Gravity's Engines: How Bubble-Blowing Black Holes Rule Galaxies, Stars, and Life in the Cosmos

by Scharf, Caleb

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  Our conclusion is that the very first black holes—large and small—can leave an imprint on all subsequent stellar generations and galactic environments. The production of new elements and the opportunities for planetary systems all hinge on these early effects, as well as on the long-term behavior of galaxies and the black holes they contain. But not all places are created equal, and most stars that are in the cores of places like galaxy clusters are quite old now. Whatever elements their dying siblings managed to spit out into the void are dispersed in the hot intergalactic gas of these vast gravitational crucibles. Very little of it is recycled back into a state from which it can become new stars or planets. Black holes bear great responsibility for this situation. Ten billion years ago, they restricted and limited what was an explosive growth of stars and elements. Since then, they have continued to hold matter at bay. The multiplicity of black holes that we found in the great dusty mountains of submillimeter-emitting material from 10 billion years ago tallies with their early formation inside the merged splatters of baby galaxies. It also tallies with a picture in which massive black holes often merge with one another, leaving telltale signs in the way that stars are spread across galactic centers. On a much smaller scale, we see aspects of the same behavior in individual galaxies. Those with great swarms of old central stars harbor the most massive black holes. These galaxies are also limited in how many new stars they have been able to make over the past few billion years, and where they could make them.

  Some of these places must present a much less fertile terrain, given what we know about the cosmic requirements for life. They may be poorer in condensing elements, and are probably poorer in fresh stars with pristine new worlds. But is this true? Are these locations really unfavorable places for life? The challenge we face is that we only have a single example to serve as context: for now, we know only one planet like Earth. But I would argue that this information still lets us learn something fundamental. We exist in a specific place at a specific cosmic time, in a particular part of a particular galaxy in a particular type of region in the universe. Since that environment is part of the conjoined evolution of black holes and their host galaxies, we can ask what special things link us directly to that history.

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  ORIGINS: PART II

  Our existence in this place, this microscopic corner of the cosmos, is fleeting. With utter disregard for our wants and needs, nature plays out its grand acts on scales of space and time that are truly hard to grasp. Perhaps all that we can look to for real solace is our endless capacity to ask questions and seek answers about the place we find ourselves in. That is not such a bad thing. Ignorance is far scarier than knowledge. One of the questions we are now asking is how deeply our specific circumstances are connected to this majestic universal scheme of stars, galaxies, and, of course, black holes. Given that we now see how the origins of black holes and galaxies are intimately linked, and how the subsequent evolution of both is tied together, it’s not unreasonable to ask what we might owe to these pinhole punctures in spacetime. Fortunately, we live in an era where we can begin to answer that question.

  Our gloriously vast universe contains at least 100 billion galaxies. Generations of careful observation, mapping, and extrapolation have gone into producing this estimate. All but a small handful of these great stellar systems are invisible to the naked human eye. Indeed, the tiniest and dimmest systems are in the majority. Dwarf galaxies, miniature versions of the fuzzy stellar swarms that we call ellipticals, are the most numerous systems in the cosmos. They’re incredibly hard to spot, though, since they can consist of as few as a couple of million stars and be only a few hundred light-years across. They’re so faint that they vanish out of sight for all but the most persistent and well-equipped observers. The big, more easily seen galaxies are broadly divided. The great disks of spirals are almost always large. They can span hundreds of thousands of light-years, and they can contain a trillion stars. Away from the intense environment of galaxy clusters, they represent more than 70 percent of all large systems. Ellipticals can also be huge, but in number they amount to only about 15 percent of all large galaxies.

  We are part of this great intergalactic jungle, and to finish my argument about the relationship of black holes to life in the cosmos, I’m first going to dig deeper into the story of our own very particular watering hole.

