"That was the remarkable thing, that the [base of the] jet was quite broad when we compared it to what was happening farther out," said John Biretta, an astronomer at the Space Telescope Science Institute in Baltimore, Md. Biretta leads the team that conducted the observations. The team's findings are published in this week's issue of the journal Nature.

One of the greatest mysteries about black holes is how they produce the narrow cosmic jets that they are associated with. Black holes are so massive that not even light can escape their tremendous gravitational pull.

As matter falls into a black hole, it forms a swirling disk of material called an accretion disk. The dynamics of the black hole and its accretion disk are poorly understood, and it has always been unclear what role each played in producing cosmic jets.

"It seems pretty clear now that the accretion disk itself is very important," Biretta said.

"If the jet were being formed entirely by the black hole itself, we would expect the jet to be fairly narrow [at the base]," he said. "But in fact we see that it's very broad, so that's telling us that something bigger than the black hole is forming the jet. The obvious thing is the accretion disk that is around the black hole."

Biretta and his colleagues suggest that magnetic field lines that are anchored in the accretion disk drive the jet.

"As material spirals in the accretion disk toward the black hole, it is pulling in magnetic field lines with material," Biretta explained.

These field lines become highly concentrated at the center of the accretion disk, providing the energy and the pathway for charged particles to stream off in jets.

"What we think is happening is that particles move out along the field lines, because that's what is easiest for them to do," Biretta said. "As the field lines are rotating, these particles slide along the field lines and get accelerated by the twisting of the field lines," Biretta said. He compared the movement to that of a bead threaded on a twisted wire. If the wire started rotating extremely fast, the bead would tend to get shot down the wire, moving in some slinky-shaped orbit determined by the shape of the wire, he said. (See illustration at right.)

M87 is one of the nearest examples of a galaxy with an active jet and a central black hole, which makes it one of the best places to study extra-galactic black holes, Biretta said. Just 50 million light years away, M87 is right down the street in cosmological terms.

But it isn't close enough to allow astronomers to see its core easily.

The high-resolution images of M87's jet were made by coordinating the activities of radio telescopes across the United States and Europe.

"We kind of fake having a big telescope by having little parts of that big telescope," said Bill Junor, an astronomer at the University of New Mexico who worked closely with Biretta on the project.

By putting the small pieces of that telescope around the world and combining the signals from them, the collection of radio-antennas worked as a telescope as large as the Earth.

While the radio images are giving astronomers and theorists a better idea of how and where cosmic jets form, they are not shaking the foundations of black-hole research. In fact, the image of the jet's wide base is confirming many of the guesses that scientists have already made.

"It says, yes our ideas about making jets are not as crazy as we might think," Junor said. "We have a reasonable grasp on how this might be done."

Steven Eikenberry, an astronomer at Cornell University in Ithaca, N.Y. who studies black-hole phenomena, agreed with that assessment.

Due to the tremendous energy and the amount of material that seems to make up cosmic jets, many scientists have thought that jets must form over a broad region and then get funneled or focused into a narrower beam, he said.

The detailed image of the jet's base seems to prove that idea correct, Eikenberry said. "You're starting out with a fairly wide-angle jet right around the black hole itself. The material isn't beamed very tightly -- it's kind of spread out -- and then it narrows up later on."

Something similar happens in water faucets, Eikenberry explained, where water splashes from the orifice in a fairly broad, chaotic stream that narrows as the water flows downward.

Biretta and Junor suggest that magnetic focusing might be the force that forces the jet into a narrow column, but that is just one of several ideas that have been proposed.

"There are different models for the focusing mechanism, and it's not clear to me that this particular observation is going to tell us which it is," Eikenberry said. "However, this is very definitely the first step in the right direction, because before we've never seen the focusing,".

The next missing piece in the puzzle of M87's galactic jet is information about how the magnetic field is organized. Biretta and his colleagues have proposed to next take observations sensitive to the polarization of the radio waves.

"If we could tell which direction the radio waves were vibrating in, we could learn something about the magnetic field geometry. That would tell us something about the exact direction of the magnetic field lines and see if they're consistent with the picture we have developed," he said.

Members of the team also hope to get even-higher-resolution images of features near the black hole by making observations at higher radio frequencies.