Astronomers may have detected for the first time the final cry from matter disappearing into a black hole.

It seems to have been spotted in the centre of a galaxy 100 million light years away.

The ASCA satellite made the vital observations

A black hole is a region of space where super-dense matter creates a gravity field so strong that not even light can escape its pull. Black holes can come in many sizes but it is believed that supermassive black holes, which have masses of up to one billion suns, are found at the centres of galaxies.

Red shift

Dr Paul Nandra, an astrophysicist at Nasa's Goddard Space Flight Center, observed the galaxy NGC 3516 using the Advanced Satellite for Cosmology and Astrophysics (ASCA), a Japanese/US X-ray satellite launched in 1993.

NGC 3516 is a so-called Seyfert galaxy. It is named after the astronomer Carl Seyfert who in 1943 published a catalogue of strange galaxies that had bright objects at their centres and peculiar spectra. Since then it has become one of astronomer's favourite galaxies, possibly holding the clue to the mystery of black holes at galactic cores.

Dr Nandra detected the emission of X-rays from iron atoms in the gas swirling around a central, dense object in NGC 3516. The gas is heated to temperatures of millions of degrees under the force of the object's extreme gravity.

Dr Nandra said this emission is typical of a black hole observation. But buried in the typical emission spectrum was a "red-shifted" absorption feature, also from iron atoms. This feature, says Dr Nandra, suggests that the matter is moving away from us, at six million miles per hour towards the black hole.

Long, hard look

"The evidence is pretty good," adds Dr Nandra, whose data came from an unprecedented five days of uninterrupted observation. "You often see evidence of matter flying out of a black hole, but never falling in. For example, with ultraviolet telescopes, one also detects absorption lines. But this is always from matter moving toward us and not into a black hole."

The Hubble Space Telescope has snapped NGC 3516

According to Dr Richard Mushotzky, also from the Nasa Goddard Center and co-author on the paper, this observation is the first clear detection of an absorption feature with a red-shift for a black hole.

"Nobody has ever seen direct evidence for inflow," Dr Mushotzky said. "We know from general physical arguments that the active galaxies are powered by accretion of gas onto the black hole, but no one's ever seen it. There's a possibility that these data indicate that we are actually starting to see direct evidence for accretion."

Accretion occurs when matter accumulates around the black hole faster than it falls in, forming a swirling disk. Friction within this disk causes it to shine brightly.

Astronomers believe that in the past our own galaxy may have had an active black hole at its core. Although it is still there, it is not now being fuelled by a giant accretion disk.