A supermassive black hole with a mass 4 million times that of the Sun lies at the heart of the Milky Way Galaxy. It is orbited by a small group of bright stars and, in addition, an enigmatic dusty cloud, known as G2, which has been tracked on its fall toward the black hole over the last few years. Closest approach, known as peribothron, was predicted to be in May 2014.
The great tidal forces in this region of strong gravity were expected to tear the cloud apart and disperse it along its orbit. Some of this material would feed the black hole and lead to sudden flaring and other evidence of the monster enjoying a rare meal. To study these unique events, the region at the galactic center has been very carefully observed over the last few years by many teams using large telescopes around the world.
A team led by Andreas Eckart from the University of Cologne in Germany has observed the region using ESO’s VLT over many years, including new observations during the critical period from February to September 2014, just before and after the peribothron event in May 2014. These new observations are consistent with earlier ones made using the Keck Telescope on Hawaii.
The images of infrared light coming from glowing hydrogen show that the cloud was compact both before and after its closest approach as it swung around the black hole.
As well as providing very sharp images, the SINFONI instrument on the VLT also splits the light into its component infrared colors and hence allows the velocity of the cloud to be estimated. Before closest approach, the cloud was found to be traveling away from Earth at about 6 million mph (10 million km/h) and, after swinging around the black hole, it was measured to be approaching Earth at about 7.5 million mph (12 million km/h).
Florian Peissker from the University of Cologne in Germany, who did much of the observing, said: “Being at the telescope and seeing the data arriving in real time was a fascinating experience.” Monica Valencia-S., also from the University of Cologne, who then worked on the challenging data processing, added: “It was amazing to see that the glow from the dusty cloud stayed compact before and after the close approach to the black hole.”
Although earlier observations had suggested that the G2 object was being stretched, the new observations did not show evidence that the cloud had become significantly smeared out either by becoming visibly extended or by showing a larger spread of velocities.
In addition to the observations with the SINFONI instrument, the team has also made a long series of measurements of the polarization of the light coming from the supermassive black hole region using the NACO instrument on the VLT. These, the best such observations so far, reveal that the behavior of the material being accreted onto the black hole is very stable and so far has not been disrupted by the arrival of material from the G2 cloud.
The resilience of the dusty cloud to the extreme gravitational tidal effects so close to the black hole strongly suggest that it surrounds a dense object with a massive core, rather than being a free-floating cloud. This is also supported by the lack of evidence that the central monster is being fed with material, which would lead to flaring and increased activity.
Andreas Eckart sums up the new results: “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.”