As with so many puzzles in science, new observations and new data would make all the difference. In 1999, and within five months of each other, two mammoth telescopes launched into orbit around the Earth, and they would prove to transform our view of the universe. One was NASA’s Chandra X-ray Observatory, and the other was the Newton Observatory of the European Space Agency’s X-ray Multi-Mirror Mission (or XMM-Newton). Both were designed to collect more X-ray light from astrophysical objects than ever before, and to make the most detailed images and spectra of that light. With this new precision, scientists could now monitor the X-rays from galaxy clusters well enough to closely track the behavior of the cooling gas by exploiting another of its unique characteristics.
In among the hydrogen and helium of this gas are the same kinds of elemental pollutants that we find everywhere in the universe. Produced by generation upon generation of stars and wafted, blasted, and blown out of individual galaxies and protogalaxies, these heavier elements permeate the denser regions of the cosmic webbing. Some of them are particularly visible in X-ray light from hot gas. Iron, for example, has a complex energy hierarchy for the electrons held around its atomic nucleus. Oxygen, the most abundant element in the universe heavier than helium, has similar properties. In these cases, even at temperatures of millions of degrees electrons can stay engaged with their individual atoms, and energetic X-ray photons are absorbed and released at very specific wavelengths or “colors.” The highly advanced instruments on board Chandra and XMM-Newton could sniff out the X-ray photons from these heavy elements. This light provides a unique fingerprint directly related to the gas temperature. It’s like having a thermometer inside a galaxy cluster, and astronomers were quick to put these tools into action, swinging the great observatories to gather up light from the brightest systems in our nearby universe.
Here was the gas, cooling. Down and down it went. And then … nothing. You can imagine the scientists’ consternation. Suspecting a mistake, they quickly reanalyzed the data. But there was no mistake. In cluster after cluster, the gas cooled down as expected, and then, just as it reached a temperature of a little more than 10 million degrees, it stopped. Not only did it stop, it didn’t even accumulate at that minimum temperature—there was no vast snowdrift of piling-up material. You might expect a great mass of this gas to be growing as more and more poured in through the outer cooling flow, but it wasn’t doing that. To all intents and purposes, it was mostly vanishing, with just a trickle carrying on down to lower and lower temperatures. It was as if the chain of cars rolling down the hill got a certain distance and then just conveniently disappeared. It felt like watching a great ocean liner gracefully sailing off to the horizon, and then suddenly turning into a dinghy before dropping out of sight.
Clearly, something was happening to all this gas. Perhaps an unknown mechanism was heating the gas in a targeted fashion, cooking up the cool stuff and getting it quickly back into the general mix before astronomers noticed it. Or perhaps, with the right arrangement of magnetic fields, thermal energy could be channeled to the cooling gas to warm it up—a great system of under-floor heating. The X-ray thermometer that astronomers were using relied on the precise mix of heavier elements like iron and oxygen with unpolluted gas. Fewer of those richer elements could skew the measurements at lower temperatures. Perhaps the just-cooled gas could be mixing up with either much hotter or much cooler gas, or could even be obscured by a hitherto unseen blanket of cool material that simply blocked the X-rays from our view.
There was another possibility. Maybe energy from a central supermassive black hole was halting the cooling. But how? The ability of a black hole to squirt out jets of particles, jets that could then splash out into great lobes of seething relativistic electrons and protons, was a tantalizing candidate. We knew that big central galaxies in clusters often harbored such structures. These were places where radio maps traced out colossal clouds and arcs of particles. Astronomers had certainly considered the impact these feeding black holes might have on the young galaxies and cooling gas we could see in the more distant universe. But something was needed in our neighborhood galaxy clusters: a clear signpost, something that could tell us exactly where to look. In the end there were several such signs, but one in particular is so big and clear that in retrospect it’s almost embarrassing that we hadn’t connected the dots before.
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The answer begins with Perseus—not the monster slayer of Greek mythology, but a huge cluster of hundreds of galaxies, named for its sky placement in that hero’s constellation but actually located about 250 million light-years from our solar system. The Perseus cluster is one of the largest such structures in our cosmic neighborhood, and the brightest such object in X-rays from our viewpoint. If we could see it with our eyes, it would cover a patch of sky four times broader than the full Moon. Adding up all the stars, gas, and dark matter in this great agglomeration, it totals a staggering 700 trillion times the mass of our Sun—a thousand times the mass of the Milky Way. This huge collection is spread over a three-dimensional region that stretches across 12 million light-years. As in all such vast gravitational wells, the bulk of the normal matter in Perseus consists of gas at extraordinary temperatures. Heated to tens of millions of degrees, it glows with X-ray photons. Just as in all the other great clusters, the gas is trying to cool off. And just as in so many clusters, within the center is the unmistakable signature of particles that have been spewed out from around a supermassive black hole, glowing with radio emission.
