6
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.
If the skies are clear tonight, go outside and take a look around. Perhaps the Moon is visible. But what you see is not the Moon, but the past Moon. It’s the Moon as it was 1.3 seconds ago. Up there in the sky is another object, tiny but brilliant. It’s the planet Jupiter. Or rather, it is Jupiter as it was forty minutes ago. Look around a little more and find the bright stars. If you can see the southern sky, one of the brightest stars of all is Alpha Centauri A, as it was a little over four years ago. Other bright stars are glowing the way they were several decades ago. If the night sky is dark enough you can make out the hazy splash of the plane of the Milky Way galaxy, the projected light of the nearest spiral arms. Most of the light you see has been traveling toward you for thousands of years.
Because of photons’ limited speed, we’re forever trapped by time, blanketed and shielded from whatever is happening in the cosmos right now. But the truth is that it is our minds that have a problem. We need to let go of our conceit that we really witness anything “as it happens.” When I drop a coin, I see it hit the ground a few nanoseconds after the coin “thinks” it does. If I watch a seagull scooping up a fish from the distant ocean, the gap in time between its hungry gulp and when I witness it can be tens of nanoseconds. It takes that long for the light reflected from these objects to reach me. The simple fact is that when events occur is all relative, something that is of course deeply embedded in Einstein’s descriptions of the physical universe. Luckily this characteristic of nature also provides us with the means to practice cosmic paleontology.
In 1962, when astronomer Maarten Schmidt discovered how to interpret the light coming from a distant quasar, he realized that those photons had been en route for 2 billion years. Since that extraordinary measurement, astronomers have striven to push further and further back in cosmic time. We’ve looked for supernovae, quasars, radio-emitting galaxies, ordinary galaxies, and clusters of galaxies at ever-increasing distances. At times the quest has been highly competitive. Scientists highlight the cosmic distance of a new discovery front and center in the title announcing their work, clearly claiming bragging rights. The following week another team of researchers will try to one-up that discovery by perhaps 100 million years’ worth of intergalactic time. Astronomers take pride in their ability to eke out new objects that are fainter and more difficult to characterize than anything that came before. Indeed, it’s the nature of the subject, an indelible part of its history and practice.
The result of all these efforts is a cosmic time line. Just like fossil hunters carefully brushing away the dusty grains of rock entombing a specimen, astronomers peel away layer upon layer of spacetime strata. In doing so, we can track the ways in which the populations of stars and galaxies have evolved as the universe has aged. It is how we know that quasars, the most massive and most actively feeding black holes, used to be far more prevalent. It is also how we know that the populations of stars and galaxies of recent times have changed from their earliest days. But why all this galactic evolution happens is still a major question. Theoretical astrophysicists use sophisticated computer simulations in their efforts to understand the steps involved. These virtual worlds incorporate the effects of gravity and gas pressure, and even attempt to model the nature of star formation and the complex interplay of energy and matter in different environments. Yet again and again many of these simulations have produced overgrown galaxies, systems the likes of which are nowhere to be seen in the universe. They contain too many stars, far more than really exist. This is a huge problem, and tells us that we are missing something, some piece of astrophysics that is not yet properly contained in our theories. But we’re making progress, and this progress is part of the story here. The bubbles blown by supermassive black holes are one critical component of the answer, but there is still much more at play that we need to understand.
It’s in “today’s” universe that we’ve clearly seen the dramatic relationships between supermassive black hole energy output and the nature of galaxies and galaxy clusters. But we need the entire cosmic time line of this phenomenon in order to understand fully how the universe got to be the way it is. Following the fossil clues from the deep past is going to help give us the answers. So let’s now take a look at the story of one particular very distant, very strange, and very revealing place in the young universe.
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My entry into the science of supermassive black holes came about because I was interested in something that I thought had nothing to do with these objects at all. For several years I had been pursuing clusters of galaxies across the universe, together with several colleagues around the world. Not literally, of course. Sadly, we were not superheroes. We chased them down the way astronomers chase down any cosmic objects: by surveying the skies with new tools and new persistence. More specifically, we were beachcombers, sifting through the sands of a large database of astronomical X-ray imagery. It all came from the orbiting X-ray telescope called the Roentgen Satellite, or ROSAT for short, a joint European and U.S. space mission. The mission was named after Wilhelm Roentgen, a German scientist distinguished by being the first person to receive the Nobel Prize for physics in its inaugural year of 1901. He was the discoverer of the mysterious “X-rays,” or Roentgen rays, that we are now familiar with at our dentist’s offices and hospitals. These were produced as a side effect during his laboratory experiments with cathode rays—beams of electrically accelerated electrons. He noticed that despite placing a thin aluminum screen, and then a sheet of cardboard, across the end of his experiment, something was still getting through and producing fluorescence in nearby materials. He called this unknown phenomenon “X-rays.” Roentgen was a talented scientist, but little did he know at the time the role that this radiation plays throughout our universe.
