Probing the heart of extragalactic jets.

Astronomers have come a little closer to finding the origin of the twin jets that shoot out from the brightest objects in the cosmos. Some 10 percent of quasars and active galaxies emit such jets, which extend in opposite directions up to thousands of light-years.

By studying the radio emissions from the jets squirted out by the distant quasar 3C279, researchers have gathered the first compelling evidence that the jets consist mainly of electrons and their antiparticles, positrons. This composition hints at the processes taking place deep within the hearts of active galaxies, where the jets are produced.

Any bright galactic core not powered by starlight is known as an active galactic nucleus. The most luminous of these are called quasars, and they are thought to be fueled by massive black holes. The new observations point to interactions between subatomic particles just outside a black hole as the source of the jets.

John F.C. Wardle and his colleagues at Brandeis University in Waltham, Mass., used the Very Long Baseline Array, a network of 10 radio telescopes, to examine the radio waves emitted by the jets in 3C279. They measured the polarization, the direction in which the electric field of the radio waves oscillates.

In the Oct. 1 Nature, Wardle's team reports finding circularly polarized radiation from the jets, meaning that the angle at which the electric field oscillates is not fixed but rotates around the wave's direction of travel. Earlier observations indicated that most of the radio emission comes from high-speed electrons accelerated by a magnetic field. Scientists had also determined that there must be roughly equal numbers of positive and negative charges in the jets, but they didn't know whether the positive charges were protons or positrons.

The pattern of polarization found by Wardle's team indicates that the electrons' positively charged companions are mostly positrons. The team has found similar results in jets from three other active galactic nuclei.

There are two ways to make beams of positrons and electrons, says Thomas W. Jones of the University of Minnesota in Minneapolis. In one scenario, material falling toward the black hole generates high-energy photons, which collide to produce pairs of electrons and positrons. These particles in turn radiate more photons, which create more electron-positron pairs. One drawback to this story line, says Jones, is that if the radiation generated by infalling material is too intense, it may act as a brake on the electrons and positrons, preventing them from attaining the high speeds observed in the jets.

A more likely possibility, he notes, is that energetic protons, too massive to be slowed by radiation, collide with either each other or photons. These reactions generate subatomic particles called pions, which can decay into electrons and positrons. The reaction also generates a small number of protons, consistent with the observation that most but not all the positively charged particles are positrons.