Monster Black Holes

By Cowen, Ron | Science News, October 31, 1992 | Go to article overview

Monster Black Holes


Cowen, Ron, Science News


Armed with ultrasensitive light detectors and state-of-the-art telescopes, astronomers during the past decade have peered deeper than ever before into the center of galaxies. And there they've found more than just stars. Some galaxy cores rank among the most brilliant and energetic entities in the universe. Spewing out jets of radiation that extend for thousands of light-years, the cores of active galaxies can accelerate particles to energies dwarfing those of the biggest accelerator on Earth.

Powerful cores require powerful energy sources. Many astronomers now believe that compact, supermassive black holes may fuel the fireworks at the center of some galaxies. Supermassive black holes are dense, collapsed objects--millions to billions of times the mass of the sun - with a gravitational tug so strong that not even light can escape their clutches. "The only way we know how to put such a big mass in a very small region is to make it a black hole," says Alan Dressler of the Observatories of the Carnegie Institution in Pasadena, Calif.

But while black holes have captured the imagination of scientists and the public alike, hunting them remains a challenge. By definition, a black hole can't be seen. This means its presence must be detected through indirect evidence. So, like an invisible houseguest who makes his presence known by the amount of food he eats, a black hole reveals its existence by the way it influences its surroundings.

Yet the mounting flurry of supporting evidence for galactic black holes can leave the average astronomy buff bewildered. Reports earlier this year about black hole candidates - many based on observations with the Hubble Space Telescope - beg some fundamental questions: What are the criteria for finding a supermassive black hole, and what constitues the nearest thing to absolute proof?

Just the place for a snark, I have said it twice, That alone should encourage the crew. Just the place for a snark, I have said it thrice, And what I tell you three times is true. -"The Hunting of the Snark"

Just as the hunters of Lewis Carroll's mythical beast had their own standards for deciding when they had found the creature, astronomers have developed some basic rules for determining when they have snared the black hole they seek. To house a giant black hole, a galactic core must meet two requirements. It must exhibit a telltale increase in the intensity of starlight from its edge to its center. And the stars surrounding the core must orbit so rapidly that the tug from visible matter alone can't account for their speed.

While searching for bright nuclei may appear easier than measuring star velocities, the tests must go hand in hand to prove the existence of a galactic black hole, says John Kormendy of the University of Hawaii in Honolulu. Adds Dressler: "Most astronomers I know are not saying any black hole is absolutely proven until the kinematic data - the motions of stars - complement studies of light intensity."

Astronomers often trace the hunt for galactic black holes to 1978, when the late astronomer Peter Young and his colleague Wallace L. W. Sargent of the California Institute of Technology in Pasadena published two seminal papers on the giant elliptical galaxy M87. The researchers suggested that an unexpected increase in the intensity of light toward the galaxy's center might signify a collection of densely packed stars orbiting a black hole. They also asserted that as higher-resolution telescopes became available, these instruments would find that the concentration of starlight closer and closer to the galaxy's center would continue to rise steeply, according to a formula that describes the density of stars surrounding a black hole.

The research articles, recalls Kormendy, drew criticism from many astronomers. But the studies also sparked fresh interest in the black hole scenario.

In their analysis, Young and Sargent had assumed that near the core of M87, located some 52 million light-years from Earth, stars move with about the same average velocity in all directions. …

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