During the early 1990s, ROSAT had taken long-exposure digitized images of X-rays emanating from all manner of astrophysical objects—thousands, in fact. As is common practice with almost any large telescope or astronomical instrument, astronomers petitioned the organizations running ROSAT about their favorite things to look at, some known and some exploratory. The telescope would eventually make the observations for the lucky few whose scientific arguments won out in a process of review by their peers. Later on, all this data was placed into a great electronic repository, archived for anyone to use. Scavengers like us could then trawl through the raw material to look for gemstones among the rough.
We were a motley crew. Three of us, myself, the English astronomer Laurence Jones, and the American astronomer Eric Perlman, all ended up spending a couple of years at NASA’s Goddard Space Flight Center in suburban Maryland just outside Washington D.C. Jones and Perlman were the first to begin sniffing around in the piles of data from ROSAT, along with Gary Wegner, an astronomer at Dartmouth College. I joined in through my related work on the large structures in the universe, what would later be known as the cosmic web. And, although he was thousands of miles away in Hawaii, we also teamed up with Harald Ebeling, a German-born astronomer with a knack for clever computer algorithms, to sieve through X-ray telescope data looking for interesting objects. Equipped with computers and physics in place of hard hats and lanterns, we were cosmic data miners.
Our goal was to try to find new and ever-more-distant examples of clusters of galaxies. Our clues came from the hot and tenuous gas harbored by the gravity wells of these huge structures. This was the same gas that would later be seen to contain the bubble-blowing black holes in our nearby universe. We looked for the wide, fuzzy smears of X-ray photons from this hot material that had been captured serendipitously in the ROSAT data archives. Once we found these features, we inspected their locations using earthbound telescopes to detect and count the galaxies that might be there. Clusters are as their name implies: their galaxies group together, making a crowded patch on the sky. Many X-ray smears turned out to be just stars or galaxies superimposed by chance, but others turned out to be the real deal: vast collections of galaxies and dark matter in a great gravitational equilibrium.
As time went by we pushed ever farther out into the cosmos, finding these close-knit galactic communities at greater and greater distances. The ultimate prize we sought was to use these objects as a surrogate set of scales for weighing the whole universe: we literally wanted to measure the mass of the cosmos. It was an ambition that many astronomers had long pursued, and in essence the idea is simple. The more mass the universe contains, the more quickly galaxy clusters should appear to grow. Most of that growth should also take place more recently, within the past few billion years, if the universe is chock-full of matter. This means that in a weighty universe we might expect to find very few, if any, clusters, at great cosmological distances. Conversely, in a universe that contains less matter, our measures of cosmic distance and time are different, and the growth of galaxy clusters appears as a weaker and more prolonged affair. By finding lots and lots of clusters, we aimed to refine the statistics and to narrow down estimates of the total content of normal and dark matter in the universe. In doing so, we would arrive at a comparison between science’s most fundamental cosmological models and nature itself.
For all of us it became a bit of an obsession. Ebeling, Jones, Perlman, and I seldom went a day without dealing with some piece of the puzzle. After a time we were also joined by Donald Horner, a hardworking graduate student from the University of Maryland. Together we’d pore over the output of the computer algorithms that sniffed through the mountains of X-ray imagery. We’d argue about what looked real and what didn’t, flinging printouts of visible-light pictures of what might be galaxy clusters at each other. Then periodically we’d go off to big telescopes to finally nail down our very best candidates, sitting bleary-eyed through long mountain nights in Arizona, Chile, and Hawaii. It wasn’t a process that wrapped up quickly, especially since pushing our observations to find ever more distant systems was paramount. There is enormous leverage in distinguishing between different cosmological models if you can see how rapidly clusters form in the young universe. Finding even a single cluster at earlier and earlier cosmic times can make or break certain theoretical scenarios. It’s like finding the fossil remains of a feathered dinosaur millions of years before you expect to. Eventually these rare and unique cases might force our ideas about evolution to change.