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  The Milky Way itself is a big system, even by the standards of spirals. Its 200 billion stars amount to a mass approximately 100 billion times that of our Sun, and its disk stretches across a diameter of 100,000 light-years. Our parent star and our home planet are positioned toward the outer edge of this vast plate, although by no means at the edge of the matter it contains. The visible stars represent just one aspect of a slowly rotating, center-orbiting wheel of dust, gas, and dark matter. Every 210 million years, we complete another circumnavigation of the Milky Way. Since the Sun formed more than 4.5 billion years ago in a long-dissipated clutch of other new stars, we have made this galactic round trip just over twenty times.

  Our biggest neighbor is the Andromeda galaxy, separated from the Milky Way by a gaping void of 2.5 million light-years of intergalactic space. Our eyes see only the barest hint of a hazy patch at its location. In truth, its light is spread out across the sky in a great band some six times the size of the full Moon. It is a giant spiral, but it is quite different from the Milky Way. While our galaxy is still actively producing a few new stars every year, Andromeda has descended into late middle age. It’s not without baby stars, but they are forming at one-third to one-fifth the rate that they do in the Milky Way. Andromeda’s central cloud of very old stars is also far more prominent than that of the Milky Way. Nestled inside this central stellar hive in Andromeda is a black hole 100 million times the mass of our Sun. Just as in most other galaxies, this hole is one-thousandth the mass of the old stars surrounding it.

  Figure 16. The spiral galaxy known as NGC 6744, widely considered to be a close match for the structure of our own Milky Way galaxy. It is 30 million light-years from our location.

  In 4 billion to 5 billion years, the curved spacetime containing the masses of Andromeda and the Milky Way will cause them to merge. In fact, they’ve already started falling toward each other. Although this encounter will happen at a velocity of more than a hundred miles a second, it will not be a collision in the traditional sense of the term. There is so much space between the tiny points of condensed matter in stars that the galaxies will simply drift and flow into each other with little violence. Exactly how intimate this vast embrace will be is unclear, and it will play out over hundreds of millions of years. But eventually the combined content of these two great systems may settle into something resembling an elliptical galaxy, and Andromeda and the Milky Way will be no more.

  Regardless of the outcome, by the time this slow collision begins our Sun will have used up the hydrogen fuel in its core, which will contract inward as gravity acts against the diminishing pressure in its center. The shrinking interior will get hotter and will flood the upper layers of the solar atmosphere with radiation, inflating them outward. The Sun will grow to a bloated and gouty red-giant star, engulfing what remains of the inner planets, including Earth. Whether or not our distant descendants are still around to witness these events, they will no doubt mark the end of our birthplace. This tiny scrap of rock and water that took life from microscopic single-celled organisms to beings like us in just a few billion years will be erased. But until then, we have a chance to understand what makes the Milky Way tick, and how it compares to all other galaxies.

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  We live in a time of unprecedented cosmic exploration. The tools of modern astronomy are unlike anything else in the history of our species. We have the technological prowess to construct exquisite instruments like the Chandra X-ray Observatory and the Hubble Space Telescope. We can also co-opt the powers of computer automation and global communications to examine previously unimaginable volumes o
f the universe. Indeed, much of the astronomy we’ll practice in the future will involve a degree of rich mapmaking and information gathering the likes of which we have never seen before. Giant new telescopes will sweep the skies, and every few nights they will produce a record of hundreds of millions of cosmic objects, from stars to galaxies. And they will repeat this again and again. They will do for the universe what something as mundane as a security camera does for a city street: constantly monitoring, constantly growing our library of data, and producing a map of the cosmos of ever-increasing levels of detail in both space and time.

  Our early steps toward this new type of astronomy have already begun. One such effort has been the project known as the Sloan Digital Sky Survey. This extraordinary enterprise has surveyed over 35 percent of Earth’s night sky since the year 2000, detecting 500 million astrophysical objects. Sloan’s modest telescope is just over eight feet in diameter, but it scans across swath after swath of the heavens above New Mexico, with its digital camera recording untold cosmic photons. More than a million of the objects it has captured are galaxies, an incredible sampling of the local universe that penetrates 2 billion years deep into cosmic time. But how to pick through such a wealth of data?

