Photon drag: new spin on making a black hole.

Some galaxies have a heart of fire, a center so luminous that it outshines the rest of the starlit body Most astronomers believe that a black hole fuels the fireworks at the core of such galaxies, known as active galactic nuclei. But astrophysicists are uncertain how a massive black hole -- an object that represents the extreme of gravitational collapse - could form, especially so early in the history of the universe.

Though black holes may be exotic, one of the puzzles in understanding their creation lies in ordinary physics, notes Abraham Loeb of the Institute for Advanced Study in Princeton, N.J. Early in the universe, random fluctuations in the density of matter may have prompted some huge gas clouds to begin collapsing. But long before becoming a black hole, a cloud's own rotation, or angular momentum, would halt the process. Just as Earth's rotation provides a centrifugal force that prevents our planet from falling into the sun, the swirling motion of the cloud prohibits complete collapse.

In order to form a black hole, the cloud must lose much of its angular momentum. Ordinary viscosity, caused by collisions between particles in the gas, won't suffice, Loeb says. But in the Feb. 1 ASTRO-PHYSICAL JOURNAL, he suggests a possible solution to the problem.

Loeb notes that the cosmic background radiation - photons left over from the universe's explosive birth -- had a high density in the young universe. He calculates that the interactions oi these photons with electrons or dust in a gas cloud could produce a drag force, slowing the rotating cloud. Like a falling water droplet that encounters resistance from surrounding air molecules, electrons and dust in the cloud lose energy as they scatter off the cosmic photons inside the cloud. Loeb says that the collisions may significantly reduce the cloud's angular momentum, enabling a black hole to form.

Photons may also play an important role later on, after the cloud has succeeded in forming a black hole and outside matter begins spiraling in, forming an accretion disk around the condensed mass. To fall into the hole, this matter must also lose angular momentum. Cosmic photons can't do the job, since their density is too low at later times in the expanding universe. However, the quasar-like radiation emitted just outside the black hole as previous matter fell into it may provide the answer, Loeb says.

As the quasar photons stream outward, they slam into electrons, enabling the radiation to carry angular momentum away from the interior of the accretion disk. This allows material robbed of its angular momentum to fall into the hole, Loeb suggests. As this material gets sucked in, it emits light and the process repeats. Loeb estimates that this photonelectron interaction increases the viscosity of gas in the accretion disk to about a trillion times that of water.

In this model, a black hole and the quasar powered by it are created first; surrounding gas eventually forms a galaxy around them. But astronomers don't yet know if this sequence is correct, in part because visible-light studies don't easily permit searches for extremely distant quasars-those that might have been born before the universe attained even 7 percent of its current age.

Loeb proposes in an upcoming ASTRO-PHYSICAL JOURNAL LETTERS that a highly sensitive array of radiotelescopes, looking for a specific wavelength of radiation emitted by singly ionized carbon atoms, may find more distant quasars. Ultraviolet light from quasars prompts surrounding gas clouds to produce such radiation, which is emitted in the far-infrared but redshifted to millimeter wavelengths. Detecting this light from the far reaches of the cosmos may indicate whether quasars and massive black holes existed before galaxies did, Loeb says.