But galaxy clusters have some tricky characteristics. While I might describe them as “objects” in space, the truth is that they’re not like planets or stars. Those bodies are highly self-contained and have a quite well-defined moment at which they finish their formation. A star is a star when its fusion engines are fully started. A planet is a planet when it ceases to accumulate a noticeable amount of material. But a galaxy cluster is a great amoeba of gas and stars, hanging at the intersections of a larger webbing of structure that extends for hundreds of millions of light-years. Matter accumulates almost continually from the surrounding universe, heating as it falls faster and faster inward. Optimistically, we might consider a cluster complete once its constituents reach a state of physical equilibrium. Hot gas will sit quietly in the deepest regions of its gravitational well once the forces of pressure and gravity balance out, and galaxies will remain within the system once their orbits are established. But we know that nature can be awfully messy. The gas cools; black holes throw out energy; incoming matter slowly adds to the system. The precise moment at which a cluster “becomes” itself is therefore open to some interpretation. In astronomy, a field so utterly dependent on observation, nothing beats going and looking. So to answer questions about the baby steps of galaxy clusters, the very best thing would be to find these infants in the act of growing up. And this was where serendipity would rear its head.
Late in the summer of 1999, I made the long journey from the United States to an astronomy conference on the volcanically hewn Greek island of Santorini. It would be tedious to go into how great a place it was for a bunch of sunlight-deprived scientists to hang out, but it was a special thing to be in such dramatic surroundings to discuss the latest science. Also attending was an old colleague, the English astronomer Ian Smail from Durham University in northern England. Smail had made his name chasing some of the most distant, and hence youthful, objects in the universe. In the latest advance, he and his collaborators were making use of an intriguing and rather new type of astronomical camera. Unlike typical devices for making images of the sky, this camera operated in a netherworld of the electromagnetic spectrum. Between infrared wavelengths and the beginnings of microwaves is a spectral region known as the submillimeter. It’s a notoriously tricky regime. At slightly higher energies and shorter wavelengths we can treat light as bouncy photons, trapping and focusing them with our mirrored telescopes. At slightly lower energies and longer wavelengths, we have to treat it as waves that require antennae for detection. Lurking in between is the realm of the submillimeter. It’s a slippery beast. (Electromagnetic radiation in this range can penetrate through a person’s clothing before reflecting off the outer layers of our skin. Because of this, it is a critical component of some of the machines used to scan your body for concealed items when you go through an airport screening. The stealthy submillimeter photons allow a discreet amount of electromagnetic frisking.)
For astronomy, it’s also a regime where our Earth’s moist atmosphere is full of potentially confounding noise from the wiggling energetics of water molecules. Submillimeter radiation from cosmic sources is mostly absorbed and overwhelmed by this curtain-like barrier. There are only certain spectral “windows” that are clear enough to look through, and less than half a dozen places on Earth where the environment is dry and dark enough for us to have a hope of peering into the submillimeter universe.
Nonetheless, new technology had produced a camera that could now make astronomical images in this tricky spectral range. The particular device that Smail and his colleagues were using was on the 2.5-mile-high peak of Mauna Kea in Hawaii, attached to the appropriately named James Clerk Maxwell Telescope. It was well known that out in the nearby universe, cold rich dust and gas was an excellent source of submillimeter radiation. The thickly blanketed and mysterious environments of star- and planet-forming nebulae, or dust-rich nearby galaxies, were perfect targets. But Smail and his group were interested in places far, far removed from these.
He and his group wanted to push back into the deep history of the universe, and the submillimeter spectrum offers a unique vantage point. As light traverses the cosmos, its wavelength gets stretched. Our universe is expanding, and spacetime is inexorably swelling. Galaxies far enough apart to have minimal gravitational interaction are flying ever farther away from one another, like raisins in an endless sea of rising yeasty dough. The very tissue of the universe that light must travel through is widening, and a photon that was once at an ultraviolet wavelength can arrive on the other side of the cosmos as one of visible light. A photon that began its journey 10 billion years ago as a blip of infrared energy will arrive as a little wave of submillimeter radiation. Incredibly, Smail’s collaborators were finding faint mounds of this cool radiation on the sky that consisted of the cosmically stretched photons of infrared light coming from the rich, dusty, gaseous mix at the very birthplaces of galaxies and stars. These were objects, perhaps protogalactic structures, whose light had taken more than 10 billion years to reach us. As we sat basking in the eight-minute-old photons of brilliant Greek sunshine, we talked excitedly about their most extraordinary finds.
They had taken images of locations that were already known to be strong sources of radio waves in the youthful universe—precisely the kind of objects associated with supermassive black holes in our own more mature cosmic neighborhood. In these places they were discovering great regions of submillimeter emission, tens of thousands of light-years across. These were the hallmarks of clouds of dust that were tens of millions of times the mass of the Sun. They were heated by the intense radiation of newly forming stars, and perhaps something else, shrouded inside. Some of these dusty places were also grouped and clustered together. The statistics used to come to this conclusion were a bit threadbare, but solid enough to take a chance on. It looked like they could be the toddlers that were going to grow into the overfed hulks of galaxy clusters found in our modern universe.
