X-Ray Universe: Quasar’s jet goes the distance

Photons left over from the birth of the universe appear to have helped generate the longest X ray-emitting jet ever found in a distant galaxy.

X-RAY REVELATIONS.Illustration shows that en route to Earth, X rays from the quasar PKS 1127-145 pass through a galaxy 4 billion light-years away. Inset: X-ray image shows that an enormous jet emanates from the quasar. Illustration: CXC/M. Weiss Image: NASA/CXC/Siemiginowska, Bechtold

Discovered by NASA’s Chandra X-ray Observatory, the jet shoots at least 1 million light-years into space, about 10 times the diameter of the galaxy in which the jet originates.

The jet emanates from a quasar–a brilliant source of light believed to be powered by a supermassive black hole–at the galaxy’s core. Composed of charged particles, the jet broadcasts faint radio waves in addition to the brilliant X-ray emissions.

The jet’s great length makes it unlikely that the quasar could provide the energy for the X rays, notes Aneta Siemiginowska of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. The relative intensity of the X-ray and radio emissions, as well as differences in their distribution within the jet, also casts doubt on several other possible energy sources, she adds.

Instead, Siemiginowska and her colleagues say the X rays are most likely generated by collisions between charged particles in the jet and photons in the cosmic microwave background (CMB)–the radiation left over from the Big Bang, which permeates all of space. When photons in the CMB collide with electrons in the jet, the electrons get a boost in energy and emit

X rays, suggest Siemiginowska, Jill Bechtold of the University of Arizona in Tucson, and their colleagues in the May 10 Astrophysical Journal.

The quasar lies some 10 billion light-years from Earth. That means the X rays that Chandra now records left the distant galaxy when the expanding universe was relatively young and compact. In a denser universe, collisions between charged particles and the CMB were more likely than they are today.

Interactions with the CMB could account for the 20 or so X-ray jets so far spied by Chandra, says Richard Mushotzky of NASA’s Goddard Space Flight Center in Greenbelt, Md. Only three were known before Chandra’s launch in 1999.

In addition to suggesting a novel role for the CMB, which astronomers usually study to glean information about the earliest moments of the universe, data about the jet may provide new clues about the formation of quasars and the black holes that fuel them, says Siemiginowska. For instance, the jet’s length indicates that the quasar has been active for 1 million years.

Bechtold and Siemiginowska found the jet by accident while using Chandra to study the X-ray spectrum of the quasar, known as PKS 1127-145.

The spectrum is itself intriguing: It reveals that en route to Earth, light from the quasar passed through an intervening galaxy. Since specific atoms absorb specific X-ray wavelengths, the spectrum carries information about the galaxy’s composition. Moreover,

X rays provide a highly accurate probe of galactic chemistry because atoms absorb this penetrating radiation regardless of whether they are ionized, attached to dust, or incorporated into molecules, says Mushotzky.

Because the intervening galaxy lies so far away, the quasar light passed through it 4 billion years ago. The spectrum shows that the galaxy has only about 20 percent as much oxygen as our solar system has today. The researchers described their study in the Nov. 20, 2001 Astrophysical Journal.

The galaxy could be a dwarf, which is a galaxy type known to have a lower abundance of heavy elements than do larger galaxies like the Milky Way, notes Bechtold. On the other hand, the oxygen shortage could be an indication of how the chemical composition of galaxies evolves over time.

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