Volume 4, Issue 26 March 2007
Illuminating Black Holes
In addition to developing theories about the physics of black holes, galaxies, and other space objects, Professor Quataert also serves as the director of UC Berkeley's Theoretical Astrophysics Center. Image credit: courtesy Eliot Quataert
In a universe filled with improbable objects, supermassive black holes are among of the oddest of the odd. They lurk at the centers of galaxies like giant spiders, gobbling up much of the matter and energy within their gravitational reach. Such an appetite leaves black holes at the centers of galaxies quite corpulent, with most topping out at a few million to a few billion times the mass of our Sun. At the same time, black holes remain surprisingly compact; a fraction of a teaspoon could weigh billions of tons. This great density arms black holes with the most powerful gravity in the universe.
Yet these galactic hubs are tricky to study. Since supermassive black holes swallow rather than emit any light of their own, scientists must instead observe the objects that surround them and the radiation produced by gas in their vicinity.
As a theoretical physicist and professor of astronomy at UC Berkeley, Eliot Quataert uses these observations to deduce how black holes and other astronomical objects work. "One of the really attractive things about astrophysics research is you can take relatively basic concepts of physics and apply them to understand the vast array of things that we observe in the universe." Though Quataert studies a wide variety of astrophysical problems, one of his major areas of expertise is how supermassive black holes grow and affect the development of their galaxies.
According to Quataert, black holes have effects on their galaxies all out of proportion to their size and mass. "It's as if a speck of dust had a huge effect on everything in a room," Quataert says.
"A Ring of Fire": Gas falling into a black hole, based on numerical simulations. Most of the light is produced just before the gas crosses the event horizon, giving rise to a ringlike structure. Over the next decade, astronomers should be able to observe this event directly, testing this and other theoretical predictions of black hole behavior. Image credit: Josh Goldston-Peek and Eliot Quataert
This far-reaching influence stems from the halo of matter and radiation streaming outward from each black hole. "Together with neutron stars, black holes emit the brightest electromagnetic radiation in the universe," Quataert says. Yet the only part of this process scientists can observe is the amount and electromagnetic spectrum of these flares.
From these rudimentary clues, Quataert and other astronomers have sketched out a compelling picture of black hole operations. The black hole's powerful gravity pulls any gases drifting nearby into a spinning pancake known as an accretion disk with the black hole at its center. With every revolution, these gases moves closer to the black hole itself. Just before crossing the black hole's event horizon—the point of no return—the gases emits a tremendous burst of radiation in the most efficient energy-producing reaction known to mankind, Quataert says. "If you had a choice for energy on Earth and could choose between building a little black hole and throwing some gas into it and performing nuclear fusion, the black hole would be a much better energy source."
Quataert is now deciphering why black holes belch out the amount and frequency of electromagnetic radiation they do. The magnetic field around each black hole likely plays a starring role. "Energy from the gas gets stored in the magnetic field," he says, slowing the rotation of any orbiting gas. Eventually, the gas loses so much momentum that the gravity of the black hole can capture the gas particles and send them spiraling toward its gaping maw.
At the same time, Quataert has found the magnetic field returns some of the energy stolen from the gas as heat and turbulence. This accelerates the gas until the matter closest to the black hole is whirling around at nearly the speed of light. The gas in the disk grows hotter and hotter until it eclipses the temperature of the sun. Then, just before it crosses the event horizon, it turns into the blaze of radiation we see from Earth. "Understanding in detail the chain of energy—how it goes from inflowing material into magnetic fields, back into the particles via turbulence, and then into radiation—needs to be understood to interpret the radiation we see," Quataert says.
An X-ray image of two giant black holes at the center of a nearby galaxy. X-rays are produced when hot gases fall into a black hole. Supermassive black holes at the centers of galaxies produce so much radiation that they dramatically influence how their host galaxies form. For scale, this image is roughly 30 thousand light-years across&mdsah;nearly a billion times larger than the gas disk illustrated in the "Ring of Fire" above. Image Credit: NASA/CXC/MPE/S.Komossa et al.
Recent observations indicate that black holes in the center of galaxies weigh almost exactly a thousand times less than the mass of the stars in their galaxy. Quataert has developed a theory explaining this strange relationship. "A black hole at the center of a galaxy eventually gets so big that the amount of radiation and light outflowing from it stops more gas from falling in, and the black hole's not going to be able to grow." Quataert says.
This phenomenon, in turn, affects the size of the host galaxy itself. "If the black hole is pushing a lot of its galaxy's gas outward into the universe, there's less matter available to turn into stars," Quataert says. "The galaxy will end up being less massive than it otherwise would have been." The resulting equilibrium defines the limits of some of the largest structures in the universe.
"If we understand the gas around black holes, and its behavior and dynamics, we'll be ultimately able to say something quantitative about Einstein's predictions of what gravity looks like around a black hole," Quataert says—and help illuminate one of the darkest mysteries in the cosmos.