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Mystery object at the centre of our galaxy may be larger than thought

Jenny Winder, News Writer
Dec 2, 2014, 18:04 UTC

Sen—Controversy over what is happening at the centre of our Milky Way galaxy has taken a new twist. The team that discovered an object labelled G2 are still insisting it is a gas cloud, and add that it is even bigger than they previously thought. 

Last month Sen reported that a rival group of astronomers was claiming the object may be an extremely large star, formed by the merger of a pair of binary stars. This was contested at the time by Stefan Gillessen, of the Max Planck Institute for Extraterrestrial Physics in Germany, who led the team that discovered G2 in 2011.


High-resolution image of the gas cloud G2 at the centre of our Milky Way with the SINFONI instrument at the VLT.  The red part of the cloud approaches the 4 million solar masses black hole (indicated with a cross). Image credit: MPE

Now Gillessen and his team have discovered that the orbit of G2 matches that of another gas cloud detected a decade ago, suggesting that G2 might actually be part of a much more extensive gas streamer.

G2 is on a highly eccentric orbit around the galactic centre and observations in 2013 have shown that part of the gas cloud is already past its closest approach to the black hole, at a distance of roughly 20 light hours.

New, deep infrared observations with the SINFONI instrument at the Very Large Telescope array (VLT) track the ongoing tidal disruption of the gas cloud by the powerful gravitational field.  A closer look into the data set led to a surprise.

“Already a decade ago, another gas cloud—which we now call G1—has been observed in the central region of our galaxy,” remarks Stefan Gillessen. “We explored the connection between G1 and G2 and find an astonishing similarity in both orbits.”


High-resolution images from the VLT. G1 and G2 are coloured blue and red, respectively. The dashed lines show the orbits of the star with the best known orbit (S2) as well as the best-fit common orbit for the two gas clouds. The cross marks the position of the 4 million solar mass black hole at the galactic centre. Image credit: MPE

G1 can be seen in the data sets from 2004 to 2008. This allowed the team to determine G1’s orbit and revealed that it  passed pericenter in 2001. The similarity of the orbits thus suggests that G1 is about 13 years ahead of G2.

“Our basic idea is that G1 and G2 might be clumps of the same gas streamer,” explains Oliver Pfuhl, lead author of the study presented in the recent paper. “In this case, we should be able to simultaneously fit both data sets and, indeed, our model captures the G1 and G2 orbits remarkably well.”

The model assumes that G1 was decelerated by a drag force due to the thin atmosphere that surrounds the massive black hole. This drag pushed it into a more circular orbit. Using this simple assumption the emission of both G1 and G2 apparently trace the same orbit. Small deviations from the fit are not surprising given the simplicity of the model, which likely is neglecting some essential physics.

“The good agreement of the model with the data renders the idea that G1 and G2 are part of the same gas streamer highly plausible,” states Gillessen. A likely source for both G1 and G2 could then be clumps in the wind of one of the massive disk stars, which could have been ejected some 100 years ago close to the apocentre of the G2 orbit. 

The current VLT data makes the suggestion that G2 is a large star, the product of merger of a pair of binary stars, highly unlikely.

The gas streamer picture could also help to explain the missing X-ray emission from the gas cloud near the black hole.