In 2011 the Very Large Telescope (VLT) discovered a gas cloud with several times the mass of the Earth accelerating towards the black hole at the center of the Milky Way. The cloud is now so stretched that its front part has passed the closest point and is traveling away from the black hole at more than 10 million km/h, whilst the tail is still falling towards it.
The black hole at the center of the Milky Way, known as Sgr A* (pronounced Sagittarius A star), is estimated to have a mass of about four million times that of the Sun and is the closest supermassive black hole known - so it is the best place to study black holes in detail.
This cloud is now making its closest approach and new VLT observations show that it is being grossly stretched by the black hole's extreme gravitational field. As the gas cloud is stretched its light gets harder to see. But by staring at the region close to the black hole for more than 20 hours of total exposure time with the SINFONI instrument on the VLT — the deepest exposure of this region ever with an integral field spectrometer — the team was able to measure the velocities of different parts of the cloud as it streaks past the central black hole.
In an integral field spectrometer the light recorded in each pixel is separately spread out into its component colors and so spectra are recorded for each pixel. The spectra can then be analyzed individually and used to create maps of the velocities and the chemical properties of each part of the object, for example.
"The gas at the head of the cloud is now stretched over more than 160 billion kilometers around the closest point of the orbit to the black hole. And the closest approach is only a bit more than 25 billion kilometers from the black hole itself — barely escaping falling right in," explains Stefan Gillessen of the Max Planck Institute for Extraterrestrial Physics, Garching, Germany and leader of the observing team. "The cloud is so stretched that the close approach is not a single event but rather a process that extends over a period of at least one year."
The distance of closest approach is about five times the distance of the planet Neptune from the Sun, way too close for comfort to a black hole with a mass four million times that of the Sun.
"The most exciting thing we now see in the new observations is the head of the cloud coming back towards us at more than 10 million km/h along the orbit — about 1% of the speed of light," adds Reinhard Genzel, leader of the research group that has been studied this region for nearly twenty years. "This means that the front end of the cloud has already made its closest approach to the black hole."
The origin of the gas cloud remains mysterious, although there is no shortage of ideas: Astronomers thought that the gas cloud might have been created by stellar winds from the stars orbiting the black hole. Or possibly even be the result of a jet from the galactic center. Another option was that a star was at the center of the cloud. In this case the gas would come either from a wind from the star, or from a planet-forming disc of gas and dust around the star.
The new observations at least narrow down the possibilities.
Due to its distance, and the fact that we see the orbit at a steep angle as the cloud falls towards the black hole, only the position, not the shape, of the cloud can be discerned in this video. The stretching of the cloud is seen in observations of its velocity, which allow astronomers to work out where on its orbit the different parts of the cloud are now located.Credit:ESO/S. Gillessen
"Like an unfortunate astronaut in a science fiction film, we see that the cloud is now being stretched so much that it resembles spaghetti. This means that it probably doesn't have a star in it," concludes Gillessen. "At the moment we think that the gas probably came from the stars we see orbiting the black hole."
The climax of this unique event at the center of the galaxy is being closely watched by astronomers around the world. Astronomers expect to see that the evolution of the cloud switches from purely gravitational and tidal to complex, turbulent hydrodynamics. Regions close to the black hole that have not been previously studied and the effects of super-strong gravity.The team is also hoping to see evidence of how the rapidly moving cloud interacts with any ambient gas around the black hole. So far nothing has been found, but further observations are planned to look for such effects.
Paper: S Gillessen, R Genzel, T K Fritz, F Eisenhauer, O Pfuhl, T Ott, M Schartmann, A Ballone and A Burkert, 'Pericenter passage of the gas cloud G2 in the Galactic Center', Submitted to The Astrophysical Journal