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Record-breaking brightness poses enigma.

Two months ago, British astronomers reported finding the brightest object so far observed in the universe -- a quasar called BR 1202-07. Now, a British-U.S. team has announced the serendipitous discovery of an object that surpasses the quasar's brightness by 40 percent, glowing with 30,000 times the luminosity of the entire Milky Way.

Unlike the previous record-breaker, which radiates strongly in visible light (SN: 5/4/91, p.276), the new title holder emits 99 percent of its energy in the infrared. While the researchers believe the emission comes from a hot, massive dust cloud, the source heating the cloud remains unclear. In the June 27 NATURE, co-discoverer Michael Rowan-Robinson of Queen Mary and Westfield College in London and his colleagues suggest two possibilities for the enigmatic heat source: a quasar buried at the cloud's core, or a primal burst of star formation in the cloud.

Either way, they may have witnessed the birth of a key celestial structure. On the one hand, Rowan-Robinson reasons, a quasar locked within a dust cloud could not remain there for more than a million years -- a short interval, astronomically speaking -- because its radiation pressure would blow away the cloud. Thus, if a dust-shrouded quasar induced the radiation, the quasar must have just switched on. On the other hand, if the radiation stems from star formation, then the cloud likely represents a galaxy undergoing its first stellar ignition, he says. "Whether quasar or galaxy, it's a key moment we're catching," Rowan-Robinson says.

The researchers made their fortuitous finding while identifying visible-light counterparts to faint infrared sources detected in 1983 by the Infrared Astronomical Satellite (IRAS). Two years ago, on the last night of a survey with the William Herschel 4.2-meter telescope in La Palma, Spain, they had their sights trained on an object thought to be associated with one of the IRAS sources. By chance, the image of another celestial body fell on the slit of their spectrograph.

In analyzing its spectra, the team deduced that the unexpected find has a redshift of 2.286 -- meaning it lies billions of light-years from Earth and must have an inherently high luminosity to be detected at all. Subsequent observations revealed that the visible-light image coincides with that of a faint radio source in the constellation Ursa Major. Both lie at the center of the infrared-emitting region found by IRAS, Rowan-Robinson says.

He calls a hidden quasar "the safer bet" to explain the infrared emission, since quasars have high luminosities. However, the team did not find a broadening of certain spectral lines emitted by the cloud, which they would expect to see if the cloud harbored a quasar. Moreover, the mass of the dust cloud may support the galaxy hypothesis, Rowan-Robinson says. The cloud is composed mostly of metals -- which are usually spewed out by stars -- and its mass may be as great as l billion solar masses, exceeding the metal mass of a typical galaxy. This would fit with a d budding galaxy rapidly undergoing its first, most brilliant phase of star formation -- a phenomenon astronomers have long sought to witness.

A finding that starbirth can proceed so rapidly, notes Rowan-Robinson, could overturn standard models, which hold that galaxies assemble gradually. But first, he says, researchers need stronger evidence that the cloud represents a new galaxy, and "we need other examples to convince outselves that this is not some absolute freak."
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Title Annotation:discovery of the brightest object ever observed in the universe
Author:Cowen, Ron
Publication:Science News
Date:Jun 29, 1991
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