Edition: U.S. / Global

Astronomers Say Proof of Black Hole Theory Is Almost Within Their Grasp

Published: January 17, 1992

Astronomers believe they are a measurement away from definitive proof of the existence of black holes, those central regions of some galaxies where matter would be so dense and the pull of gravity so strong that nothing, not even light, could escape from them.

Einstein's theory predicted black holes, and current astrophysical theory assumes that they exist, but the hunt for proof of their existence has been going on for two decades without clear success.

New observations reported here today at a meeting of the American Astronomical Society were seen as the strongest evidence so far of a supermassive black hole residing at the center of a galaxy, sucking in a whirlpool of surrounding stars. A Vast Black Hole

A team of astronomers led by Dr. Tod R. Lauer of the National Optical Astronomy Observatories in Tucson, Ariz., said images taken by the Hubble Space Telescope showed an unusual concentration of stars at the center of the galaxy M87. They appeared to be drawn there by the gravity of a black hole whose mass was estimated to be 2.6 billion times that of the Sun.

Scientists said this finding and other characteristics of the galaxy had made them 90 percent certain that they were observing the effects of an extraordinarily massive black hole.

Calling the results "very, very important," Dr. James E. Hesser, director of the Dominion Astrophysical Observatory in Victoria, British Columbia, said the images were "making more and more credible the concept of black holes."

Dr. Lauer emphasized that the images of especially dense starlight alone did not prove conclusively the black hole's presence. "It looks like a duck, but we haven't heard it quack yet," he said at a news conference.

The "quack" may be forthcoming, scientists said. Spectroscopic observations by the orbiting Hubble telescope in the next few months will measure the velocity of the stars orbiting within the galactic nucleus. High velocities should be conclusive evidence of a black hole and would provide astronomers with a direct measurement of its mass.

Officials of the Space Telescope Science Institute in Baltimore said they were concerned that the Hubble's flawed mirror might prevent them from making the detailed observations required for such confirmation. Plans to install corrective optics in the telescope, however, could yield the required velocity measurements within three years.

"Then the black hole should be undeniable," Dr. Lauer said.

Dr. Holland Ford, an astronomer at the space telescope institute, said: "In the minds of astronomers, there's no doubt that black holes do exist. What we really want to know is their mass." A Major Hubble Goal

Discovering the existence of black holes was a primary objective of the Hubble telescope, which was launched into an Earth orbit in April 1990.

The giant elliptical M87 galaxy is at the center of a cluster of galaxies in the constellation Virgo, 52 million light-years away. It is one of the brightest galaxies in the nearby universe, containing more than 100 billion stars, and is visible as a round fuzzy object even through the telescopes of amateur astronomers.

M87's nucleus has long been considered a strong candidate for a black hole, as hypothesized in Einstein's theory of general relativity. Early in this century astronomers discovered a plume, or jet, of hot gases issuing from the galaxy's core.

Dr. F. Duccio Macchetto, an astronomer at the space telescope institute, said no one had ever explained those jets -- seen in many galaxies -- in any way other than as a result of a black hole, creating the temperatures and pressures that cause these eruptions of electrically charged gases, or plasma.

"If you have jets, you have to have a black hole," Dr. Macchetto said, explaining current theory.

Other observations in recent years have detected strong bursts of radio and X-ray radiation from M87's nucleus and jet. These radiations tell of cataclysmic forces roiling the galactic center, perhaps caused by a black hole's tremendous gravitational appetite for surrounding stars and gases.

On the basis of this evidence, scientists at the California Institute of Technology postulated in 1978 that the central part of M87 appeared to be dominated by the gravity of a black hole. But subsequent ground-based telescopic studies and theoretical research failed to confirm this conclusion.

By getting a clearer image of starlight at the center of M87, Dr. Lauer said, the Hubble telescope showed clearly that the intensity of light rises sharply at the nucleus. If the core was merely inhabited by a concentration of stars, without a black hole, the intensity of light should have risen and then leveled off as a kind of plateau, not a sharp peak, he said. Density Beyond Predictions

Dr. Lauer reported that the density of stars at the core was at least 300 times greater than expected for a normal elliptical galaxy, and more than a thousand times denser than the distribution of stars in the neighborhood of the Sun. The density may be even greater, he said, but its measurement is beyond the power of the Hubble telescope.

The black hole itself, by definition, cannot be seen. What is being observed presumably is the congregation of stars orbiting the galactic core and being pulled toward the center by the black hole's relentless gravity. Many of the stars are destined to be consumed by the black hole, fueling its further growth and power.

The estimated mass of the apparent black hole, Dr. Lauer said, is sufficient to power a quasar, a brilliant object that looks like a star but is believed to be caused by powerful radiations coming from centers of galaxies. Since there is no detectable quasar associated with M87, he said, this was the "first good case near us of a dead quasar."

Working with Dr. Lauer in analyzing the Hubble images were Dr. Sandra M. Faber of the University of California at Santa Cruz and Dr. C. Roger Lynds of the National Optical Astronomy Observatories. The images were taken by the Hubble's wide field-planetary camera.