Ablaze from afar: astronomers may have identified the most distant "blazar" yet

Natural History, Sept, 2004 by Charles Liu

Imagine standing on a hilltop on a foggy night, with a powerful flashlight in each hand. You point one flashlight forward and one backward, then turn them both on. If a friend is watching from far away, what would she see? It depends, of course, on what direction she's looking from. From either side, she would observe two cones of light--illuminated fog, really--shining from one spot, in opposite directions. Observing from behind or in front of you, though, she'd see a single bright source, aimed directly at her.

This example illustrates the quandary we astronomers face when we study the superenergetic systems known as quasi-stellar objects, or QSOs. According to current theory, all QSOs lie far outside our Milky Way and harbor at their center a supermassive black hole, several million to several billion times the mass of our Sun. All around the black hole are enormous swirling clouds of matter, which the black hole's great mass drags inward. The infalling matter liberates tremendous amounts of energy--often more in a few hours than the Sun will produce in its entire projected ten-billion-year existence.

Much of the energy gets channeled into two powerful, oppositely aimed jets of electromagnetic radiation and subatomic particles, plowing outward at nearly the speed of light. So depending on whether, from our vantage point here in the Milky Way, the jets of a QSO are head-on, sideways, or diagonal to our line of sight, we observe a single powerful beam, two expanding jets of glowing gas, or something in between. Viewing angles may thus account for the observed variety of QSOs. If so, each view--each kind of QSO--affords the chance to study a different aspect of supermassive black holes and their environs.

One member of the QSO menagerie is called a blazar, and it appears to be a QSO viewed right "down the barrel" of one of its jets. Now a research team led by Roger W. Romani of Stanford University has reported the discovery of the most distant blazar ever identified, some 13 billion light-years from Earth.

Regardless of the viewing geometry, all QSOs reside at the centers of distant galaxies. The closest QSOs are about a billion light-years from Earth. (Plenty of supermassive black holes lie closer by, but they and their environments are much less luminous.) The central energy source of a QSO is so bright and concentrated that, from our vantage, it drowns out the light of its host galaxy. That's why, in any typical picture of the sky, QSOs look like ordinary stars.

The resemblance creates a problem for astronomers. With millions of foreground stars for every QSO in the sky, identifying the latter can be harder than finding miniature black pearls in a barrel of peppercorns. The only way to be sure that an object is a QSO is to measure its full spectrum, and that can take a lot of telescope time. There aren't enough telescopes in the world to permit astronomers to measure the spectra of every starlike object in the sky, hoping to discover QSOs by chance. So astronomers have to find clever ways to improve the odds of finding these black-hole superengines.

One way is to search for electromagnetic radiation other than visible light. Powered by nuclear fusion at their cores, ordinary stars generally emit most of their energy as visible, ultraviolet, and infrared light in well-known output ratios, determined by their composition and temperature. QSOs, by contrast, are powered by gravity, not nuclear fusion. They emit copious quantities of X rays and radio waves, whereas typical stars produce only minute amounts. So QSO hunters often make X-ray images of large areas of sky, then match them up with radio and visible-light images. If an object shines brightly in all three pictures, it's a good bet that it's a QSO.

Romani and his colleagues added another dimension to this multiwave-length strategy--one particularly suited to identifying blazars. QSOs emit gamma rays, the most energetic type of electromagnetic radiation, near their centers, but this radiation seems to be directed largely along the jet. So if you happen to be staring head-on at a QSO jet--that is, when you're looking at a blazar--the gamma rays should be visible. To pinpoint likely blazars, Romani's team assembled gamma-ray data obtained with the Compton Gamma Ray Observatory, and compared them with X-ray, radio, and visible light data to find probable QSOs. Then, with the 9.2-meter Hobby-Eberly Telescope 450 miles west of Austin, Texas, they measured the spectra of the blazar candidates to confirm their identities.

The technique has enabled Romani and his colleagues to pinpoint a number of blazars, all billions of light-years from Earth. One of them, in an area of the sky off the end of the bowl of the Big Dipper, stood out. With a redshift of 5.47 it is so far away that when the light we now observe left the blazar, the universe was only 15 percent of its present size and "only" about a billion years old. The object thus affords astronomers an unprecedented view of a QSO jet early in cosmic history, and may illuminate how such jets affected the development of the universe.