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Double Hell

Astronomers catch the first step in the depth spiral of supermassive black holes and prepare to look for the last step

by Erin Wells Bonning and Gregory A. Shields

StarDate magazine Nov/Dec 2004 coverAfter traveling through the vast and empty universe for eight billion years, a feeble signal reaches the orbiting Hubble Space Telescope. At a glance, a team of astronomers that is using HST to study the chemistry of distant quasars knows that it has found something special. Just how special soon becomes obvious: The team has discovered the death spiral of two monstrous black holes.

Quasars are extremely bright, remote early galaxies with supermassive black holes at their hearts. The gravity of the black holes, which are millions or billions of times as massive as the Sun, rips apart nearby stars and pulls in great gulps of gas. As the gas spirals closer and closer to a black hole, it forms a disk that's heated to millions of degrees. This disk is no larger than our solar system, but it produces a great outpouring of energy. As a result, the heart of a quasar is bright enough to see across billions of light-years of space.

An artist's concept shows the turbulent environment around a quasar. [ESA]
An artist's concept shows the turbulent environment around a quasar. [ESA]

The quasars under study were unusual. Earlier observations suggested that they produced torrents of outflowing gas containing much larger amounts of heavy chemical elements like carbon, nitrogen, and silicon than in normal cosmic gas. There was a less exotic possibility, though. The outflowing gas might be arranged in an unusual pattern, and the amounts of these elements could be perfectly normal. Hoping to resolve these questions, the team of astronomers -- led by Vesa Junkkarinen of the University of California, San Diego, and including one of us (Shields) -- observed eight quasars with an instrument aboard HST, called the Space Telescope Imaging Spectrograph (STIS).

In July 1999, the telescope took a picture of the quasar LBQS 0103-2753. (LBQS stands for Large Bright Quasar Survey, and the numbers indicate its location in the sky.) When Junkkarinen received the image, he was stunned. It showed not one quasar, but two. Earlier observations by ground-based telescopes had not "split" the object because the two quasars are so close together.

But pictures are flat -- they squash the three dimensions of space into two. Just because two objects appear next to each other in a picture doesn't mean they're close together in space. It's unlikely that the two quasars would appear side by side just through chance, though, leaving two possibilities: Such a close pair of quasars might be a binary (two quasars bound together by gravity) or a gravitational lens.

A gravitational lens occurs when the gravity of a galaxy acts like a lens, bending the light of a more-distant quasar to produce several images of it. Analyzing the light from these multiple images of the same object should show that they have nearly identical properties. In a stroke of luck, for these observations STIS captured the light of the two quasars separately, and analysis showed that they are quite different. This is a true binary quasar. It's not the first one ever discovered, but the spacing of the two quasars is closer than in any other binary yet discovered.

We also took pictures of the object with another HST instrument. If this was indeed a binary quasar, it was likely to show tidal arcs from the merging galaxies -- large streamers of stars stripped away by the gravitational interaction of the galaxies. The observations revealed a large tidal arc. The binary nature of LBQS 0103-2753 thus seems assured. Also assured are the celestial fireworks that inevitably result when such a binary is formed.

Binary quasars result when two galaxies -- giant, organized agglomerations of sometimes hundreds of billions of stars -- collide and merge, causing the disk of hot gas surrounding the giant black hole in the core of each to blaze with the light of a thousand Milky Ways. Such a celestial merger reflects a long history.

Almost fourteen billion years ago, the universe was just cooling from its fiery birth in the Big Bang. Cosmic gases, thinning rapidly with the expansion of the universe, were spread with almost perfect uniformity through space. Over time, though, slight differences in the density of the gas grew stronger. Big clouds of gas contracted under gravity to form the first galaxies. These were small, but over time the galaxies collided and merged to build bigger and bigger ones, eventually leading to the big galaxies we see today. Our own Milky Way is undergoing this process. It is gobbling up two small dwarf galaxies, and eventually will pull in the Large and Small Magellanic Clouds, two small galaxies that orbit the Milky Way. And in several billion years, the Milky Way may merge with the equally impressive Andromeda galaxy to form a galactic giant.

When galaxies collide, stars in either galaxy feel the gravity of both galaxies, and move in strange orbits. Streams of stars and gas are flung far out into space, forming graceful "tidal arcs." Energy from the motion of the two galaxies is taken up by the agitated motions of the stars, leaving the two galaxies gravitationally bound together. Their fate is now set: They will loop around each other a few times in a shrinking ellipse and then coalesce into one.

In the innermost light-year of our own galaxy, and in many other galaxies, stars orbit the center at startlingly high speeds. From analyzing the orbits of these stars, astronomers can measure the mass of the central object, and the results are large: the central few light-years of the galaxy contains a mass millions, sometimes billions of times more massive than the Sun. There is only one likely explanation for such a huge mass in so a small volume: a black hole.

Black holes are a prediction of Albert Einstein's theory of general relativity. If enough mass is compressed within a small enough volume, then neither matter nor light can escape the gravitational field from within that volume. The boundary where light cannot escape the gravity of the object is called the "event horizon." All matter within the event horizon is compressed to a point that approaches infinite density -- the "singularity."

Black holes with masses a few times that of the Sun can form when the largest types of stars die. However, no known star has a mass higher than about 100 times the Sun's mass. So how did the supermassive black holes in the centers of galaxies get so big? Astronomers speculate that the process begins with a seed black hole in a galaxy's center with a mass of perhaps 1,000 times that of the Sun, possibly formed by the merger of several close-packed stars. It grows by capturing gas and dust from the center of the galaxy.

Gas falling into giant black holes will not likely go unnoticed. Since the middle of the twentieth century, astronomers have known that some galaxies have a brilliant, tiny source of energy in their cores. These are called active galactic nuclei. The most powerful examples are the quasars, which are among the most energetic phenomena in the universe.

