Black Holes: Supermassive Guardians of the Galaxies
Black holes are the darkest and most mysterious objects in the known universe. In casual conversation, they serve as a stand-in for any unseen force capable of making things disappear. Tell your friends you lost a sock in the dryer or can’t find your keys, and they might well invoke a black hole as the guilty party.
It is easy to understand why black holes have this reputation. After all, they are so massive and dense that not even light can escape their immense gravitational pull. Any matter that wanders too close is irreversibly drawn past the black hole’s event horizon—the point beyond which it is impossible for anything to escape. The gravitational field of a black hole is so powerful it can bend light and warp the fabric of space and time.
But black holes are so much more than matter-hungry cosmic juggernauts. Although their relatively small size makes them very tricky to study, especially at large distances, black holes hide key details about the nature of matter and the origin of the universe.
“We now know that black holes are a ubiquitous component of galaxies,” says Suvi Gezari, an assistant professor of astronomy at the University of Maryland who tracks the evolution of black holes over time. “These exotic manifestations of strong gravity are so powerful, the physics alone is a compelling area of study. The largest of them most likely shape the evolution of their host galaxies and go through their own distinct phases of development.”
The largest black holes Gezari refers to are supermassive black holes, which can reach 10 billion times the mass of the sun. At least one of these behemoths is believed to occupy the center of every massive galaxy, where they can exert a strong influence on their host galaxy’s size, shape and life history. Those who have seen the 2014 blockbuster movie Interstellar will have some idea of the forces at play, as a supermassive black hole played a key role in that film’s plot. Supermassive black holes have been spotted in several dozen nearby galaxies—including Earth’s own Milky Way.
Smaller black holes exist, but supermassive black holes are particularly tantalizing targets for research, largely because of their suspected role in the evolution of galaxies. These giant black holes are also thought to be the perfect natural laboratories to study gravitational waves and may one day yield further secrets about the nature of gravity.
The more scientists learn about these massive beasts, the closer they get to understanding the origin of the universe and the nature of matter itself. And UMD astronomers are leading the way, with a group of researchers dedicated to finding and demystifying supermassive black holes.
Nowadays, astronomers are quite confident that black holes exist. But that was not always the case. An object that is totally dark is terribly difficult to confirm, let alone study.
The idea of black holes first surfaced when Albert Einstein published his theory of general relativity in 1915. Although Einstein was initially quiet on the subject, his theory implied the existence of “dark stars” in purely mathematical terms: any object with sufficiently high mass and density would be expected to create a staggeringly powerful gravitational field. According to some accounts, the very idea made Einstein uncomfortable, and for decades the concept sat on the proverbial shelf.
Then, in the early 1970s, observational technology caught up with theory. One of the first key discoveries was Sagittarius A* (pronounced “A-star”), a powerful source of radio emissions situated near the center of the Milky Way. For years, astronomers struggled to explain this object, which is now widely accepted to be the Milky Way’s supermassive black hole.
“There’s almost nothing feeding Sagittarius A* at the moment. It’s starving, the poor thing,” says Sylvain Veilleux, an astronomy professor at UMD. “While our galaxy’s central black hole isn’t very exciting now, it was probably a lot more interesting in the past.”
A LIGHT IN THE DARKNESS
Luckily, when black holes consume matter they become active, and a spectacular show ensues. When astronomers search for active supermassive black holes at the centers of distant galaxies, they are looking not for the black hole itself, but for telltale evidence of the black hole’s dining habits.
Active supermassive black holes have a large “accretion disk” of matter—mostly gas—which is drawn toward the black hole.
As the gas becomes more densely packed, it produces friction on an epic scale. This friction sets off some impressive fireworks, emitting radiation across the entire electromagnetic spectrum.
This explosive dance between a central supermassive black hole and its accretion disk creates an active galactic nucleus. The most energetic active galactic nuclei are quasars, which are the brightest objects in the known universe. Finding these objects is not exactly easy, but technology and methodology have advanced by leaps and bounds over the last several decades.
“One of the main areas of my research is to find how many active galactic nuclei are there, in what type of galaxies they are located and what effect they seem to have on their host galaxies. Theorists can take it from there,” says Richard Mushotzky, an astronomy professor at UMD. To distinguish an active nucleus from any of a number of other bright objects in the sky, one needs to rely on a combination of three main sources of evidence. The first is X-ray emission.
“Almost all active galaxies are luminous X-ray sources. Once you get to a certain luminosity—about a billion times that of the sun—the object is always an active black hole,” Mushotzky explains.
Second, there are also key differences between the spectrum of the light emitted by active galactic nuclei and by stars and more typical celestial objects.
“The nature of the light that a quasar emits is very different. If a star emits red, green and blue, you can think of a quasar as emitting maroon, chartreuse and puce,” Mushotzky says.
A third line of evidence is radio wave emission, which enabled the detection and study of Sagittarius A* decades ago. But only some quasars are strong radio sources. Others are referred to as “radio quiet,” making the first two sources of evidence all the more important.
