Ablaze from Afar

By Liu, Charles | Natural History, September 2004 | Go to article overview

Ablaze from Afar


Liu, Charles, Natural History


Astronomers may have identified the most distant "blazar" yet.

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. …

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