The Johns Hopkins Gazette: March 19, 2001
March 19, 2001
VOL. 30, NO. 26


Long Search Ends: Astronomers Find Missing Type of Quasar

By Michael Purdy
Johns Hopkins Gazette Online Edition

Astronomy's "most wanted" list shortened by one last week when researchers announced that they had spotted the first type II quasar, an object on the fringes of the known universe whose existence they had suspected for two decades.

The finding is an important step forward in efforts to understand how black holes and galaxies formed in the early universe, according to astronomers at the Krieger School of Arts and Sciences, the Space Telescope Science Institute and the European Southern Observatory.

Researchers anticipate that many more such quasars will soon be found through a special set of observational data that NASA released to the general astronomy community last week. The Chandra Deep-Field South Exposure, taken through NASA's orbiting Chandra X-ray Observatory, covers a patch of sky about the size of the moon and reveals sources of X-ray radiation in the very distant universe in unprecedented detail.

Like a conventional quasar, a type II quasar is an extremely "bright" source of X-ray and other radiation located in the most distant regions of the universe. Astronomers think both types of quasars are powered by massive black holes. Unlike a type I quasar, though, a type II quasar is surrounded by or buried in an occluding cloud of gas and dust that reduces its brightness in visible light. The cloud, though, has little effect on X-ray radiation from the quasar.

"The thinking is that these may be quasars in the early stages of their evolution," said Colin Norman, professor of physics and astronomy at Johns Hopkins. "As the quasar matures, the extremely powerful radiation it emits disperses the shroud of gas and dust."

Astronomers first postulated the existence of type II quasars in the early 1980s, when they were beginning to agree on a unified model of quasars and other distant, bright objects thought to be powered by massive black holes.

To detect the type II quasar, a team that included Norman; Riccardo Giacconi, research professor at Hopkins and president of Associated Universities; former Hopkins postgraduate fellow Piero Rosati; and several other researchers compared data from the Deep-Field South Exposure with optical observations from the Very Large Telescope in Chile.

The Very Large Telescope is one of the most powerful ground-based telescopes, and is operated by the European Southern Observatory, where Rosati is now a program scientist.

"We found just what the theory was predicting--an object that has very narrow emission lines in the optical spectrum readings from the VLT but is giving off very hot X-rays in the readings from Chandra," Rosati said.

Norman and Rosati both expressed eagerness to get a better sense of the prevalence of type II quasars as compared to type I quasars.

"This may just be the tip of the iceberg," said Rosati, who made many of the arrangements for observations from the VLT and Chandra.

Light from the new type II quasar, located in the Southern Hemisphere constellation of Fornax, has traveled approximately 9 billion years to reach the Earth, meaning the activity that researchers detected took place when the universe was approximately one-tenth of its current age. They hope to be able to study their type II quasar and others in a few years with the Space Infrared Telescope Facility, a new orbital NASA observatory to be launched at the end of next year. SIRTF may help them learn more about the contents of the clouds surrounding the quasar.

Norman and Giacconi were on a panel of astronomers commenting on the release of the Chandra Deep-Field South at last week's NASA Space Science Update in Washington, D.C.

For Giacconi, the release of the new data concludes a decades-long process of transforming a distant, diffuse X-ray glow that he first spotted nearly 40 years ago into millions of pinpoints of distant X-ray light.

"Our ability to image distant sources of X-ray radiation has improved enormously since I first detected the X-ray background radiation in 1962," Giacconi said. "The background is now resolved in individual sources which are mostly active galactic nuclei and quasars. The identification of a type II quasar is just the first of many exciting discoveries that I am sure will come from this data."

Giacconi stressed that the exposure, taken over 1 million seconds, is the deepest ever obtained.

"It will allow us to study X-ray emissions from low luminosity galaxies and groups of galaxies at early times in their formation," Giacconi said. "The gas surrounding these objects is thought to contain a large fraction--perhaps most--of the normal matter in the universe."

Both the Deep-Field exposure and a paper on the identification of the Type II quasar have been submitted to The Astrophysical Journal. Additional authors on the quasar paper are Guenther Hasinger, Roberto Gilli, Roberto Gilmozzi, Lisa Kewley, Mario Nonino, Gyula Szokoly, Paolo Tozzi, Junxian Wang, Wei Zheng, Andrew Zirm, Jacqueline Bergeron, Norman Grogin, Anton Koekemoer and Ethan Schreier.