February 1, 2003

Two Israeli astrophysicists may be able to explain the appearance of quasars, which are some of the brightest and most distant objects in the visible universe.

Professors Rennan Barkana of Tel Aviv University in Israel and Abraham Loeb of Harvard University in Cambridge, Massachusetts, claim they have found what could be the “fingerprints” of the quasar’s formation.

Barkana and Loeb’s work on quasars was published in the January 23 issue of Nature Magazine.

Quasars are extremely powerful forces: some quasars emit more energy than a thousand ordinary galaxies. Astronomers think that this comes from a violent process in which gas is sucked into a black hole that is more massive than a billion stars, located at the center of the galaxy that hosts the quasar.

There is now evidence that at least two galaxies, including our own, host a central supermassive black hole, and astronomers think this may be true of many or all other galaxies. It makes the prevailing view of quasars seem satisfyingly plausible. But there is a problem, and it lies in the age of the quasars.

Some quasars date back 12 billion years – to within a billion years of the Big Bang. That just doesn’t seem to leave long enough for the host galaxies of quasars to come together and gather their supermassive black holes from a cloud of primordial gas and dust.

The usual solution to this conundrum invokes invisible material called dark matter. Astronomers believe that the galaxies we can see are pervaded and surrounded by vast clouds of matter that we cannot see. No one knows what this matter consists of, but if it wasn’t there, rotating galaxies would fly apart. The gravitational pull of the dark matter keeps them together.

If quasars and their host galaxies assembled within surrounding haloes of dark matter, that would explain how they formed so quickly. Unfortunately there hasn’t been any evidence for these dark-matter haloes – until now.

Barkana and Loeb claim they have found what could be the fingerprint of dark-matter haloes in the light from quasars

As a quasar’s black hole sucks in gas from the surrounding space, say the researchers, it collides with the edge of its dark-matter halo and forms a shock wave. This impact heats the gas suddenly and breaks up its atoms to form electrically charged ions.

The infalling gas absorbs some of the light streaming from the quasar. But when it gets ionized, the gas becomes transparent. This sudden appearance of transparent gas at the shock boundary, Barkana and Loeb predict, breaks up the spectrum of light from the quasar such that it is split into two intensity peaks with different heights. Without ionization, the light intensity would rise and fall smoothly as the wavelength changes.

It’s still a tentative result, and other astronomers may demand more corroborating evidence before they are ready to embrace the idea.

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Jason Harris

Jason Harris

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