Smail and his colleagues were keen to determine whether or not there really were supermassive black holes lurking in and around their distant sightings of warm material. Such behemoths would also help heat up the masses of dust that were producing the submillimeter light, and would be critical ingredients to understand. I was keen to find out whether these really were the locations of youthful galaxy clusters, the beginnings of those great cathedral-like structures of stars, gas, and dark matter. If they were, then perhaps we could learn something of those structures’ initial building blocks.
There was an obvious way to pursue all these goals, and that was through an X-ray telescope. Only the most energetic and penetrating X-ray photons stood a chance of drilling out through the dusty cloak surrounding a massive black hole in such a place. And the fog-like X-ray emission of hot gas getting trapped inside a growing cluster’s gravitational bowl was also the best way to measure that great warp in spacetime. We clearly needed to look for X-rays from one of these submillimeter mysteries, and luckily our timing was good. Chandra, NASA’s high-performance, $2 billion X-ray space telescope, had just been launched a few months before I met with Smail in Santorini. It was the ideal instrument to chase these distant objects. We just needed to choose the best target. It wasn’t a hard choice to make: it had to be the brightest of the distant and dusty blobs, with the uninspiring name of 4C41.17. This mysterious form was a mind-boggling 12 billion light-years away.
It was a long shot that we’d be able to detect anything, but it was a tremendously exciting plan. If we succeeded it would be the most distant detection of this kind of structure yet made, and it really seemed that it was within our grasp. We finally got our chance when we were given the go-ahead after two years of nail-biting anticipation. In late September of 2002, Chandra settled into position to point at our unfathomably distant target. For a total of 150,000 seconds, or about forty hours, it captured and counted X-ray photons streaming in from the universe. Most of these photons were the equivalent of noise on a radio or the fuzzy speckles in an image of a badly tuned TV. X-ray photons, like any other electromagnetic radiation, can traverse the universe. They come from all over; from stars, neutron stars, black holes, great shock waves of supernova explosions, and the hot gas of galaxy clusters. It’s a cosmic forest full of rustling leaves. But in among this barrage of random bits and pieces was a copse of noble trees. A grand total of about 150 of these photons had traversed 12 billion years of cosmic time in a direct path from our mysterious 4C41.17.
* * *
And so here again is the scene where we began, with pixels on a screen. In this case, from a display in the middle of my desk covered in scattered papers and coffee stains in my office in New York, these particular pixels formed an image. They conveyed the message that Chandra, high above the Earth’s surface, had indeed obtained our precious cargo of data. For the last couple of days it had stared at the sky as it silently circumnavigated the Earth. The finest mirrors and instruments that humans could produce had pointed toward a small patch of the cosmos, close to the constellation of Auriga—the Charioteer. In this direction the glorious view across the bows of our Milky Way galaxy took us all the way to 4C41.17, in the deep cosmic past.
It was mid-morning on the island of Manhattan, and the sound of traffic echoed up through its canyons of rock and steel. I stared at the picture on my screen and squinted at the noisy spread of pixels. This was a preliminary view, before the data were properly processed and massaged to remove spurious features. Fast-moving particles like electrons and protons had ripped through Chandra’s frame high in orbit, spewing energy into its sensitive digital camera. This was nothing unusual, just an occupational hazard of space-based astronomy. But there was a shape in that mess of pixels, and I could see it clearly: a pinpoint of X-ray light, along with something else. I sent the image to a printer down the hall and trotted after to grab the hot paper as it spilled from the machine. Subdued by the heat-fused ink, the noisy features in the image were dissipated. There in stark relief was the extraordinary light of something unknown, a thick streak of brightness poking out from either side of a spot of intensity. It looked like dragonfly wings attached to a compact little body, an entomological picture from a long-forgotten era.
My delight at finding something crazy and interesting in the data soon turned to puzzlement. At the center of the image was the bright pinpoint of X-ray light, and this was fairly easy to explain. Its spectrum had the fingerprint of intense X-ray emission from around the accretion disk of a supermassive black hole, buried somewhere inside the thick dust that we knew existed in this system. That problem was solved—there was no doubt now about the presence of a monster in the midst. But there was this other mysterious stuff: the thinly spread wings of light. Translating their length on the image into real distances revealed that altogether they spanned over three hundred thousand light-years from end to end. If this was the X-ray light from hot gas, heated as it flopped into a baby galaxy cluster’s gravity well, then it would also have a very particular flavor—that of the bremsstrahlung radiation from hot electrons we encountered before. The spectrum of X-ray photons would obey a particular pattern, and there would be lots of lower-energy photons and few higher-energy ones. Instead, as I wrestled with the data, I found a much more egalitarian spread of energy. That was all wrong—this meant it could not be coming from just hot gas. That wasn’t the only thing that was puzzling. If I computed the total power of this radiation, it was a hundred times greater than the X-ray emission of a normal galaxy cluster. This was also at odds with the radiation originating from hot gas in a baby system, yet here was a vast cloud of something merrily pumping out X-ray photons. I sent a worried message to Smail: things appeared funny, off-kilter, and I didn’t understand.
