COSMIC BELCH from supermassive black hole stuns boffins

Rare high-speed gas absorbs X-ray radiation, say boffins

A black hole

Pic The dimming of the bright centre of an active distant galaxy may be the result of unexpected behaviour from a supermassive black hole at its heart, according to NASA.

A black hole

Bad gas ... Artist's impression of a black hole

The scientists, using NASA and ESA space observatories including the Hubble Space Telescope, noticed last year that there were strange things happening at the centre of galaxy NGC 5548. They now reckon that the odd gloom is the first direct evidence for a long-predicted shielding process that allows powerful gas winds to be accelerated to high speeds out of a black hole.

"There were dramatic changes since the last observation with Hubble in 2011," said team member Gerard Kriss of the Space Telescope Science Institute.

"This is a milestone in understanding how supermassive black holes interact with their host galaxies," added Jelle Kaastra of the SRON Netherlands Institute for Space Research, who led the research team. "We were very lucky. You don't normally see this kind of event with objects like this.

“It tells us more about the powerful ionised winds that allow supermassive black holes in the nuclei of active galaxies to expel large amounts of matter. In larger quasars than NGC 5548, these winds can regulate the growth of both the black hole and its host galaxy."

A long process leads to the creation of these powerful gas streams. As matter spirals into a black hole, it forms a flat disc known as the accretion disc, which gets heated so much that it emits X-rays around the hole and less energetic ultraviolet radiation further out. The UV radiation can create winds that are strong enough to blow away some of the gases that would otherwise have fallen into the black hole – but only if their starting point is shielded from the X-rays.

Boffins have seen before the effects of both X-rays and UV radiation on warm gas far away from the black hole, but this time they spotted a new gas stream between the disc and the original cloud. That stream is absorbing most of the X-ray radiation before it reaches the original cloud, leaving only the UV radiation behind. It’s also shielding the gas closer to the accretion disc, making strong winds possible.

"The new wind reaches speeds of up to 18 million kilometres per hour, but is much closer to the nucleus than the persistent wind," said Kaastra. "The new gas outflow blocks 90 per cent of the low-energy X-rays that come from very close to the black hole, and it obscures up to a third of the region that emits the ultraviolet radiation at a distance of a few light-days from the black hole."

The galaxy NGC 5548

The galaxy NGC 5548. Credit: ESA/Hubble and NASA. Acknowledgement: Davide de Martin.

Because of this shielding, the persistent wind far away from the nucleus receives less radiation and cools down.

"Because of this cooling down, new features arise in the Hubble spectrum of the wind. These features allow us to pinpoint the location of the strongest persistent wind component," co-author Nahum Arav said.

Passing clouds are another possibility that has previously been seen for this kind of strong X-ray absorption by ionised gas, but combining the data from XMM-Newton and Hubble, along with information from Swift, NuSTAR, Chandra and INTEGRAL, has convinced the boffins that this is a fast stream of outflowing gas very close to the nucleus.

"It may originate from the accretion disk," added team member Pierre-Olivier Petrucci.

Studying the activity could provide new insights into the interactions of supermassive black holes and their host galaxies. The full study, "A fast and long-lived outflow from the supermassive black hole in NGC 5548", was published in Science. ®

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