The Super-Massive Black Hole Stays on a Diet

Chandra_image_of_Sgr_A

At the very center of our galaxy lives a gigantic black hole with a mass more than four million times that of the Sun. The existence of such an immense monster does not constitute an anomaly in our Universe but rather a common occurrence since most (if not all) galaxies host huge black holes in their center. These super-massive black holes exhibit two different kinds of behavior: they are either actively swallowing large amounts of matter or they are quiet with almost no material falling into them.

The super-massive black hole of our galaxy is of this second kind and it is well known to astronomers because it is surrounded by several stars that orbit around it. It is thanks to the orbital motion of these stars that it has become possible to measure the huge mass of this monster. Indeed since the black hole is not devouring any material, it is basically impossible to directly spot it. Gas falling into a black hole would become extremely luminous and emit radiation in a process known as accretion. 

However, In 2011 a surprising discovery was made by a team of astronomers lead by Stefan Gillessen: a big cloud of gas was found to be headed towards the black hole at high speed, with a closest approach predicted to be happening in early 2014. The cloud was seen to be moving in an orbit that would have brought it so close to the black hole to be tidally disrupted by the huge gravitational force exerted upon it. This event sprouted a lot of excitement among astronomers because part of the disrupted cloud could have fallen into the black hole and/or could “light-up” in the process, revealing precious information about the super-massive monster and the environment around it.

The closest approach of the cloud is finally happening in these days and the first surprising results have been reported in a brief communication released today and lead by astronomer Andrea Ghez: with great surprise the cloud of gas has remained intact after its closest encounter with the super-massive black hole. This implies that the cloud is really not what it seems and the most likely hypothesis now is that a star is embedded in the center of the cloud. The gas obscures the light emitted by the star thus making it invisible, but the gravity of the star has kept the gas together even after passing very close to the black hole.

Now astronomers are following the behavior of the cloud (plus the embedded star) at Keck observatory, hosting one of the biggest telescopes on the planet. More exciting and probably unexpected news will certainly come in the next future as the interaction between these two objects keeps developing: stay tuned for more news!

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