Call it the case of the noisy neighbor.
Since the nearby galaxy Cygnus A was first discovered in the 1960s, scientists have puzzled over how it churns out tremendous amounts of radio energy, making it the second strongest source of radio waves in the cosmos.
Now three astronomers using the Hubble Space Telescope have stumbled onto evidence that a quasar -- a mysterious object that can emit a trillion times as much energy as the sun -- nestles at Cygnus A's core, broadcasting all that radio babble.
That would make Cygnus A's quasars one of the closest quasars ever discovered, a mere 600 million light years away. Most quasars are billions of light years distant.
"I was stunned when we realized we had a quasar, it was a total surprise," said Anne L. Kinney, an astronomer with Baltimore's Space Telescope Science Institute. While some theoreticians had predicted that the galaxy contained a quasar, she said, "it wasn't the point of our observation."
"That shows how many surprises there are in our own backyard of the universe," said Stephen P. Maran, a spokesman for the American Astronomical Society.
Dr. Kinney, Robert Antonucci and Toddy Hurt, both of the University of California at Santa Barbara, published their findings in this week's issue of the British journal Nature.
Quasars look like single stars to astronomers using optical ground-based telescopes. But when measured by instruments that detect forms of energy other than visible light, they appear hundreds or thousands of times brighter than our Milky Way galaxy, which may contain 100 million stars.
Astronomers think quasars may be a jumbo version of an object called a black hole, a collapsed star so heavy and compact that its crushing gravity prevents light from escaping.
There is growing suspicion that quasars sit at the center of many or even most galaxies.
Using Hubble before its repair last December, Dr. Kinney and her colleagues decided to study Cygnus A for evidence that the galaxy was powered by a dense region of hot, massive stars in the processing of exploding, not a quasar.
Instead, in looking at ultraviolet light reflected off the galaxy's center, they found evidence that the material in the core is swirling at tremendous speeds -- speeds that could only be generated by material accelerating into a black hole.
The team spent three months puzzling over their data before coming to the conclusion that they had stumbled on evidence of a quasar.
Dr. Kinney hopes the new evidence will help astronomers learn why about one out of ten quasars produces as much radio noise as Cygnus A, while the rest are quiet.