Galactic Black Hole disrupts Gas Cloud
December 14, 2011
Over the next few years, astronomers will be able to observe first-hand how the super massive black hole at the centre of our Milky Way is being fed: an international team of astronomers led by the Max Planck Institute for Extraterrestrial Physics has found a gas cloud that is falling towards the black hole in the galactic centre. While some distortion due to the huge gravitational pull of the black hole can already be seen, the gas cloud will be completely disrupted and ultimately swallowed by the black hole, resulting in largely increased X-ray emission. The observations and analysis are described in a Nature paper, published online on 14 December 2011.
At the heart of our Milky Way resides a black hole with about 4.3 million solar masses, as has been shown by long-term observations of the motions of stars orbiting this gravitational monster. Even though it is a very extreme and interesting object - it is the only super massive black hole close enough to be observed in detail - most of the time the black hole lays dormant, emitting modest flares only occasionally. By their very nature, black holes do not emit radiation directly, the emission originates from matter falling towards the event horizon, releasing potential energy and heating up.
Analysing very sharp images and detailed observations of the galactic centre, the MPE astronomers have now detected for the first time a gas cloud that is falling into the accretion zone of the black hole. The orbit of the cloud is highly eccentric and it will be closest to the black hole in 2013 with a distance of 40 billion kilometres - a very close encounter in astronomical terms (1).
"Only two stars so far have come that close to the black hole since we started our observations in 1992," says Stefan Gillessen, lead author of the paper describing the detection and analysis of the gas cloud. "The stars passed unharmed through their closest approach; the crucial difference to them is that the gas cloud will be completely ripped apart by the tidal forces around the black hole. As a result the gas inflow into the black hole should increase substantially, as should the level of radiation from it."
The gas cloud can be seen in all long-wavelength infrared images from 2002 onwards, and for the past three years already shows signs of being disrupted. As the cloud falls towards the black hole - its current velocity is about 2350 kilometres per second - it will interact with the hot gas present in the accretion flow around the black hole and become disrupted by turbulent interaction.
Simulation gas cloud
"Because the mass of the gas cloud is larger than the mass of the hot gas within the area of closest approach to the black hole, the accretion near the event horizon will be temporarily dominated by the accretion of the cloud itself," explains Reinhard Genzel, MPE director and head of the galactic centre research group. "This will provide stringent constraints on the physics of black hole accretion, since we have an unusually good knowledge of the mass available."
Due to the long-term observations at many different wavelengths, the astronomers can constrain the properties of the cloud very well. The temperature of the warm dust cloud is about 550 Kelvin (~280 °C) and its density is 300 times larger than that of the surrounding hot gas, with a total mass of about three Earth masses (1.7 × 1025 kg). With this information, the scientists were able so simulate the time evolution of the size and velocities in a model, the main effects being the gravitational pull of the super massive black hole and the interaction with the surrounding hot gas (see animation).
From this simulation and hydrodynamic calculations, the astronomers predict that the temperature of the gas cloud should increase rapidly to several million Kelvin (2) near the black hole, leading to X-ray emission that should initially be somewhat larger than the current X-ray luminosity of the galactic centre. Over the following years it could potentially brighten by a large factor.
"Detailed observations of the radiation from the galactic centre over the next years will give us the unique opportunity to probe the properties of the accretion flow and observe the feeding process of a super massive black hole in real time," predicts Stefan Gillessen.
In 2013, the distance of the gas cloud to the black hole will be 36 light hours, which is about 3100 times size of the event horizon of the black hole. This is about 250 times the distance Earth-Sun, and the event horizon of the black hole is about 20 times the size of the Sun.
As the gas cloud falls towards the black hole, the hot gas present in the accretion disk around the black hole is expected to drive a shock wave, which will slowly compress the cloud. This will lead to a growing, dense shell surrounding the inner zone of the gas cloud. Due to the black hole's tidal forces, the cloud becomes elongated along its direction of motion, until it is completely disrupted due to instabilities at the contact area. Just before pericentre the gas cloud intersects with the shock front and the post-shock temperature may increase rapidly to several million Kelvin. This should lead to increased emission in particularly in high-energy X-rays.