Black Hole: Evidence Supports Existence In Active Galaxy


AN international team of astronomers has provided the most compelling evidence yet of a massive black hole at the center of an energetic galaxy 21 million light-years from Earth.

The research, presented this week at the American Astronomical Society meeting in Tucson, Ariz., comes 60 years after an astrophysicist from India proposed to the Royal Astronomical Society in London that certain stars could collapse into dense, gravitationally muscular neutron stars.

It is from neutron stars that, under the right conditions, black holes are thought to develop.

Subrahamanya Chandrasekhar's idea was scorned by more-senior astronomers at that 1935 meeting. The reaction to these latest results, which were reported in this week's issue of the journal Nature, stands in sharp contrast to Chandrasekhar's rebuff.

``My mind is blown,'' says astronomer Vera Rubin, of the Carnegie Institution in Washington. ``These are phenomenal results.''

A black hole is an object with gravity so strong that even light, traveling at 186,000 miles a second, cannot escape its grip. Its presence is inferred by what it does to the material that surrounds it. One of its calling cards is the presence of a rapidly rotating disk of material circling a relatively small volume of space.

The most likely candidates for so-called stellar black holes, thought to form after massive stars exhaust their fuel and erupt in a supernova, are binary-star systems that emit copious amounts of X-rays. As material from the companion star is sucked into the black hole, compression heats the material to the point that energy radiates as X-rays.

But the nuclei of some galaxies also emit vast amounts of X-rays, leading many astronomers to suspect that extremely massive black holes may lie in the galactic cores. These jumbo black holes are thought to be the engines driving a variety of bizarre phenomena, including jets of energy that can appear to be traveling faster than the speed of light.

The galaxy NGC 4285 had been observed to emit thin jets of energy from its core, visible in a range of wavelengths. In 1982, radio astronomers noted that the galaxy also emitted intense beams of microwave energy, known as masers. Ten years later, three Japanese radio astronomers discovered that the masers were moving rapidly when compared with the velocity of the galaxy. They theorized that a black hole might lie at the galaxy's center.

The three joined forces with four colleagues from the United States and last year used the National Radio Astronomy Observatory's Very Long Baseline Array, a set of radio telescopes that stretch from the Virgin Islands to Hawaii, to take another look. The VLBA was linked to yet another dish array in the New Mexican desert, giving the sharpest renderings yet of a disk 1.5 light-years in diameter spinning around the center of a galaxy 90,000 light-years across and 21 million light-years from Earth. The disk is extremely thin - only 2 percent of its diameter.

Based on the disk's rotation, the central object's mass worked out to be 36 million times that of the sun, says James Moran, an astronomer with the Harvard-Smithsonian Center for Astrophysics and one of the members of the international team. Yet the density worked out to roughly 100 million solar masses per cubic light-year, at least 10 times the density of any other galactic black-hole candidate.

Some astronomers suggest that such densities also could be achieved by clusters of stars rather than black holes. ``If you want to explain this as a star cluster that provides the mass that binds our disk, you need 100 million solar-like stars,'' Dr. Moran says. The distance between those stars, he adds, would have to be relatively small, roughly 100 times the sun-to-Pluto distance.

The evidence was strengthened by a second group that looked at the same galaxy using the ROSAT X-ray satellite.

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