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Search for `dead' quasars may strengthen case for black holes

Evidence of the existence of black holes -- enigmas that tantalize scientists trying to unravel the mysteries of the universe -- has been strengthened by a recent survey of nearby galaxies. The case for these exotic objects, thought to be the super dense remains of collapsed stars, remains circumstantial. But new evidence for ``dead'' quasars at the center of a number of spiral galaxies supports the current contention that quasars -- the brightest burning objects in the known universe -- are directly related to black holes and, in fact, get their staggering power from them.

Proving the existence of black holes would be a boon to scientists, giving them a potential testing ground for physical laws, including Einstein's theories. These objects also may play a major role in the still poorly understood process of galaxy formation.

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The survey, reported in the current issue of the Astrophysical Journal Supplement Series, was conducted by California Institute of Technology graduate student Alexei Filippenko and astronomy professor Wallace L. W. Sargent.

Conducted on the 200-inch Mount Palomar telescope, it was the first major effort to identify quasar-like light signals in the nuclei of normal galaxies.

Quasars are featureless points of light visible over billions of light years. They consume so much power that the most plausible explanation today is that their energies come from gigantic black holes, millions, even billions of times the mass of the Sun.

Black holes are among the strangest objects yet conceived. When massive stars run out of nuclear fuel, they explode. This explosion is thought to leave behind a core collapsed to the size of a planet, with gravity so strong that not even light can escape.

Put such an object in a cloud of gas, and it should act like a celestial vacuum cleaner. As the gas is pulled into the black hole, it would be violently heated and compressed. In the process, this gas should emit light with the spectral fingerprint seen in quasars, theoretical calculations confirm.

Because light travels at a fixed velocity, the more distant an astronomical object the older it is. So the fact that more quasars are found billions of light years away than at closer distances suggests they were far more abundant in the early epochs of the universe.

``This is the main reason that people think there should be `dead' quasars around,'' explains Dr. Sargent. By dead quasar he essentially means a massive black hole that no longer has the quantities of gas near it to produce the brightness of an active quasar.

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This possibility has prompted a number of astronomers to inspect galactic centers for the spectral fingerprint of a quasar. Several positive results have been reported.

Sargent and Mr. Filippenko examined the nuclei of 75 nearby galaxies (within 50 million light years). They say they found faint evidence of quasar-like emissions from at least 19 and perhaps as many as 28 of them. The reason for the range is because this signal is very difficult to distinguish from ordinary starlight. -- 30 --{et