A puzzle about Perseus had emerged in the early 1980s with its first detailed X-ray images. Perseus was great to look at—it was big and bright, and its glowing gas could be seen spread across hundreds of thousands of light-years, brighter toward the center and getting ever fainter toward the edges. But there was a strange dark zone in one corner of this huge structure, an area that looks like a dirty thumbprint on these images, blotting out perhaps a hundred thousand light-years of X-ray glow. A decade later, new telescopes and instruments revealed the same thing. Perseus had a gap in it. In 1993 the German astronomer Hans Boehringer used the latest data to begin to finally unravel the mystery. Within the core region of Perseus were two other vast dark hole–like gaps inside the hot, glowing gas. When Boehringer and his colleagues placed a map of the radio emission from Perseus over their new X-ray image, the two prominent lobes of radio light lined up almost perfectly with these voids in the gas.
There had been growing suspicion over the years that the high-speed particles injected into a cluster from a central black hole jet would have to push aside the cluster gas. Not only that, but they would constitute a far lower-density material than even the tenuous cluster plasma. This suggested that the inflating lobes of radio-emitting particles should be buoyant. If so, then they should quite literally float in the cluster. But it was not yet clear whether this could really happen. Perhaps these lighter bubbles would simply dissipate and fizzle out. Along with the enormous cooling flow puzzle, this was a huge question to answer. It was evident that we needed to take a very, very careful look deep inside one of these systems.
Fabian and his colleagues set out to study the Perseus cluster in unprecedented detail, using the Chandra observatory. After some intriguing initial results, they decided to go for broke. They needed the most precise image of Perseus possible. Only this would allow them to peel apart the data to get at the gold nuggets inside. Over the course of two years, they accumulated the data they needed, until they had almost 280 hours’ worth of photons—nearly a million seconds altogether—and from those they finally generated a new image of Perseus.
And what an image it was. With this incredible new visual fidelity, Perseus took on a whole new texture and flavor. It looked like a pond after a giant pebble has been thrown in, interrupting its smooth surface. There are clear bubble-like gaps, and there are ripples—actual waves through intergalactic space. All these are marching outward from the supermassive black hole at the core.
Fabian’s team look
ed at the data every way they could, holding it up like a faceted diamond to see the shifting colors and projections from within. The mysterious gaps high up in the outer regions of the cluster were indeed rising bubbles. Whatever highly energized particles the black hole had squirted out millions of years ago to inflate these forms had long since cooled down, invisible even to sensitive radio telescopes. But the particles live on, holding the cluster gas at bay. These are ghostly cavities in Perseus’s body, buoyant structures in a sea that is hundreds of thousands of light-years across. Down toward the core there are new bubbles, these still filled with hot electrons shedding radio-wave photons. The ripples between these floating bubbles are subtle, gentle structures. They are actual sound waves, the booming call of a leviathan. The time it takes for light to travel from one wave crest to the next is equivalent to the passage of all recorded human history.
Figure 14. An X-ray image of the hot gas in the inner regions of the Perseus galaxy cluster, showing the dark bubbles that have been blown by the black hole at the center and the great ripples of sound waves set in motion across the structure. At the very center are signs of some darker, cooler gas, looking like dirty strands of coagulated matter.
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Fabian himself is an avid rower. More often than not, if you’re taking a morning stroll along the banks of the River Cam passing through Cambridge, you’ll spot his boat skimming along the gently flowing water, its V-shaped ripples expanding outward, lapping softly at the riverbank. Watch him move upriver, and you see the same processes at work that are taking place in Perseus. As the boat pushes and displaces the fluid around it, some of that energy dissipates across the water, far away from its source. The lap and splash of the shoreline waves comes from energy that is generated within the muscle fibers of human arms and transferred across a river’s surface. The supermassive black hole in a cluster core does the very same thing.
These structures in Perseus help us understand how the central supermassive black hole holds cooling matter at bay. The buoyantly rising bubbles can lift and push cooling gas aside, preventing it from funneling all the way down to the core. And like a vast musical organ, the bubbles set sound waves in motion that can disperse energy throughout Perseus, keeping it at a perfect simmer. We don’t yet know if these rippling pressure waves, like the rolling thunderclaps of a distant storm, are entirely responsible for halting the flow of cooling gas into Perseus’s inner core. But if we carefully measure these waves and compute how much energy they can push out across the cluster, it is certainly enough to balance out the energy that the cooling gas loses in X-rays.
A simple physics experiment brings the principle to life: place an open loudspeaker from a music system so that the speaker is on its back, forming a shallow cup. Put a sprinkling of sand or rice grains on the speaker. They roll and slither down to the middle. But then play music through the speaker with the volume turned up, perhaps a good bit of Bach or some heavy metal. The bass notes (or longer sound waves) vibrate the speaker and push at the grains. If you find the right pitch and volume, they’ll spread back out up the sides of the cup, bouncing and agitated and unable to slump to the center, just like the gas in Perseus.
Every few million years a supermassive black hole in Perseus is being fed matter. When this happens, an outburst of energy from its jets and radiation inflates bubbles of high-pressure, fast-moving particles into the comparatively cooler and denser gas of the cluster. As these bubbles inflate, they act like the vibrating blast from a massive pipe organ, pushing off a rippling pressure wave that spreads out across the system. As it passes it releases energy into the gas, helping to prevent it from cooling and rushing inward. The black hole is driving the ultimate subwoofer, an audiophile’s fantasy. The note it’s playing? Fifty-seven octaves below B flat above middle C, in case you were curious. That’s approximately 300,000 trillion times lower in frequency than the human voice. And the power output is a planet-disintegrating 1037 watts. Supermassive black holes can make you a very, very nice sound system.
