An international team of astronomers¹, using data from several NASA and European Space Agency (ESA) space observatories², has discovered unexpected behavior from the supermassive black hole at the heart of the galaxy³ NGC 5548, located 244.6 million light-years from Earth. This behavior may provide new insights into how supermassive black holes interact with their host galaxies⁴. Immediately after NASA's Hubble Space Telescope observed NGC 5548 in June 2013, this international research team discovered unexpected features in the data. They detected a stream of gas flowing rapidly outward from the galaxy's supermassive black hole, blocking 90 percent of its emitted X-rays.

 

"The data represented dramatic changes since the last observation with Hubble in 2011," said Gerard Kriss of the Space Telescope Science Institute (STScI) in Baltimore, Maryland. "I saw signatures of much colder gas than was present before, indicating that the wind had cooled down due to a significant decrease in X-ray radiation from the galaxy's nucleus⁵（核心）."

 

The discovery was made during an intensive observing campaign that also included data from NASA's Swift spacecraft, Nuclear Spectroscopic Telescope Array (NuSTAR), and Chandra X-ray Observatory⁶, as well as ESA's X-ray Multi-Mirror Mission (XMM-Newton) and Integral gamma-ray observatory (INTEGRAL).

 

After combining and analyzing⁷ data from all six sources, the team was able to put together the pieces of the puzzle. Supermassive black holes in the nuclei⁸ of active galaxies, such as NGC 5548, expel large amounts of matter through powerful winds of ionized gas. For instance, the persistent⁹ wind of NGC 5548 reaches velocities¹⁰ exceeding 621 miles (approximately 1,000 kilometers) a second. But now a new wind has arisen, much stronger and faster than the persistent wind.

 

"These new winds reach speeds of up to 3,107 miles (5,000 kilometers) per second, but is much closer to the nucleus than the persistent wind," said lead scientist Jelle Kaastra of the SRON Netherlands Institute for Space Research. "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."

 

The newly discovered gas stream in NGC 5548 -- one of the best-studied of the type of galaxy know as Type I Seyfert -- provides the first direct evidence of a shielding process that accelerates the powerful gas streams, or winds, to high speeds. These winds only occur if their starting point is shielded from X-rays.

 

It appears the shielding in NGC 5548 has been going on for at least three years, but just recently began crossing their line of sight.

 

"There are other galaxies with similar streams of gas flowing outward from the direction of its central black hole, but we've never before found evidence that the stream of gas changed its position as dramatically as this one has," said Kriss. "This is the first time we've seen a stream like this move into our line of sight. We got lucky."

 

Researchers also deduced that in more luminous¹¹（发光的） quasars（类星体）, the winds may be strong enough to blow off gas that otherwise would have become "food" for the black hole, thereby¹² regulating both the growth of the black hole and that of its host galaxy.

 

These results are being published online in the Thursday issue of Science Express.

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