I knew about hot gas in galaxy clusters, but not enough about strange structures emanating from what had to be a massive black hole on the other side of the visible universe. I started poring over articles in astrophysical journals and cautiously bringing up the data with other colleagues. A few ideas bounced around. Then I noticed two papers that helped shed light on the problem, literally. One of them was recent and written by Dan Schwartz, an astronomer at Harvard. The other one was written in 1966, by Jim Felten and Philip Morrison, two physicists then at Cornell. Felten had played a pivotal role early on in recognizing that galaxy clusters could emit X-ray radiation from their hot gas, and Schwartz was an expert on black hole jets and X-ray astronomy. Despite the time span between these two works, they had something critical in common: both papers talked about the astrophysical manifestations of a phenomenon that I dimly remembered from my undergraduate physics classes, which had the rather nondescript name of “inverse Compton scattering.” I soon realized how important this was in explaining our mysterious object in the distant universe.
In 1922, the American physicist Arthur Compton had discovered that X-rays could bounce off freely floating electrons and change their energy, or wavelength, in the process. In essence, this phenomenon is simple. Particles like electrons can interact with photons. If an electron is just sitting quietly and a photon comes zooming along and bounces off it, like a pebble off a rock, then some of the photon’s energy will get transferred to the electron, moving it slightly. The photon will carry a bit less energy afterward, shifting to a lower frequency, and the electron will gain a little motion. But if the electron already has a lot of energy, then the outcome is different, and it is the photon that stands to gain.
Out in the cosmos, there are phenomena that can accelerate particles like electrons to huge velocities that are significant fractions of the speed of light. The jets of matter squirting from black holes are an excellent example. These particles carry an exceptional amount of energy of motion, or kinetic energy. Relativity also tells us that the apparent mass of these particles will increase with speed, and that just further boosts the energy they represent—like a sponge getting more massive as it soaks up water. In this case, when a photon happens to bounce or scatter off one of these speedy electrons, then the inverse effect can occur. The fast and hot electron gives up some of its energy to the photon, which is a process known as “inverse Compton scattering,” and the photon emerges a new, more energetic beast.
The cosmos is filled with photons. We saw this in our map of forever. Many of these are cosmic microwave photons left over from the early universe, and they make up a large fraction of the background electromagnetic soup of the universe. This means that if fast-moving electrons are generated by a phenomenon like a black hole jet, there’s a pretty good chance that as they zip along they’ll encounter the photons in the cosmic soup and give them a boost of energy. Incredibly, if the electrons move fast enough they can boost a photon from the microwave domain all the way up to the X-ray and even gamma-ray domain. That’s like taking a cup of coffee and boosting its temperature high enough to drive the steam turbines of a power plant.
Here was a way to generate X-ray photons with just the spectral signature that I was seeing. All I needed were fast-moving electrons and lots of low-energy photons. It already looked like a supermassive black hole lived in the system. This could certainly be spewing out electrons in the form of fast-moving jets. Indeed, 4C41.17 was a potent source of radio emission, just as one would expect from spiraling relativistic electrons as they splashed into the surrounding universe. I knew that Smail had a map of the radio emission. In it there was a central bright point, and there were two possible dumbbell-shaped zones, almost in line with the wings of X-ray light. It was a good bet that fast-moving electrons were filling these regions.
But what about the supply of those low-energy photons? In the present-day universe there are about 410 cosmic microwave background photons in every cubic centimeter of space at any given time. We found these earlier on, seeping out of our hypothetical sack full of universe. The problem I was now facing was that this was nowhere near enough to account for the huge output in energy we were seeing. This simply didn’t represent enough photons to be boosted to an appreciable number of X-rays. I twiddled my thumbs and stared out the window, trying to imagine myself in that distant place, and the pieces suddenly started fitting together.