  In 2007, a consortium of astronomers launched a project called Galaxy Zoo. The idea was simple, but challenging to implement. The galaxies within the Sloan survey needed to be classified—somehow we needed to assign the correct physical label to all the detected systems in order to extract robust statistical facts about them. The characterizations are familiar: spirals and ellipticals, and subdivisions within these kingdoms. This kind of classification might sound like a straightforward task—surely a computer could be used to “recognize” galaxies. However, nature is tricky, and mistakes made at the level of just a few percent can mess everything up. There is an enormous range of natural variation in structures, as well as confusing quirks—and just plain hiccups in the data. Even very clever computer algorithms can be fooled, especially when you have a million systems to work on.

  Since the advent of telescopic astronomy four hundred years ago, the human eye and brain have proved themselves to be remarkable image analyzers. With just a small amount of training and practice, a human being can distinguish galaxy types with incredible efficiency. It’s like looking at dried flowers or squashed bugs—after a while you can race through samples with little hesitation. There’s a daisy, a lily, a rose, another daisy. There’s a beetle, a fly, another fly, a mosquito—the human mind is a brilliant pattern-recognition machine. The real problem facing the Galaxy Zoo project was the sheer scale of its goals. The scientists wanted to classify a million galaxies; they also required at least twenty duplicate identifications for each possible galaxy, in order to weed out mistakes. Not even a dedicated group of scientists could find the time or perseverance to accomplish this.

  The solution was to harness the power of the human hive, to “crowdsource” astronomy like never before. Soon after it launched, Galaxy Zoo put out a request for volunteers on the Internet. Within a month of its call for human eyes, eighty thousand people had carved out time to look at the million galaxies ten times over. They were scientists, students, bus drivers, retirees, amateur astronomers, kids, athletes, writers, artists, doctors—individuals from all walks of life. It was an amazing example of the joyful and satisfying spirit of cooperation. Just a year later, a staggering 150,000 people had made more than 50 million classifications. The project continues to this day, expanding into the details of galaxies and into new data from the huge archives of the Hubble Space Telescope’s two decades in orbit.

  Huge sets of data like the information gathered through Galaxy Zoo have allowed astronomers to tackle questions that used to be near-impossible challenges. It is like having census data from an entire continent instead of from a few quirky and obscure neighborhoods. Exactly how we will find ourselves interpreting the results will likely play out over the coming decades, but for now we can ruminate on some of the most compelling discoveries. For us, a key one is the link between galaxy properties and the supermassive black holes that they host. At last we have a way to avoid the confusing peculiarities of individual galaxies and to look instead at how they match up against a million other systems.

  We find that there is a significant difference between the supermassive black holes inside elliptical galaxies and those inside spiral galaxies. In today’s least-massive elliptical galaxies, the least-massive black holes are also the most active black holes—they are still eating and producing energy. The opposite is true in spiral galaxies: in these systems it is the most-massive black holes that are producing the most energy. This sounds like a stunning reversal in behavior until we realize that the most-massive black holes in spiral galaxies are in fact about the same size as the least-massive black holes in elliptical galaxies.

  We can interpret this to mean the following. In today’s universe the most-massive black holes are effectively has-beens. It doesn’t matter where they are—most of them have eaten their fill and are certainly not going to light up like quasars again. They are starving. Whatever activity they exhibit is typically modest: enough, for example, to regulate the flow and cooling of matter deep inside a galaxy cluster. The lower-mass black holes, from a few million to a few tens of millions of times the mass of the Sun, are the main players in our surrounding universe. They are still growing, albeit rather gently and sporadically. Thus, the quasars and the great elliptical galaxies have exhausted themselves by leaping out of the starting gate, while the spirals and their more modest black holes have been biding their time. It is the ultimate race between the tortoises and the hares. In fact, some observations suggest that the level of black hole growth we see in these tortoises today is larger than it was a few billion years ago. Only after nearly 14 billion years are they finally hitting their stride.

