Swiftly moving gas streamer eclipses supermassive black hole

An international team of astronomers has discovered that the supermassive black hole at the heart of the galaxy NGC 5548 has recently undergone strange, unexpected behavior rarely seen in the heart of active galaxies.This activity may provide new insights into the interaction of supermassive black holes and their host galaxies.

Discovery

The researchers detected a clumpy gas stream flowing quickly outward and blocking 90 percent of the X-rays emitted by the supermassive black hole at the center of the galaxy.This discovery was accomplished through an intensive observing campaign with the major ESA and NASA space observatories: XMM-Newton, the Hubble Space Telescope, Swift, NuSTAR, Chandra, and INTEGRAL. An international team led by scientist Jelle Kaastra, of the SRON Netherlands Institute for Space Research conducted the most extensive monitoring campaign ever of an active galaxy in 2013 and 2014.

‘There are other galaxies that show gas streams near a black hole, but they haven’t changed as dramatically. This is the first time we’ve seen a stream like this move into the line of sight,’ said Gerard Kriss of the Space Telescope Science Institute in Baltimore, Md. ‘We just happened to get lucky. With most objects like this, you don’t normally see this kind of event.’

The researchers say that this is the first direct evidence for the long-predicted shielding process that is needed to accelerate powerful gas streams, or ‘winds,’ to high speeds. The team reports that this is a milestone in understanding how supermassive black holes interact with their host galaxies.

These results are being published in Science, 19 June 2014.

Winds

Matter falling onto a black hole gets heated and emits X-rays and ultraviolet radiation. The ultraviolet radiation can launch winds outward. The winds may be so strong that they can blow off gas that otherwise would have fallen onto the black hole. Black hole winds can therefore regulate both the growth of the black hole and its galaxy.

But the winds only come into existence if their starting point is shielded from X-rays. The newly discovered gas stream in the archetypal Seyfert galaxy NGC 5548 -- one of the best-studied sources of this type over the past half-century -- provides this protection. It appears that the shielding has been going on for at least three years.

Right after the Hubble Space Telescope had observed NGC 5548 on June 22, 2013, the team discovered unexpected features in the data. ‘There were dramatic changes since the last observation with Hubble in 2011. I saw signatures of much colder gas than was present before, indicating that the wind had cooled down, due to a strong decrease of ionizing X-ray radiation from the nucleus,’ Kriss said.

After combining and analyzing data from the six observatories, the team was able to put the pieces of the puzzle together. Supermassive black holes in the nuclei of active galaxies, such as NGC 5548, expel large amounts of matter through powerful winds of ionized gas. For instance, the persistent wind of NGC 5548, known for two decades, reaches velocities exceeding 1000 km/s.

But now a new wind has arisen, much stronger and faster than the persistent wind. ‘The new wind reaches speeds of up to 5,000 kilometers per second but is much closer to the nucleus than the persistent wind,’ Kaastra said. ‘The new gas outflow blocks 90 percent 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 few light-days distance from the black hole.’

Because of this shielding, the persistent wind far away from the nucleus receives less radiation and cools down. Science paper co-author Nahum Arav said: ‘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.’

Strong X-ray absorption by ionized gas has been seen in several other sources, and it has been attributed for instance to passing clouds. ‘However, in our case, thanks to the combined XMM-Newton and Hubble data, we know this is a fast stream of outflowing gas very close to the nucleus,’ said team member Massimo Cappi.  ‘It may originate from the accretion disk,’ added team member Pierre-Olivier Petrucci.

(19 June 2014)

Further information

These results are published in Science: A fast and long-lived outflow from the supermassive black hole in NGC 5548’, by Kaastra et al.

Movie

A new creation by Renaud Person, one of the Worlds Director of the famous video game Assassin's Creed © Ubisoft. The movie brings us in a journey to the inner regions of NGC5548, and helps us in visualizing the findings presented in this study.

An animated journey through the active galaxy NGC 5548

At its center is a supermassive black hole that is 40 million times heavier than our Sun, all concentrated in a region smaller than the Earth’s orbit around the Sun. Gas swirls around this black hole and is sucked into it, heating up its surroundings and producing strong energetic X-ray radiation. This X-ray hot corona is fuelled through a rapidly rotating accretion disk. The disk also produces strong patchy winds of warm gas that is thrown out into space. It contains denser parts that may obscure the X-rays emitted in the direction towards the Earth, shown by the green line. Further outwards we see the winds produced by the outer parts of the rotating disk, where the so-called broad line clouds are. At light years away from the black hole the warm winds also absorb some of the X-ray and ultraviolet light from the nucleus. These winds can cool down when the obscuring inner clouds block the light from the nucleus. The power emitted by the nucleus is so strong that it affects major parts of the host galaxy. The galaxy has a size of hundred thousand light years and is at a distance of 240 million light years away from us.

See also

Website SRON
Prof.dr. Jelle Kaastra

Last Modified: 20-06-2014