We now know that Perseus is not the only place where this is happening: there is evidence for bubbles in 70 percent of all galaxy clusters. Here, too, the deep booming notes of these systems and their rippling undercurrents help moderate and regulate the cooling and inflow of gas. Here, as well as in isolated galaxies, we see evidence for a dynamic balance in the struggle between matter trying to build structures and the forces of disruption. In engineering this is called a “feedback loop.” A simple example is a device used for centuries to regulate the speed of engines, from those driven by windmills on into the era of steam power. The centrifugal or “flyball” governor is a pair of opposing metal balls hanging like twin pendulums on stiff wires from a vertical rod. The rod is connected to the spinning axle of an engine. The faster the engine runs, the faster this vertical rod spins, and the higher the two metal balls rise as centrifugal forces push at them. But as they rise, the wires holding them can transmit that movement to a valve or throttle that slows down the engine and prevents it from running too quickly.
We think a similar thing happens with feeding black holes. In essence, the greater the amount of matter that falls toward the hole, the greater the energy output, and the harder it becomes for matter to reach the hole in the first place. This is the effect of the ripple-producing bubbles in a galaxy cluster. Although this serves to slow down the black hole gravity engine, it’s rather sporadic and jerky. Unlike a beautifully smooth mechanical governor, a piece of high engineering from the Industrial Revolution, the fluid and fickle nature of gas and astrophysics results in something unique every time. Occasionally, a particularly dense patch of gas or stars manages to cool through to the core of a galaxy and descend into a black hole’s grasp. The hole flares up brilliantly and squirts out a bubble-inflating jet for a million years before running out of fuel. At other times, there may be just enough gas to trigger a mild response.
In clusters where a trickle of gas manages to cool all the way down before being heated and rearranged, there are ethereal fingerprints. Astronomers’ images in optical, ultraviolet, and infrared light reveal wispy threads and filaments of dense, warm material, cobweb-like structures of luminosity around the central zones of these clusters, no longer emitting X-rays. The cluster Perseus contains such forms within its central elliptical galaxy. Reaching across tens of thousands of light-years, they appear like strands of curdling milk in a great vat of matter. At the enormous distances of galaxy clusters it is impossible for us to see individual stars. They all merge into fog-like hazes. Nonetheless, by examining the spectral fingerprint of these milky filaments, we can tell that in among them are young large blue stars. The only viable explanation is that they are forming here, the end point in the journey of intergalactic gas that has settled its way down through the cluster. For now, escaping the petulant fist-waving of the supermassive black hole, it finishes as a warm nebula that then cools further to make new stellar systems. Eventually, these may be fodder for the next cycle of black hole activity, but not yet.
In the places where this happens, as much as a few times the mass of our Sun is converted from gas into stars every year. This rate provides a good match to the overall proportions of stars to gas in these central cluster galaxies. This is strong evidence that the reason we see the number of stars that we do in these galaxies is because the black holes are controlling the production line. The balance set by nature in these great systems of feedback is literally written in the stars. And that point of impasse stems from the fundamental nature of black hole physics, from electromagnetism to curved, spinning spacetime.
Here then, in the special environments of galaxy clusters, are clues to one of the major routes by which the universe builds new stars and galaxies. It is a startling display of how a supermassive black hole functions as a cosmic regulator, making sure the porridge of intergalactic matter is not too hot and not too cold. There is still much to learn about these mechanisms. There are indications that the supermassive black holes in
the cores of clusters like Perseus may also be the fastest spinners. Faster-spinning holes are more efficient at producing energy. It makes sense: cluster-bound holes are the end recipients of a vast reservoir of intergalactic matter. If they weren’t big and efficient at pushing back, we would see far more cooling gas converted into stars, and it would be a very different universe if that had been the case.
Massive black holes are also clearly present in the cores of other galaxies, including those that are not part of bigger clusters. The most powerful jets in these systems have little surrounding medium into which to blow bubbles, and much of that energy simply sprays out to intergalactic space. But the fearsome glow of radiation and particles from the material accreting around the holes must still impact the surrounding environment. The next step for us is a big one. If we want to complete the story of black holes and the cosmic struggle between construction and destruction, we need to travel even farther. To finish the recipe for apple pie, we need to visit the remotest edges of the universe itself.
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A DISTANT SIREN
When John Lennon sang that images of broken, dancing light were calling him on and on across the universe, he was thinking metaphorically. But for astronomers and cosmologists, the extremely distant parts of the universe represent a treasure trove of insights into the workings of nature. The finite speed of light is in this case a remarkable gift, opening up billions of years of history for us. As photons pass through the cosmos they carry the imprint of the moment they were emitted, or last reflected. Each one is a messenger that we can query.
Gravity's Engines: How Bubble-Blowing Black Holes Rule Galaxies, Stars, and Life in the Cosmos Page 14