Artist's impression of a quasar, a super-heated disc of material spiralling into a super-massive black hole.

Credit: NASA

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SAN FRANCISCO: For the first time ever, astronomers have looked inside quasars - the brightest objects in the universe - and have seen evidence of black holes, U.S. researchers say.

Their study lends further confirmation to what scientists have long suspected - that quasars are made up of super-massive black holes and the super-heated discs of material that are spiralling into them. The results were reported this week at the meeting of the American Astronomical Society’s High Energy Astrophysics Division in San Francisco.

"There are many models that try to describe what's happening inside a quasar, and before, none of them could be ruled out. Now some of them can," said co-author Xinyu Dai, from Ohio State University. "We can begin to make more precise models of quasars, and gain a more complete view of black holes."

Seen from Earth, quasars, or quasi-stellar objects, look like stars. They are extremely bright, which is why we can see them even though they are among the most distant objects in the universe. Astronomers puzzled over quasars for decades before deciding that they most likely contain super-massive black holes that formed billions of years ago.

Black holes cannot be directly observed, because they are so massive that even light cannot escape their gravity. The material that is falling into a black hole, on the other hand, glows brightly. In the case of quasars, the material shines across a broad range of energies, including visible light, radio waves, and X-rays.

Along with their colleagues, Dai and co-author Christopher Kochanek, professor of astronomy at Ohio State University, studied the light emanating from two quasars.

Quasars are so far away that even in the most advanced telescopes, they look like a tiny pinpoint of light. The interior structures of the two quasars in this study only became visible when a galaxy happened to line up between them and the Earth, and magnified their light like a lens.

The astronomers likened the effect to being able to look at the quasars under a microscope.

Einstein predicted that massive objects in space can sometimes act like lenses, bending and magnifying light from objects that are behind them, as seen by an observer. The effect is called gravitational lensing, and it enables astronomers to study some objects in otherwise unattainable detail (see Cosmos, Issue 5, p34).

"Luckily for us, sometimes stars and galaxies act as very high-resolution telescopes," Kochanek said. "Now we're not just looking at a quasar, we're probing the very inside of a quasar and getting down to where the black hole is."

They were able to measure the size of the so-called accretion disc around the black hole inside each quasar.

In each, the disc surrounded a smaller area that was emitting X-rays, as if the disc material was being heated up as it fell into the black hole in the centre.

That's what they expected to see, given current notions about quasars. But the inside view will help them begin to refine those notions, Dai said.

Key to the project was U.S. space agency NASA's Chandra X-Ray Observatory, which allowed them to precisely measure the brightness of the X-ray emitting region of each quasar. They coupled those measurements to ones from optical telescopes that belong to the Small and Moderate Aperture Research Telescope System Consortium.

The astronomers studied the variability of both the X-rays and visible light coming from the quasars and compared those measurements to calculate the size of the accretion disc in each.

They used a computer program that Kochanek created especially for such calculations, and ran it on a 48-processor computer cluster. Calculations for each quasar took about a week to complete.

The two quasars they studied are named RXJ1131-1231 and Q2237+0305, and there's nothing special about them, Kochanek said, except that they were both gravitationally lensed. He and his group are currently studying 20 such lensed quasars, and they'd like to eventually gather X-ray data on all of them.