Astronomers have 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 archetypal Seyfert galaxy NGC 5548, located 244.6 million light-years from Earth. This strange and unexpected activity may provide new insights into the interaction of supermassive black holes and their host galaxies.
This image shows the archetypal Seyfert galaxy NGC 5548. Image credit: ESA / Hubble / NASA / Davide de Martin.
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 gas stream in the galaxy NGC 5548 – one of the best-studied of the type of galaxy know as Type I Seyfert – provides this protection. It appears that the shielding has been going on for at least 3 years.
“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. We just happened to get lucky. With most objects like this, you don’t normally see this kind of event,” said Dr Gerard Kriss, an astronomer of the Space Telescope Science Institute in Baltimore and a co-author of a paper published in the journal Science (arXiv.org pre-print).
This discovery is the result of an intensive observing campaign using major ESA and NASA space observatories, including Hubble Space Telescope, XMM-Newton, Swift, NuSTAR, Chandra, and INTEGRAL.
“This is a milestone in understanding how supermassive black holes interact with their host galaxies. You don’t normally see this kind of event with objects like this. It tells us more about the powerful ionized 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,” said first author Dr Jelle Kaastra of the SRON Netherlands Institute for Space Research.
Right after the Hubble Space Telescope had observed NGC 5548 in June 2013, Dr Kaastra, Dr Kriss and their co-authors 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,” Dr Kriss said.
After combining and analyzing data from the six observatories, the astronomers were 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 reaches velocities exceeding 1000 km/s. But now a new wind has arisen, much stronger and faster than the persistent wind.
This is an artist’s impression of a clumpy gas stream flowing quickly outward from the supermassive black hole at the heart of NGC 5548. The arrow shows NASA/ESA Hubble Space Telescope’s line of sight. Image credit: NASA / ESA / A. Field, STScI.
“The new wind reaches speeds of up to 5,000 km per second but is much closer to the nucleus than the persistent wind. 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,” Dr Kaastra said.
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,” said co-author Dr Nahum Arav of Virginia Tech in Blacksburg.
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. It may originate from the accretion disk,” the astronomers said.
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J. S. Kaastra et al. A fast and long-lived outflow from the supermassive black hole in NGC 5548. Science, published online June 19, 2014; doi: 10.1126/science.1253787
