In 2013, scientists discovered that the brilliant nucleus of a galaxy not so distant from ours dimmed dramatically for no obvious reason. Now, astronomers believe that the galaxy's heart of darkness was caused by a rare stream of fast-moving stiff winds that seems to be blowing from somewhere near the center of the galaxy.
NGC-5548 is one of the most frequently observed Seyferd galaxies, galaxies that have a bright center. It is located some 244.6 million light years away in the constellation of Bootes the Herdsman. At its center is a black hole about 65 million times as massive as the sun. It emits a bright, hot light that is actually x-rays emitted by an accretion disk, a flat band of matter that is drawn towards the massive black hole. On the outer edges of the disk are lower-energy ultraviolet rays that create winds strong enough to blow away winds that would have otherwise spiraled into the black hole.
But this isn't the same wind that now blows out from the center of NGC-5548. Scientists have long known that a persistent wind of velocities going up to 3.5 million kilometers per hour have spewed forth from the nucleus of NGC-5548, but the new stream is five times more powerful than the old wind. It is so powerful, in fact, that at extremely high speeds of 18 million kilometers per hour, it blocks out 90% of the x-rays that come from the center of the black hole's accretion disk.
Astronomers combined data gathered from May 2013 to February 2014 by six different NASA and European Space Agency observatories, including the Hubble Space Telescope, XMM-Newton, Swift, NuSTAR, INTEGRAL and Chandra. Their findings seem to point to one conclusion-a rare wind that doesn't seem to have been observed in decades.
Jelle Kaastra, lead researcher and astronomer at the SRON Netherlands Institute for Space research says the new stream of wind has "travelled a distance of at least 100 billion kilometers" and "probably has an elongated structure." She estimates that the stream's width is only one-tenth or less than its entire length.
"This new stream is most likely gas that comes from the accretion disk, the disk of gas that is swirling in toward a black hole. The disk is turbulent, filled with all kinds of bubbles and instabilities that can launch gas from the disk," Kaastra says.
Team member Gerard Kriss of the Space Telescope Science Institute adds that the stream showed "signatures of much colder gas" that was not seen in the older stream. This indicates that there has been a significant decrease in ionizing x-rays from the center of NGC-5545, contributing to the cooler stream that is blocking out the light that was once seen at the heart of the galaxy.
The researchers believe that their discovery is the first direct evidence of a shielding process, one that astronomers have predicted years ago, and is needed to push black hole gas streams or winds into ultra-high speeds. When powerful enough, black hole winds can blow away matter that would otherwise have fallen into the black hole. The findings, the researchers hope, will lay the foundation for future research into how supermassive black holes grow and influence the growth or death of their host galaxies.
