Jet Set: Astronomers identify the makeup of quasar streams

The particle jets streaming from the neighborhood of a supermassive black hole punch their way out of their home galaxies and extend hundreds of thousands of light-years beyond. Astronomers this week reported that they have finally identified the particles in the jets as electrons and protons and have found that those streams of particles carry much more energy than some astronomers had theorized.

The composition of jets has been debated ever since they were detected in the 1950s. The jets were known to be electrically neutral, but astronomers weren’t sure what they were made of. One theory held that they contained mainly electrons and their antimatter partners, positrons. The other theory was that the jets were made of electrons and protons. Protons are about 1,800 times as heavy as positrons.

“The question is very important,” says theorist Roger Blandford of Stanford University, because “jets carry energy and momentum from the vicinity of the black hole out into intergalactic space.” By redistributing matter and energy, the jets enable black holes to wield influence beyond their own galaxies.

Recent studies had hinted that jets might be primarily electrons and protons, but astronomers lacked solid evidence. Determining the composition required observations of high-energy X rays, which no telescope had clearly recorded.

Rita Sambruna of NASA’s Goddard Space Flight Center in Greenbelt, Md., and her colleagues used NASA’s recently launched Swift satellite, which is exquisitely sensitive to energetic X rays, to examine jets emanating from two quasars more than 10 billion light-years from Earth.

Quasars are the powerful beacons of light at the cores of galaxies. They’re powered by black holes that have crushed the equivalent of millions to billions of suns into a region about the size of the solar system. Most quasars have particle jets, which seem to arise from a disk of matter that swirls around a supermassive black hole.

“Thanks to the spectra from Swift, we were able to see all the [elementary] particles in the jets,” Sambruna says. Her team found that the X rays peaked at an energy of 10,000 electronvolts. Using that information, the team calculated the energy carried by particles in the jets.

Computer models developed by Fabrizio Tavecchio and Gabriele Ghisellini at the Merate Observatory in Italy indicate that jets composed of electrons and positrons would not contain as much energy as the X rays indicated. In fact, such jets would fizzle near the black hole instead of streaming into space. Therefore, the jets are probably made of electrons and protons, Sambruna reported Oct. 5 at a meeting of the American Astronomical Society in San Francisco.

“What Rita [Sambruna] is saying is that in her jets, she needs more power. … A protonic jet will carry more power,” comments Blandford. He adds that the jets probably also include pairs of positrons and electrons, as well as photons. “Undoubtedly, all are involved [in carrying energy], and there is probably a transformation from one to the other along the jet,” he says.

Sambruna and her colleagues calculate that each jet moves at 99.9 percent of the speed of light and carries as much mass as that of Jupiter.