DO NOT ATTEMPT THIS AT HOME: Looking down the barrel of a jet from a supermassive black hole is not something you would be advised to do. The jet packs enough punch to create bubbles of hot gas with an energy equivalent to millions or billions of supernova explosions. Black Hole BLOWBACK A single black hole, smaller than the solar system, can control the destiny of an entire cluster of galaxies By Wallace Tucker, Harvey Tananbaum and Andrew Fabian 42 SCIENTIFIC AMERICAN f you drew a large-scale map of the universe, it would look rather like a map of the U.S. Interstate Highway System. Galaxies line up in fi laments that criss- cross intergalactic space like freeways. In between the roads are regions of I relatively low density: the cosmic countryside. And at the crossroads, where mul- tiple fi laments converge, are clusters of galaxies: the cosmic megacities. The size of these clusters is daunting. It takes light a little more than a second to reach Earth from the moon and eight minutes to reach Earth from the sun. Light from the center of our Milky Way galaxy must make a journey of 25,000 years to reach us. Even that is fairly quick compared with the time required for light to cross a galaxy cluster—about 10 million years. In fact, clusters are the largest gravitation- ally bound bodies in the universe. The roadlike fi laments may be larger in sheer size, but they are not coherent bodies held together by gravity. SCIENTIFIC AMERICAN 43 The gravitational binding means that the galaxies and other the more populated region: it contains more mass than all the material within a mature cluster have settled into an overall dy- stars in all the galaxies in the cluster. namic equilibrium. Galaxies buzz around within it and are kept The gas, which is heated primarily by the slow gravitation- from fl ying apart by dark matter, a mysterious form of matter al collapse of the cluster, gives off x-rays. Optical telescopes that has eluded detection except through its gravitational ef- cannot see the gas, and x-rays cannot penetrate Earth’s atmo- fects. The interactions of these components produce a rich array sphere, so the discovery and study of this gas has depended on of phenomena that astronomers are only beginning to grasp. orbiting observatories. Two decades ago astronomers peering Like metropolises on Earth, clusters are more than the sum with NASA’s Einstein X-ray Observatory and other instru- of their inhabitants. Processes occurring at the scale of a clus- ments noticed that the x-rays carry away so much energy that ter can dictate events on much smaller scales, such as the the gas should steadily cool off and settle into the center of the growth of galaxies and the fueling of the supermassive black cluster—thus the term “cooling fl ow.” One of us (Fabian) led holes at the hubs of those galaxies. In turn, the black holes the way in investigating these fl ows using Einstein and later blow out huge amounts of high-speed material that can drive Germany’s ROSAT x-ray satellite. He and his colleagues cal- the evolution of the entire cluster. At fi rst glance, these inter- culated the fl ows would have quite dramatic effects. If they For a black hole to affect an entire galaxy cluster would be like a blueberry affecting the entire EARTH. connections between large and small are enigmatic. The di- persisted for a billion years, the gas deposited in the central re- ameters of the black holes in question are smaller than the gions of the cluster could form trillions of new stars. solar system. For them to affect an entire galaxy cluster would The only trouble was, no one could fi nd them. Observers be like a blueberry affecting the entire Earth. looked in vain for large amounts of cool gas and hordes of new- ly formed stars. If a black hole had swallowed them all, it would The Case of the Disappearing Gas weigh as much as a trillion stars, and not even the biggest black these interactions explain some long-standing para- hole is that massive. Another one of us (Tucker) maintained doxes in the urban life of the cosmos. One is the so-called that large-scale, long-term cooling fl ows do not exist. A pos- cooling fl ow problem, which has to do with multimillion- sible explanation was that long-lasting outbursts of energy degree gas that fi lls the space between the galaxies in a cluster. from the central galaxy of the cluster heated the gas enough to If the galaxies within a cluster are the urban cores of the offset the radiative cooling. Radio astronomers had for years megacity, this gas is the suburban sprawl. Like the suburbs been accumulating