Soon after the first quasar's discovery in 1963, astrophysicists guessed that their energy might come from gas spiraling into supermassive black holes. Outside a black hole's event horizon, orbiting matter moves at speeds close to the speed of light. Matter moving at such speeds has an enormous amount of energy. Astronomers suspect that gas from the galactic nucleus gradually spirals closer to the hole in a flattened "accretion" disk. As the gas approaches the black hole, the gravitational energy heats it, causing it to glow in visible light, ultraviolet light, and other wavelengths. Magnetic fields trapped in the spinning disk create tremendous electric forces that accelerate electrically charged particles. These contribute to a quasar's radiation.

At some point, the black hole runs out of gas and dies down. What determines if a galactic black hole will blaze as a quasar or remain quiet? Some galaxies have very little gas, and in others the gas is located too far from the black hole. One way to trigger a quasar outburst is to have a collision between two galaxies. If one of the back holes receives a fresh charge of gas -- flung into its vicinity by gravitational forces in the merger -- it springs to life as a quasar. If both black holes receive fuel at once, a binary quasar results. Only a dozen or so binary quasars are known. In most cases, the two quasars are separated by tens of thousands of light-years -- about the width of a large galaxy. These are cases where the galaxy merger is still in an early stage, with the two galaxies still well separated. In the course of many millions of years, the galactic nuclei and their black holes spiral together and merge.

LBQS 0103-2753 is eight billion light-years from Earth. The two components of this binary quasar are only about 6,500 light-years apart -- much closer than any other known binary quasar. For comparison, the Sun is about 27,000 light-years from the center of the Milky Way. Thus, LBQS 0103-2753 represents a more advanced merger than the more widely spaced binary quasars. In another 10 million years or so, the stars and gas in the two galactic nuclei will merge.

When the galaxies first begin to interact, the black holes will stay nested in their galactic hosts. They will not yet feel the attraction of the other. After a couple of orbits, the cores of the galaxies will coalesce. At this point, the black holes in the new nucleus will begin sinking toward each other.

As the black holes interact gravitationally with passing matter, they will lose energy. This process is known as "dynamical friction." As a result, the distance at which the holes orbit each other will shrink, and their orbits will speed up. At some point, they will be moving quickly enough that dynamical friction is ineffective. The two holes will remain in this state, orbiting perhaps 300 light-years apart, for a long but uncertain time. The binary may slowly lose energy to passing stars, or it may be perturbed by gas falling into the center of the new galaxy. This will bring the black holes closer to each other, ever inwards to a separation at which a new and completely different effect takes over.

When the black holes are less than 0.1 light-year apart, they will begin to lose energy to gravitational radiation. This is another prediction of Einstein's theory of relativity, which describes how masses curve and warp the space around them. An accelerating mass will produce a ripple in space. Just like ripples in a pond propagate away from a pebble dropped into it, so do ripples in space speed away from their source, traveling at the speed of light.

Orbiting masses produce gravitational radiation, and since this radiation is a form of energy, the orbiting black holes will lose energy. But only when the two black holes are very close together and moving very fast will the radiation be strong enough to bring the black holes closer together at a significant rate.

The black holes will spiral toward each other until they reach a point (perhaps a little more than the diameter of the black holes) where their orbits will become unstable. Here, the lifetime of the binary will end. The black holes, unable to remain in orbit, will abruptly plunge toward each other until they lie within a common event horizon. A single bigger black hole will form, accompanied by a huge burst of gravitational radiation. The horizon of the new black hole will oscillate like the surface of a ringing bell, continuing to produce gravitational radiation. Eventually it will settle down, and the new black hole will quietly spin along in the center of its host galaxy, waiting for something new to come by and light it up.

The final burst of gravitational radiation carries off energy at the expense of the mass of the new black hole. A theorem in relativity states that the horizon of the new black hole must have a surface area greater than or equal to the sum of the areas of the original two holes. As a result, the final black hole can lose up to 30 per cent of the original two colliding holes' masses in the form of gravitational-wave energy. In the final year of the in-spiral, the radiation reaches its maximum. The power of the waves is millions of times greater than the light output of the quasars at their height.

The amount of energy carried away in a gravitational wave from a merging binary, billions of light-years away, is on the same order as the light energy received at Earth from a full Moon. However, while the amount of energy is large, the observable effect is small -- the gravitational force is weak, and the ripples in spacetime spiraling out from the colliding black holes are extremely small by the time they reach Earth. Even today they have never been observed directly.

That may be about to change, however. Some instruments are currently operating, and more are being developed, that will be able to tell when a gravitational wave passes through Earth. They will measure the tiny changes in length caused by space itself compressing and expanding as the ripple passes through the instrument. These distortions are small; even for a strong source, a gravitational wave passing through Earth will alter the planet's diameter by 1/1000th the diameter of an atom.

The technology to detect such minuscule ripples in spacetime already exists -- an instrument called a laser interferometer. NASA and the European Space Agency plan to launch such an instrument into space in a few years. Called the Laser Interferometer Space Antenna, it should be able to detect gravitational radiation from black holes in merging galaxies that are near their final collision.

A detection of gravitational waves would be an amazing discovery and a powerful confirmation of Einstein's general theory of relativity. Even more exciting for astronomers, it will open up a new way of observing the universe. Astronomers will be able to peer into the very depths of merging galaxies and see for themselves what gravitational forces are at work when their colliding black holes shatter the fabric of space and time.

Erin Wells Bonning devises computer models of merging black holes and recently completed her Ph.D. in physics at The University of Texas at Austin. Gregory A. Shields is the Jane and Roland Blumberg Centennial Professor in Astronomy at The University of Texas at Austin.