If there’s one thing nearly everybody knows about a black hole, it’s that they voraciously gobble up anything and everything that drifts nearby. It’s not true, of course, since they’re powered by nothing more mysterious than gravity, so plenty material falling inwards manages to miss the black hole and gets sling-shotted back into space. But even so, astronomers studying Sgr A* (the supermassive black hole at the centre of the Milky Way Galaxy) were surprised to notice that less than 1% of the gas and dust drawn into its gravitational field ever get consumed – almost everything else gets ejected. Who knew that a black hole could be such a picky eater!Supermasive Black Holes (SMBH) have been observed in the cores of most nearby galaxies, and astronomers believe that all galaxies have them. Some of these SMBH’s have large amounts of material falling inwards so that, via the mechanisms hinted at above, they end up ejecting powerful streams of superheated matter at enormous speeds along their North and South poles. But even the nearest galaxies lie many millions of light years away, so that the only SMBH we can study in any details is Sgr A*. Unfortunately, there are so many stars, and such vast quantities of gas and dust between us and the core, that we cannot view that region directly. Instead, we have to use telescopes that see in non-visible wavelengths of light, such as radio telescopes on the ground, and infrared and Xray space telescopes in orbit.
One such telescope is the Chandra Xray Telescope, which detects the Xrays emitted by superheated matter and other sources. When astronomers used Chandra to study Sgr A*, in one of its longest ever observations, they found that more than 99% of the infalling material was ejected long before reaching the event horizon (the point of no return around a black hole, from which not even light can escape) and was unusually cool, and therefore quite dim in the Xray spectrum. These results were published in a paper published in Science.
Thanks to the conservation of angular momentum, anything falling in towards a black hole has to lose some of its energy if it is not to miss the black hole. Particles within dense clouds lose kinetic energy easily, due to friction within the cloud. Similarly, the particles within cool clouds have little individual energy to start with, so are more easily captured by the black hole. But the Chandra observation, collected over five weeks in 2012, revealed that the material in the vicinity of Sgr A* is not only very thin and diffuse, but is also extremely hot. As a consequence, it escapes the black hole while it is still far away, and before it has time to form a superheated accretion disk, which explains the coolness of the ejected material.
“This new Chandra image is one of the coolest I’ve ever seen,” said co-author Sera Markoff of the University of Amsterdam in the Netherlands. “We’re watching Sgr A* capture hot gas ejected by nearby stars, and funnel it in towards its event horizon.”
“Most of the gas must be thrown out so that a small amount can reach the black hole”, said Feng Yuan of Shanghai Astronomical Observatory in China, the study’s co-author. “Contrary to what some people think, black holes do not actually devour everything that’s pulled towards them. Sgr A* is apparently finding much of its food hard to swallow.”