  Where, then, does our galaxy, the Milky Way, sit among these grand tortoises? The answer reveals something quite profound, but first we have to understand how to get there. When astronomers talk about matter being fed into supermassive black holes, they talk about “duty cycles,” just like the episodic sloshing of clothes inside a washing machine. The speed of a black hole duty cycle describes how rapidly it changes back and forth from feeding on matter to sitting quietly. The periodic distribution of the great bubbles floating up through clusters of galaxies is an excellent example of a duty cycle made visible. Detecting the presence of black holes is far easier when they are “switched on,” and the faster this cyclical behavior, the more black holes you will detect at any instant in a region of the cosmos. It’s like being in a completely darkened room full of very hungry mice. If you toss out some pieces of cheese, the fastest runners will quickly scurry from crumb to crumb, and you will count many of them simply by listening. The slow ones take big pauses between snacks, and you will count far fewer at any given moment.

  The results of surveys like the Sloan and the Galaxy Zoo indicate that this duty cycle is related to the overall stellar contents of a galaxy. These contents are a critically important clue to the nature of a galactic system. The stars in a galaxy can be reddish, yellowish, or bluish; blue stars are typically the most massive. They are therefore also the shortest-lived, burning through their nuclear fuel in as little as a few million years. This means that if you detect blue stars in the night sky, you’re catching sight of youthful stellar systems and the indications of ongoing stellar birth and death. Astronomers find that if you add together all the light coming from a galaxy, the overall color will tend to fall into either a reddish or a bluish category. Red galaxies tend to be ellipticals, and blue galaxies tend to be spirals. In between these two color groups is a place considered to be transitional, where systems are perhaps en route to becoming redder as their young blue stars die off and are no longer replaced. With nary a sense of irony, or indeed color-mixing logic, astronomers call this intermediate zone the “green valley.”

  Surprisingly, over the past billion years it is the largest g
reen valley spiral galaxies that have the highest black hole duty cycles. They are home to the most regularly growing and squawking giant black holes in the modern universe. These galaxies contain 100 billion times the mass of the Sun in stars, and if you glance at any one of them, you are far more likely to see the signs of an eating black hole than in any other variety of spiral. One in every ten of these galaxies contains a black hole actively consuming matter—in cosmic terms they are switching on and off constantly.

  The physical connection between a galaxy being in the green valley and the actions of the central black holes is a puzzle. This is a zone of transition, and most galaxies in the universe are either redder or bluer than this. A system in the valley is in the process of changing; it may even be shutting down its star formation. We know that supermassive black holes can have this effect in other environments, such as galaxy clusters and youthful large galaxies. It might be that these actions are “greening” the galaxies. It might also be that the circumstances causing the transformation of a galaxy are feeding matter to the black hole.

  As we study other spiral galaxies in the nearby universe, we do find evidence that the black holes pumping out the most energy have influenced their host systems across thousands of light-years. In some cases, the fierce ultraviolet and X-ray radiation from matter feeding into the holes can propel wind-like regions of heated gas outward. These wash across a galaxy’s star-forming regions like hot-weather fronts spreading across a country. Exactly how this impacts the production of stars and elements is unclear, but it’s a potent force. Equally, the trigger for such violent output from the central hole can influence the broader sweep of these systems. For example, the inward fall of a dwarf galaxy captured by the gravity well of a larger galaxy stirs up material to funnel it toward the black hole. It is like fanning the embers of a spent fire to relight it. The gravitational and pressure effects of that incoming dwarf galaxy can also dampen or encourage the formation of stars elsewhere in the larger system. Some or all of these phenomena could help link a supermassive black hole to the age (and hence color) of the stars around it.