km/h (over 6 million mph). After it swung around the black hole it was measured speeding towards Earth at around twelve million km/h (nearly 7 million mph).

Eckart said in a statement, “We looked at all the recent data and in particular the period in 2014 when the closest approach to the black hole took place. We cannot confirm any significant stretching of the source. It certainly does not behave like a coreless dust cloud. We think it must be a dust-shrouded young star.”

However Gillessen responded critically to the new findings, when asked by Sen to comment, and said he believes the team have misinterpreted their own observations.


Advanced analysis techniques using data sets for years since 2008 show the ongoing tidal disruption of G2, becoming more extreme, according to Gillessen. Image credit: Stefan Gillessen

He told us: “In summary, I don’t believe their paper. It is based on a much smaller data set than what we got in the same year. The team was not taking all data available, but all data available to them. That is a huge difference.”

In addition, Gillessen said he was concerned that the conclusion reached by Eckart’s paper had changed since it was first published in the pre-print archive astro-ph.

He told Sen: “As you might imagine, I am unhappy with this press release.”

Last year, Sen reported the work of another team, led by Andrea Ghez, Professor of physics and astronomy at UCLA, California, which also studied G2, using the Keck Telescope on Hawaii. Ghez suggested that G2 was an extremely large star, the product of a pair of binary stars having merged.

Gillessen also contested that team’s findings. He told Sen at the time that, using the VLT, he had discovered that G2’s orbit matches that of another gas cloud, called G1, that was detected a decade ago. His view was that G2 and G1 were clumps forming part of the same extended gas streamer.

The rival teams will continue to watch G2. Eckart told Sen, “Our plan is to continue monitoring this region. It is the closest supermassive black hole we have and it deserves special attention. Here we can learn facts we will never have a chance to witness in the nuclei of other galaxies.”

The dusty cloud G2 passes the supermassive black hole at the centre of the Milky Way. Credit: European Southern Observatory (ESO)