evidence for such activity. But it was ques- that surround most American downtowns, the gas is actually tionable whether the outbursts provided enough energy distrib- uted over a large enough volume to halt the cooling fl ows, so Overview/Monster Bubbles the paradox remained: the hot cluster gas must cool, but the end product of the cooling mysteriously escaped detection. ■ Using a combination of radio and x-ray telescopes, Resolving this paradox was a major goal of two powerful astronomers have discovered vast bubbles of high- x-ray telescopes launched in 1999: NASA’s Chandra X-ray energy particles hundreds of thousands of light-years Observatory and the European Space Agency’s XMM- across. The energy required to create these structures Newton. Because the gas in clusters radiates away its energy beggars belief—it is as if 100 million stars went fairly slowly, it preserves a record of activity in the clusters over supernova at once. the past few billion years. For instance, it retains the elements ■ The only thing capable of creating such a monster is a and energy injected into it by supernova explosions in the clus- giant black hole. Not all matter that comes close to a black ter galaxies. Like archaeologists unearthing the past, astrono- hole is doomed. As magnetized hot gas swirls in an mers have used the new telescopes to excavate the relics pres- accretion disk toward a hole, strong electromagnetic ent in galaxy clusters and piece together their history. forces build up and spit out some of the gas in a narrow jet. ) ■ Not only can the jet create bubbles, it adds heat and Bubble, Bubble magnetism to the intergalactic gas in a galaxy cluster, the brightest cluster observed by x-ray instruments explaining long-standing puzzles in astronomy. The is the Perseus cluster because of its high intrinsic luminosity process appears to be a part of a multimillion-year and relative cosmic proximity to Earth (about 300 million pages preceding cycle that regulates the growth of supergiant galaxies light-years). In the 1990s ROSAT discovered two vast holes in the centers of clusters. in the x-ray gas in the central 50,000 light-years of the cluster. They look like an hourglass centered on the giant galaxy DON DIXON ( 44 SCIENTIFIC AMERICAN MARCH 2007 ANATOMY OF A CLUSTER The largest things in the universe worthy of being called hive and prevented from dispersing by gravity. At the core of “things” are galaxy clusters. They consist of 1,000 or so the cluster is an especially large galaxy—the site of the most galaxies, buzzing through a ball of hot gas (red) like bees in a violent processes in the modern universe. Jet Colliding galaxies Ripples (sound waves) 100,000 light-years Giant cavity (bubble) Central galaxy 10 million light-years As the x-rays carry off energy, the cluster gas should cool Cycle of heating and cooling explains why those stars and fl ow inward. Over billions of years, it should form are not seen. Black hole jets return energy to the gas and trillions of new stars. Yet few such stars are seen. choke off the inward fl ow. X-rays Black hole swallows Gas cools down and matter starts fl owing in ) bottom Black hole spins up, producing jets Black hole exhausts fuel; jets stop Cooling fl ow ); JANA BRENNING ( BRENNING JANA ); top Jets dissipate, Gas heating gas stops fl o w i n g i n ALFRED T. KAMAJIAN ( KAMAJIAN T. ALFRED www.sciam.com SCIENTIFIC AMERICAN 45 NGC 1275. With Chandra, Fabian and his colleagues went bubbles—a strong indication that bubbles keep the gas from back for a closer look. Their Chandra data showed the cavi- cooling down. But there is a missing link in this argument: ties in exquisite detail, revealing that they were aligned with How does the energy get transferred from bubble to gas? previously observed radio jets emanating from the center of the giant galaxy [see illustration on page 48]. The x-ray cavi- Sub-Subwoofer ties are not empty but are fi lled with magnetic fi elds and en- an obvious answer is that the bubbles generate strong ergetic particles such as protons and electrons. These ener- shock waves in a manner similar to atmospheric explosions on getic, low-density bubbles are rising buoyantly and pushing Earth. As the energetic material from an explosion pushes into aside the x-ray-emitting hot gas. the atmosphere at supersonic speeds, it sweeps up surrounding Other clusters have bubbles, too. Chandra observations re- air into a thin shell. Collisions between the swept-up particles vealed x-ray cavities with associated radio emission in the Hy- convert the kinetic energy of the explosion into heat.