Black hole-bound gas cloud 'stretched like spaghetti'

BBC News online -- 17 July 2013

The gas cloud is being stretched out by the gravity of our galaxy's central black hole

The giant gas cloud heading for the black hole at the centre of our galaxy has begun its death spiral.

The cloud, known as G2 is now being stretched out like a piece of spaghetti by the black hole's extreme gravity.

This gravitational field has caused the head of the cloud to accelerate around the black hole and to speed back towards us.

Astronomers have been closely observing G2, hoping to catch it being ripped apart and eaten by the black hole.

Details of the latest observations are outlined in the Astrophysical Journal.

Black holes

• Black holes are incredibly dense objects with gravity strong enough to trap even light
• A 'medium' black hole could have the mass of 1,000 Suns but be no bigger than Earth
• Supermassive black holes are thought to be at the centre of most large galaxies - including ours

Source: BBC Science

The cloud of gas - three times larger than Pluto's orbit but with a total mass just three times that of the Earth - was first spotted on its course toward the galaxy's centre in 2011. 

The mass of the black hole at the centre of the Milky Way is estimated to be four million times that of the Sun and is formally known as Sagittarius A (Sgr A*). It is the closest known "supermassive" black hole and is therefore considered the best places to study these dense objects in detail.

"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," said Reinhard Genzel, from the Max Planck Institute for Extraterrestrial Physics in Germany.

"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 unclear, although a variety of ideas have been proposed.

These range from its recent formation due to a collision between stellar winds and the interstellar medium to its origins as a jet emerging from the galactic centre to a faint star that is losing increasing amounts of gas.

The head of the cloud is now travelling much faster than the tail

 

The new observations argue against the cloud possessing a stellar core that would constantly be supplying new gas.

"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," said Stefan Gillessen, also from the Max Planck Institute, who has been leading the observing team.

"At the moment we think that the gas probably came from the stars we see orbiting the black hole."
Due to the tidal forces stretching G2, the front of the cloud is now moving about 500 km/s faster than its tail.

The astronomers have been using the Very Large Telescope (VLT) in Chile to study G2.

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 VLT's Sinfoni instrument, the team was able to measure the velocities of different parts of the cloud as it streaked past the central black hole.