Strands of the Stone Age.

Most quasars that astronomers detect lie billions of light-years from Earth. But scientists have now confirmed that one of these brilliant beacons resides at the heart of Cygnus A, a galaxy just 600 million light-years away.

Finding a quasar in the Milky Way's backyard should enable scientists to study in unprecedented detail a powerhouse that spews hundreds of times more energy than a typical galaxy, says Anne Kinney of the Space Telescope Science Institute in Baltimore. She and her colleagues analyzed the spectra of ultraviolet light recorded by the Hubble Space Telescope to confirm that a quasar lies at the center of Cygnus A, the brightest radio galaxy in the northern sky. Kinney and her coworkers, Robert Antonucci and Todd Hurt of the University of California, Santa Barbara, report their work in the Sept. 22 NATURE.

Astronomers have long suspected that most radio galaxies, though varying widely in appearance, have more in common than meets the eye. They theorize that these galaxies harbor a quasar shrouded by a doughnut-shaped cloud of gas and dust. Depending on the galaxy's orientation in the sky, observers on Earth either view it edge on, through the dust in the doughnut, or face on, a clear view through the doughnut's hole.

Those radio galaxies seen edge on, including Cygnus A, appear to emit only narrow bands of visible light. But seen face on, radio galaxies usually sport the brilliant, broadband emissions typical of gas swirling rapidly around a quasar.

Observations over the past several years support this scenario. In studying certain edge-on radio galaxies, astronomers have found that they can peek indirectly into the core. That's because material -- often electrons -- sitting above the hole in the dusty doughnut acts as a mirror, reflecting some of the light emitted by the naked core into Earth's line of sight. This light has the broadband spectra characteristic of a quasar. But the spectra of visible light reflected from the core of Cygnus A have no such pattern, casting doubt on whether the galaxy truly harbors a quasar.

Rather than look in visible light, Kinney and her colleagues examined Cygnus A in the ultraviolet, using Hubble's faint-object spectrograph. They were searching for massive stars, but instead they found that some of the spectra showed the broadband emissions typical of a quasar.

Why are the telltale emissions seen in ultraviolet light but not in visible light? Kinney speculates that the material reflecting radiation from the galaxy's core is dust, not electrons. Dust preferentially reflects shorter-wavelength radiation, such as ultraviolet light, toward an observer.

The new findings, Kinney says, complement an older study by other researchers, who partially penetrated Cygnus A's dusty shroud by observing the galaxy in the infrared. Dust doesn't absorb as much infrared radiation as it does visible light, which enabled the astronomers to get a more detailed view of the galaxy's core. The infrared images showed a bright, point-like nucleus highly suggestive of a hidden quasar (SN: 6/1/91, p.343).

Together, says Kinney, results from the two studies clinch the argument over whether the Milky Way has a quasar for a neighbor.