We already knew that the region we were looking at had a great output of lower-energy infrared photons from hot dust. That could certainly contribute to the reservoir being boosted to higher energies. But there was something else that I’d overlooked, and it was a huge contributing factor—indeed, it made all the difference. It hinged on the fact that the universe itself was very different 12 billion years ago. At that epoch it wasn’t yet 2 billion years after the Big Bang, and back then the cosmos was a much smaller place. Spacetime was quite literally more compact; there was much less space between everything. The distance between the baby forms of galaxies was certainly growing, but it was less than a quarter of what it is today. This also meant that the cosmic background photons had not yet been stretched as much as they would be over the next 10 billion years. At that stage in the history of the universe they were almost five times more energetic than they are today, their little wavelengths that much smaller. If I combined these factors I found that these cosmic photons had provided an electromagnetic ocean around 4C41.17 that was more than five hundred times richer than it is now. It was quickly clear that this could be the answer! The thick sea of photons would bounce and scatter off the electrons pouring out from around a black hole, get boosted in energy, and would light up the region with X-rays. It was like shining a searchlight into a fog—the volume of the beam itself would glow with scattered light. The only difference was that this was a beacon we could see across the known universe, and it would only get more efficient the further back in cosmic time we went.
This meant something else, too, something wonderful. If this was indeed the birthplace of a galaxy cluster, then we were witnessing a central black hole blowing bubbles just as its descendants do in the present-day universe. Except the bubbles were not dark voids, they were lit up as they boosted photons into the X-ray band. Push the cosmic clock back far enough, I realized, and you could invert the color scheme of bubbles in a structure, and a negative could become a positive. It was a beautiful and elegant manifestation of basic, fundamental physics. I called Smail in the U.K., bursting with excitement. We might not have seen the hot gas of a baby galaxy cluster directly, but we’d found the stuff inside that gas. We’d found the glowing bubbles driven by the massive heart of a black hole.
Now there were some big questions that we had to tackle. We wanted to know exactly what was happening in this extraordinary environment. Our measurements told us there was lots of warm dust. Altogether this dust represented a hundred million times the mass of the Sun and was made up of tiny, microscopic grains. It was spread out across a hundred thousand light-years. Something was heating it up, perhaps scores of young bright stars, perhaps the supermassive black hole as it wolfed down more matter. The hole was squirting out relativistic particles and inflating bubbles within an unseen medium of gas, and this gas was being gathered up in the growing gravity well of the system. But it also had to be getting dense and cool enough to produce all the big and short-lived stars that were in turn making all the dust, and it had to be feeding the black hole. We were missing a crucial piece, something that would tie it all together, something that would show us exactly where the rest of the matter was in this system.
A few weeks went by and serendipity raised its head again. This time it was in the form of a chance meeting with the Dutch-born astronomer Wil van Breugel, from the Lawrence Livermore National Laboratory and the University of California. Van Bruegel’s specialty and passion was tracking down the most distant and massive galaxies. He also happened to have access to the two great Keck telescopes that are perched on Mauna Kea in Hawaii. At an altitude of thirteen thousand feet, these enormous hunks of steel and glass could gulp down photons from across the universe. They were the perfect tools for capturing visible light from ancient structures. Mentioning our investigation provoked a strong response in van Breugel, who told us he had something we’d want to see.
Wil van Breugel and his colleagues had used custom-built light filters to sniff out the photons that came from very specific changes in the energy hierarchy of electrons within hydrogen atoms. Like any element, hydrogen has a number of electromagnetic scents, and this was one of the key ones. If you could hold these atoms in your hand, they would glow with a distinct ultraviolet light. Place them across the universe, though, and the expansion of spacetime would stretch that light out to visible wavelengths. To exploit this, Wil van Breugel’s special filters were tuned to catch precisely these photons from various cosmic objects whose distance was already known. What he showed us took our breath away.
He and his colleagues had captured our object, 4C41.17. It had taken one of the Keck telescopes, with its more than thirty-foot-diameter mirror, over seven hours of exposure time to produce an image. That alone was quite an achievement, but it was the view that stunned us. There was the hydrogen gas, recovering from some as yet unknown buffeting, cooling off by emitting photons of ultraviolet light. It was a colossal structure, and in its center were bright clumps and specks, each thousands of light-years across. But they were inside an even larger canopy, stretching out across the same space that our X-ray light came from. And that canopy had sweeping shapes and forms, cusps and spurs of light. What looked like a huge band of dust seemed to obscure part of the hydrogen gas, as if a belt were tightly adjusted around an overflowing midriff. There was an hourglass shape to the whole thing, hundreds of thousands of light-years across. It gave the impression of matter both coming and going, flowing inward, but also being propelled outward. It was a picture of a tempest from 12 billion years ago. Smail and I knew that we now had the other piece of the puzzle. We just had to put it all together.
Figure 15. The incredible structure of warm hydrogen gas seen around the distant object 4C41.17. End to end, it spans over three hundred thousand light-years. Near the center great bands of darker, dust-rich material seem to wrap around like a belt. The gas coincides with the spread of X-ray light seen by the Chandra Observatory. Also a source of radio emissions and intense infrared light (seen as submillimeter radiation), this remote place is a gigantic, busy construction site for a galaxy and its billions of stars.
