Scientists Cast Light on Quasars’ Brilliance

One of the most baffling mysteries in astronomy may have been solved with new evidence of where quasars, the most brilliant objects in the universe, get their fuel.

Scientists told the American Astronomical Society in Pasadena on Thursday that they have “extremely conclusive” evidence that gas and dust from colliding galaxies fuel at least some--and probably all--of the starlike objects that shine hundreds of times more brightly than the billions of stars in the Milky Way.

David Sanders of Caltech, who presented the findings to the largest gathering of astronomers ever held in North America, said the evidence came from studies of 10 celestial bodies that are evolving into quasars. He acknowledged that other astronomers will want further evidence, but he said he is convinced that his team has solved the riddle.

Although the field of study includes only 10 celestial objects, “10 out of 10” support the evidence, he said.

Since the first quasar was discovered 25 years ago, astronomers have puzzled over how virtual pinpoints in the universe could shine so brightly that they are more luminous than entire galaxies. The evidence reported Thursday supports the widely accepted theory that colliding galaxies somehow become supercharged, with much of the energy concentrated near the center of the galaxies in a dazzling display of luminosity.

Sanders’ team sort of came in through the back door in an effort to prove the theory.

Light and Heat

Quasars radiate most of their energy in visible light, so they can be seen by astronomers using optical telescopes powerful enough to see billions of light years away. In 1985, astronomers studying data sent back by an orbiting infrared observatory, the Infrared Astronomical Satellite, discovered several objects that are as energetic as quasars but release most of their energy as infrared radiation, or heat, rather than as visible light.

“Here were infrared objects as luminous as quasars,” Sanders said at a press conference at the Pasadena Center.

The discovery intrigued scientists at Caltech, who thought that the similarity had to be more than coincidence. A team organized to study the objects included Sanders, B. Thomas Soifer, Gerry Neugebauer, Nicholas Z. Scoville, Jonathan Elias, Barry Madore, Keith Matthews and Anneila Sargent, all of Caltech, and Judith Young of the University of Massachusetts.

Using the 200-inch Hale telescope, the scientists visually examined 10 of the highly energetic bodies, as well as the surrounding sky. They found that all 10 bear the astronomical signature of galaxies and were in an area that is literally crowded with quasars.

Then, using three of Caltech’s radio telescopes in the Owens Valley that are especially sensitive to molecular gas, the astronomers determined that the galaxies are extremely rich in dust and gas.

Further analysis showed that each object is not just one galaxy but two spiral galaxies that collided long ago. Since the galaxies were spiraling in opposite directions, the gravitational tug of war sent violent storms of intergalactic gas and dust streaming toward the center of the two galaxies.

That left the galactic center hidden by dust, but the scientists concluded that they were seeing the birth of a quasar whose dynamics were being revealed by the very dust that hid it from view.

As the gas and dust plunge into the center, they are energized by whatever lies at the heart of every quasar--generally believed to be a black hole. The resulting firestorm releases the stored energy of the dust in the form of heat, or infrared radiation, that can be read by astronomers on a distant planet as the object radiates like a hundred billion stars.

But, Sanders said, the dust and gas that give it enormous energy also keep it from being a quasar because they absorb the visible light that is part of the signature of a quasar. In time, he said, the dust that is not captured by the quasar’s “engine” will be blown away by radiating energy.

“We call that house cleaning,” he said.

That would remove the source of the infrared radiation, leaving a quasar burning brilliantly--and clearly visible--just as a quasar is supposed to do, Sanders said.

The evidence for this also supports another part of the theory of quasars, he added.

About 3,600 quasars have been discovered; most are on the outer fringes of the universe, so they originated when the universe was very young. The universe was also much more compact then, according to the generally accepted idea of an expanding universe, meaning that the galaxies were much closer together.

Since they were closer, it was easier for them to bump into each other, Sanders said. Thus most quasars should be found in the most distant past, and that has turned out to be the case.