Unexpectedly, black hole spares gas cloud; Gas cloud points to main meal instead

January 5, 2016

As Milky Way’s central black hole, SgrA*, failed to devour the gas cloud called G2, astronomers were disappointed their predictions were wrong. In an effort to find out what happened, they observed changes in G2’s orbit and discovered that SgrA* instead feeds on gas from other source - circumnuclear disk. 

Galaxy’s central region around the supermassive black hole, SgrA* (marked, in the centre). Dim black hole is surrounded by brighter stars and gas clouds.

A few years ago, when astronomers detected a cloud of gas heading toward our galaxy’s central supermassive black hole (SgrA*), they predicted the black hole would devour it in 2015. This was to result in bipolar jets of fast-moving charged particles, occurring when a black hole ejects the excess material it could not eat, offering a unique opportunity to watch SgrA* in action. However, since the predictions were wrong and nothing happened, astronomers looked into the reasons and made the best of the situation. 

Although the gas cloud, called G2, didn’t end up spiraling down into the supermassive black hole, it helped astronomers find out a lot about what is actually going on at the centre of our galaxy.

In a new study, Michael McCourt and Ann-Marie Madigan observed G2 and realised it interacts with some sort of local matter as it orbits SgrA*. By doing so, G2 changes its orbit and this gave them insight into how the supermassive black hole feeds.

Milky Way’s central supermassive black hole is huge, about 4 million times heavier than Sun. However, it appears to be less hungry than comparable central black holes in other galaxies. Since SgrA* is obscured from our view by the torus-shaped molecular gas, known as circumnuclear disk, it is very hard to directly observe how it feeds. This torus of cold gas is also hard to notice as it accretes toward SgrA*. Here’s where the change in G2’s orbit helped.

Gas cloud G2 is relatively light, with about three times the mass of Earth, but it seems to be compact in a sense that it lost only small amounts of its matter as it travelled near SgrA*. G2 is accompanied by G1, which is a similar gas cloud also orbiting the black hole. It is possible that they both have stars inside them. If so, G2 and G1 are stars’ extended gaseous shells, but they evolve independently as gas clouds. However, their respective stars’ gravity keeps the clouds from spiraling into SgrA*.

Since orbiting small gas clouds like G1 and G2 don’t seem to be black hole’s main source of matter, there had to be another local supplier in the vicinity of SgrA*. By modeling changes in orbits of G1 and G2 as they interact with this local matter, astronomers concluded that the main source of food for the black hole is a big torus of molecular gas some 5 light-years from SgrA*, and not stellar winds from nearby stars or orbiting gas clouds like G2. The model of these changes also indicates that the gas from this disk accretes in clockwise direction.

If their assumptions are right, telescopes like Event Horizon Telescope should verify it in the next 5-10 years by observing orbits of G1 and G2, as well as geometry of emissions around the SgrA* rim.

Study was published in Monthly Notices of the Royal Astronomical Society by Michael McCourt, of the Institute for Theory and Computation, Harvard University, Center for Astrophysics, Cambridge, USA, and Ann-Marie Madigan, of the Astronomy Department and Theoretical Astrophysics Center, University of California, Berkeley, USA.

Image credit: Stefan Gillessen and the Max-Planck Institute fur Extraterrestrische Physik

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