We needed to form an accurate mental picture of this distant, primordial environment. A few phone calls later, and over in the city of Leiden in the Netherlands, one of van Breugel’s collaborators, Michiel Reuland, found himself tasked with making a visual representation of all our data combined. It was like constructing a painting in translucent layers, each representing a different part of the electromagnetic spectrum, and each overlapping with the others. Experimenting again and again with different color schemes, and by morphing the maps into smooth and simplified forms, Reuland finally came up with a portrait.
It wasn’t pretty. In fact, with its artificial neon colors and overlapping shapes, it looked like a colossal mess. I nicknamed it “cosmic roadkill.” But despite the aesthetic issues, it was wonderfully informative. Here was just about everything we knew about this remote object in one single view. In the core was a thick shroud of obscuring dust, tens of millions of times the mass of our Sun. It betrayed the intense formation of new stars and new elements hidden within. The vast spread of ultraviolet light from hydrogen gas looked like an unfortunate spill of curdling milk, splattered across three hundred thousand light-years. On a similar scale was the fearsome X-ray glow from cooling but still relativistic electrons as they powered up cosmic background photons and the infrared photons from the warm dust. This X-ray light seemed intertwined with the ultraviolet glow of gas. In a line following the main axis, the main thrust of all this radiation and matter were the dumbbell clouds of radio wave emission. Here the very highest-energy electrons were flowing and corkscrewing outward, glowing with the synchrotron radiation that we see in today’s systems. They were still quite fresh, just a few million years old. Their origin could only be from a young supermassive black hole at the very core of this chaotic space. And unseen, but hinted at by the very edges of these structures, there had to be an outer cocoon of gas cooling and flowing inward from the cosmic web.
Particularly intriguing to me was the transfer of energy from the jet-driven particles of the black hole to the cosmic photons in order to produce the glowing X-ray forms. Some of this inverse Compton energy had to be absorbed by the very same hydrogen gas that we were seeing all across the structure in van Breugel’s images. That energy could help heat and strip electrons away from this material, ionizing the hydrogen gas and slowing down its cooling. This would in turn put the brakes on the condensation of raw material into stars. As the atoms sought to regain those particles, they would glow with ultraviolet radiation when the captured electrons settled back into the atomic energy levels. This was a new mechanism for a black hole to reach out and tweak the cosmic structure. It would only happen in the youthful universe, where spacetime was still compact enough for the soup of photons to efficiently seed this transfer. If this was correct, then we had found a new way for black holes to sculpt and mold the world around them, to disrupt the formation of new stars and structures.
This evidence, together with the incredible mechanical effects of black hole–inflated bubbles, indicates that black holes play a crucial role in the evolution of cosmic structures across time. They regulate the production of new stars in the giant galaxies of today’s universe, and have likely done so throughout cosmic time. And in the very distant past, they could limit how big those galaxies could ever hope to grow in the first place, acting like frustrated farmers trying to keep their weeds under control. What we saw in 4C41.17 was the turbulent youth of a giant galaxy in the beginnings of a great cluster, and despite all the young stars pouring out radiation, and the extraordinary shroud of stellar dust from their brief lives, at the core of it all was still a point of infinite density and relentless force. Because of it this galaxy could never grow beyond a certain size. The more matter fed into its core, the more food there was for the black hole, and the more the black hole would pump out disruptive energy. The X-ray glow of cosmic photons that had been boosted by this energy was as luminous as a trillion suns, and could easily be the tipping point for stemming the growth of the galaxy. It could explain the difference between the number of stars we thought the universe should make in these places and the number that it actually does.
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After the rush to analyze and then report our findings, there was enough of a lull to meditate on what we had discovered. While our interpretation seems reasonable, there is always room for future investigation. Regardless, the most remarkable aspect for me was my own shifting sense of what a massive galaxy in the young universe might be like. The mental picture that I’d held up to this point was rather static. Surely something like a galaxy would form serenely and majestically from the gentle condensation of material out of the pristine sea of hydrogen, helium, and dark matter? Of course, we knew that there were quasars out there. They were chewing up matter voraciously and spitting out enough energy to change the balance of atoms, ions, and molecules in the young universe. But they were still quite sparse, and half a century after we’d begun to understand their physical nature, they continued to seem a little disconnected from the galaxies that hosted and fed them. Yes, we also knew that there were generations upon generations of new stars in those early eons. A vigorous amount of stellar birth, life, and death was pumping raw elements into the cosmos. These new components were condensing out as the massive clouds of dust that shrouded so much of the first stellar light, hiding it away from us. But despite all this, the great galaxies still felt like noble and vast castles, steady and reliable.
