Writing in Astrophysical Journal Letters, the team at NASA's Goddard Space Flight Center in Greenbelt, Maryland, said they were looking at X-ray emissions from iron found very close to the edge of the black hole, at the center of a galaxy 100 million light-years away.
A light-year is the distance light travels in one year at the speed of light, 186,000 miles (300,000 km) a second.
``We were looking at one line which is from iron,'' astrophysicist Paul Nandra, who helped lead the study, said in a telephone interview.
``This line, we think, comes from very, very close to the black hole and it comes from matter that is orbiting the black hole in a disk,'' he said.
In a black hole, the force of gravity is so strong that nothing, not even light, can escape. Some black holes are formed by collapsed stars, but others are ``supermassive,'' containing as much mass as a million to a billion suns compressed into a tiny region.
The only way scientists have been able to ``see'' them up to now is by looking at the accretion disks -- the swirling matter circling around as it is being pulled into the black hole.
As matter swirls around a black hole, it sends out all kinds of energy, which can be seen by X-ray telescopes like the one the Goddard team used. Once it gets past a certain point, known as the event horizon, it cannot be seen.
It is visible before that point. Usually, however, what astronomers have seen is matter being bounced around in the accretion disk. Until now they have not seen it on its final downward plunge into the black hole.
Nandra and colleagues think they have finally seen it.
They were looking at the galaxy NGC 3516 using the Advanced Satellite for Cosmology and Astrophysics (ASCA), a Japanese -U.S. X-ray satellite launched in 1993 to look at the superheated gas in the accretion disk.
Buried in the X-rays emitted by the gas was a strange feature -- energy that has been ``red-shifted'' in an astronomical Doppler effect.
Just as stretched and compressed sound waves cause the sound of a train's whistle or a truck's horn to rise and fall as they pass an observer, light is stretched, or red-shifted, as it speeds away from the Earth.
In this case, Nandra said, an analysis of this altered light indicated the iron was moving at a clip of about 6.5 million mph (10.5 million kph) toward a black hole.
``The evidence is pretty good,'' he said. But, he added in a telephone interview, ``this is our interpretation of the data.''
Richard Mushotzky, an astrophysicist at Goddard who worked with Nandra, said this was the first time anyone had reported seeing evidence of matter falling into a black hole.
``Nobody has ever seen direct evidence for inflow,'' he said.
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