Matter's Final Plunge in to Black Hole
The final cry from material streaming toward a black hole at more than 6 million miles per hour -- possibly the first evidence of matter actually falling into a black hole -- was detected by scientists at NASA's Goddard Space Flight Center in Greenbelt, Md. This is distinct from the readily observed phenomena of matter swirling around or away from a black hole. The result, gathered from the evidence of X-ray signatures from hot gas in a galaxy 100 million light years away, appears in an upcoming article in Astrophysical Journal Letters.
A black hole is a region of space where the force of gravity is so powerful that nothing, not even light, can escape its pull. Black holes range in size from stellar black holes to supermassive black holes, the latter of which have mind-boggling masses of one million to one billion suns compressed into a region smaller than our solar system.
Dr. Paul Nandra, an astrophysicist in Goddard's X-Ray Branch, observed the galaxy NGC 3516, which is thought to harbor a supermassive black hole in its core. Using the Advanced Satellite for Cosmology and Astrophysics (ASCA), a Japanese/US X-ray satellite launched in 1993, he detected the emission of X-rays from iron atoms in the gas swirling around a central, dense object. The gas is heated to temperatures in the millions of degrees under the force of the object's extreme gravity.
Nandra said this emission is typical of a black hole observation. Buried in the typical emission spectrum, however, was a rare glimpse at a red-shifted absorption feature also from iron atoms. The absorption feature, Nandra explained, is information from the light spectrum suggesting that matter is moving away from us -- and, in this case, about 6.5 million miles per hour towards a black hole.
"The evidence is pretty good," said 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."
Scientists have many tools to understand the properties of a distant light source. Among these tools are the emission spectrum and the Doppler effect. The emission spectrum is a breakdown of light into its component colors, much like a prism separates white light into a rainbow. This provides information about the type of gas emitting the light, as well as its velocity, temperature and pressure. The emission from NGC 3516 comes from a blur of gas molecules or atoms moving in many directions.
The Doppler effect is the same phenomenon that distorts the sound of the horn on a speeding car as it approaches and passes you a highway. The Doppler effect distorts light waves as well as sound waves. Because light sometimes acts like a wave, each color on the spectrum has a characteristic wavelength. Objects moving away from us emit light with wavelengths shifted towards lower frequencies (red light instead of blue, hence the term "red-shift").
X-rays are a kind of very energetic light. Although the "colors" of an X-ray spectrum are invisible to the human eye, the change in wavelength due to the Doppler effect is detectable by instruments.
Key to his recent ASCA observation, Nandra said, was the detection of a red-shifted absorption feature in the emission spectrum. Absorption means that something stood in the path between the light source and the observer, absorbing the light beam while en route towards earth. The Doppler shift of the absorption, therefore, pins down the direction of the light source as either moving towards us or away from us, directly in our line of sight. Because Nandra detected a red-shifted absorption feature, he knew the matter was moving away from us. Because the absorbing material was in between the Earth and the black hole, the matter must be falling into the black hole.
Nandra said he essentially observed a red-shift inside a red-shift. The galaxy NGC 3516 is moving away from the earth, producing a slight red-shift in its emission. The bit of matter that Nandra interpreted as falling into the black hole had a much greater red-shift than the galaxy, produced by the high speeds it attained before falling into the black hole void.
According to Dr. Richard Mushotzky, a Goddard astrophysicist and co-author on the paper, this observation is the first clear detection of an absorption feature with red-shift for a black hole, all with an excellent signal-to-noise ratio. "Nobody has ever seen direct evidence for inflow," Mushotzky said. "We know from general physical arguments that the active galaxies are powered by accretion, 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 a black hole bites off more than it can chew. Material (usually gas from a nearby star) piles up around the black hole faster than it falls in, typically forming a swirling disk surrounding the black hole. Friction within this disk causes it to shine brightly, forming a warning beacon for the invisible black hole lurking in the center.
Mushotzky, one of the six Interdisciplinary Scientists for the Chandra X-ray Observatory, said even more convincing data will come soon with the launch of two X-ray spectroscopy satellites next year, Astro-E and XMM. Nandra is with the Universities Space Research Association. Other scientists involved in the observation include Ian George, Jane Turner and Tahir Yaqoob, all of whom appear on the Astrophysical Journal Letters paper.