News in Science

Fast-flowing gas curtails black hole growth

Galaxy sheild Astronomers have discovered a powerful gas stream that limits how big a black hole can grow, preventing it from growing faster than the galaxy around it.

The discovery, reported today in Science, is the first direct evidence of a high-speed gas stream that blocks X-rays.

Scientists have hypothesised that the growth of a black hole could be limited by some kind of wind from the black hole itself, which would blow material away before it can fall into the black hole.

But the wind can only be generated if it is shielded from X-ray radiation coming from the black hole itself.

"This observation identifies one of the mechanisms that you need to develop these high velocity winds that expel the gas," says one of the study's authors, Associate Professor Jerry Kriss of the Space Telescope Science Institute in Baltimore Maryland.

"This is good evidence for a mechanism that regulates the growth of a black hole and the growth of its host galaxy."

Self regulation

As matter spirals into a black hole, the in-falling material forms an accretion disk where it's crushed and ripped apart, releasing huge amounts of radiation including ultraviolet light and X-rays, which signal that the black hole has become active.

The ultraviolet radiation can generate powerful winds exceeding 1000 kilometres per second, which can blow away in-falling material, slowing down or even cutting off the black hole's food supply.

These winds can be strong enough to blow gas out of the galaxy, preventing the formation of new stars and limiting the galaxy's size.

While studying the spiral galaxy NGC-5548, located 245 million light years away in the northern sky constellation Bootes, Kriss and colleagues noticed a prolonged dimming of the galaxy's nucleus, with soft X-ray emissions becoming 25 times weaker than previously observed.

The authors discovered the dimming was caused by a large clumpy stream of gas travelling at 5000 kilometres per second, acting like a shield to absorb 90 per cent of X-rays.

"The cloud must be fairly large because it's still going on now and we first had inklings of this last June," says Kriss.

"There's archival data that something's been going on there since at least 2011."

The stream's location, only a few light days away from the galaxy's nucleus, implies it may come from the disk surrounding the black hole.

This finding could also provide clues about the workings of quasars, powerful energy beams produced by supermassive black holes, and which are among the most distant objects in the visible universe.