By stark contrast, in the dull name of 4C41.17 we’d looked into a maelstrom. In a billion or two years this would indeed be another giant galaxy, almost certainly an elliptical fuzzball of hundreds of billions of stars. But we were witnessing a stage where it was a pit of seething radiation and particles, much of that driven by the thrashing forces of a growing supermassive black hole. As much as gravity was trying to draw the galaxy together, it was being resisted by this outflow of energy. Clearly, that resistance was ultimately going to be futile, or else the local cosmos of today would be a very different place. This is part of the very same problem facing our theories and simulations of the growth and evolution of cosmic structures, the simplest of which predict far more stars than we actually find in the universe. What we see in 4C41.17 is a direct clue to the way in which nature limits and restrains the growth of the most massive galaxies. The energy generated by the comparatively microbial specks of the black holes in their cores helps hold them in check.
There was one more thing that this extraordinary colossus on the other side of the universe had to reveal to us, and it came from the environment it occupied. When Smail and his colleagues had taken their original deep images in the lurking submillimeter part of the electromagnetic spectrum, they had found something else. Around the distant radio-emitting galaxies like our 4C41.17, there would often be other glowing mounds that were visible in the pictures. In keeping with the grand scheme of a cosmic web of matter in the universe, these young agglomerations of mass tended to cluster together. See one and you were a little more likely to see another nearby, and another near that, and so on.
What this meant was that in any good X-ray image targeting one of the distant dusty places, there would likely be others. If they harbored supermassive black holes, then these too might show up as bright points of X-ray light as those energetic photons burrowed their way out through the thick, dusty shrouds. Again, Smail and I picked through the data we had for 4C41.17, as well as examining images of two other structures that were also snapshots of young systems, just a couple of billion years old. To our surprise, not only did we find points of intense X-ray emission apparently inside the dusty shrouds of the young galaxies, but in some cases they came in pairs: twin pinpoints of light next to each other. In one, at the hairy edge of uncertainty in among noisy data, there was evidence of even a triple arrangement—three points of X-ray light peeking out. They were tiny clusters of photons on the screen, just edging into statistical believability, but what might elicit a fatigued shrug from other scientists can excite an astronomer to sleepless nights. Luck and hunches play a huge role in the exploration of the universe.
We realized that we were seeing supermassive black holes that might have originally belonged to separate youthful galaxy structures, stellar teenagers adrift in the young universe. These wayward creatures were now merging and coalescing, setting off frenetic waves of star formation and pumping out dust. Through all this, our X-ray images were penetrating down into their hearts and cores. We know that pairs of supermassive black holes are not something we see much of in today’s universe. Only some 4 percent of galaxies are thought to harbor multiple giant black holes. This meant that the ones we had found 12 billion years in the past were likely en route to merging with each other, eventually hiding their origins within a single event horizon. Such arrangements had been posited for some time as a way to smooth out and reorganize the orbits of stars in galactic cores to better match what astronomers were seeing in the nearby universe. Two orbiting black holes are a dynamical blender, reshaping the paths of stars around them.
Given a little more time, perhaps a few million years, these distant giants would find one another through their gravitational pulls. Swirling around as an ever faster orbiting pair, they would eventually combine in a crescendo of gravitational radiation, sending ripples in spacetime ringing out across the universe. It looked like this was clear evidence for one route to growing supermassive black holes in the universe: simply have them eat each other. In doing so, they should also leave their fingerprints on the galaxies that host them, disrupting and rearranging the motions of stars around them, leaving an extremely important set of telltale crumbs around the cookie jar.
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Our distant colossus and its brethren represent the extreme end of the galactic flora and fauna. They are the giant trees in the richest parts of the cosmic tropical rain forests. In these environments there is now little doubt that supermassive black holes have played a major, likely dominant, role in sculpting the forms that we see. Twelve billion years ago, and even earlier, they served as regulators and law-keepers to stem the flood of new stars as matter cooled and condensed. Since then they have continued to hold matter at bay. The great bubbles inside clusters have mixed together elements and steadied the transformation of raw hot gas into new stars and planets. But this happens in synchronization with the inflow of that matter, an astonishing symphony of feedback and balance. These great systems breathe in and breathe out.
Elsewhere, off in other galaxies, supermassive black holes are also making their presence felt. But in these other copses and clumps of galactic trees and shrubs, the interplay between construction and destruction is more complex. We happen to find ourselves living in a very large spiral galaxy, the Milky Way. It’s an interesting terrain, neither a backwater nor one of the universe’s greatest cathedral-like structures—the giant elliptical galaxies within clusters. It’s natural to wonder what influence black holes have had on this place, and what role they might continue to play. This brings us to the final part of our story, the search for the origins and nature of our own galactic environment and perhaps even of life itself.