All told, more than 100,000 active galactic nuclei have been identified to date.
Gezari, Veilleux, Mushotzky and other UMD colleagues have had a hand in some of these discoveries. But the hunt continues, and each new discovery provides another chance to ask some big questions. One of the most compelling questions is also one of the most basic: How, exactly, do these giants exist?
AN EXISTENTIAL PUZZLE
Light from the most distant quasars has been traveling for a very long time, meaning that many of these objects existed as early as a billion years after the Big Bang. (The universe is around 13.8 billion years old.) The very existence of supermassive black holes that early raises some huge questions. According to standard model physics, the early universe should not contain any one object—let alone many of them—with that much mass. Simply put, a billion years is not long enough for anything to have grown so big. “It’s as weird as seeing a bunch of eggs that are bigger than the chicken that laid them,” says Gezari. “We see a lot of supermassive black holes out there, but not enough time has passed for them to accrete. There must have been some very big seeds to produce them.”
Much like a water main has a maximum capacity, and thus can only carry a certain amount of water per second, all black holes have a built-in “speed limit” that defines how quickly they can collect matter. Sir Arthur Stanley Eddington first described this limit in the context of massive stars, long before black holes were definitively known to exist, but the concept applies to any body that exerts a strong gravitational field.
This speed limit bears Eddington’s name and is a balance between two competing forces: the gravity drawing matter toward a black hole and the outward radiation pressure generated by this process.
“If we look at quasars that formed not long after the Big Bang, we find objects on the order of billions of solar masses,” says Christopher Reynolds, an astronomy professor at UMD whose work straddles the line between observation and theory. “So it seems the universe might have had a ‘race’ to make supermassive black holes within the first few billion years. It turns out this is quite hard. They would have to grow at the Eddington limit for the entire early history of the universe.”
Such a scenario is difficult to explain, largely because it is hard to model how a stand-alone black hole could be fed with enough matter to maintain the Eddington limit for that long. So, astronomers have looked for other explanations. It’s possible that supermassive black holes are byproducts of galaxy formation, seeded by giant central stars that collapsed and formed smaller black holes. Sitting at the center of a young, turbulent host galaxy might provide such a “seed” with enough raw material to grow bigger and quickly reach the Eddington limit.
Reynolds is leading an effort to simulate black hole seeding as a principal investigator on a National Science Foundation-funded project called the Theoretical and Computational Astrophysics Network (TCAN). With nodes at UMD, Georgia Tech and Yale, TCAN researchers are harnessing the significant computing power available at these institutions to run complex computer models of the accretion process.
“We plan to make models of these huge gas balls to see if they do actually form supermassive black holes,” Reynolds explains. “It’s not a given that they will. The high levels of energy could blow them apart. There are lots of ways the process could go wrong, actually.”
GREATER THAN THE SUM OF ITS PARTS
Some astronomers argue that simple accretion, even for a prolonged time at or near the Eddington limit, still does not adequately account for supermassive black holes. Instead, an alternate theory suggests that smaller black holes formed by accretion at the centers of young galaxies long ago. Then, some of these galaxies merged together, combining their central black holes to form larger black holes. This process continued, leading to progressively larger black holes that eventually reached supermassive status.
Galaxy mergers are known to happen. Combine that with the assumption that every galaxy has a black hole at its center, and this scenario starts to make a lot of sense.
“Small galaxies merge to form bigger ones, so it is reasonable to think that the black holes will merge as well,” says Gezari. “But so far as we know, there is only one supermassive black hole at the center of most galaxies, and evidence for two or more black holes has been difficult to find.”
Gezari and graduate student Tingting Liu recently published a study in The Astrophysical Journal Letters documenting what they believe to be a pair of supermassive black holes at the center of a large galaxy named PSO J334.2028+01.4075. If they are correct, this black hole “binary” most likely represents the final stage of a galaxy merger. The black holes are very close together—closer than anyone has seen before—leading the researchers to suspect that the two black holes are gravitationally bound to one another.
If this is the case, it is likely that the two giant black holes will soon merge to become one—possibly within the next 20 years or so. Confirming a black hole merger would lend a lot of weight to the idea that supermassive black holes formed in the same way, from a series of black hole mergers beginning shortly after the Big Bang.
“Previously, we were only able to look at one picture of the system, one moment frozen in time, like a single snapshot,” Liu explains. Now, with the advent of new data collection techniques, such as the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS1) survey that provided the researchers with their data, Liu says, “we’re looking at a movie of how these systems evolve.”
A GALACTIC THERMOSTAT
Regardless of how they form, it’s clear that supermassive black holes have a close bond with their host galaxies. One of the newest and most exciting areas of black hole research aims to understand the precise nature of this relationship. It is quite likely that these monster black holes are a major influence, strongly modifying the dynamics and evolution of the entire system.
“Galaxies and supermassive black holes are all wrapped up together, a lot like an ecosystem,” adds Mushotzky. “It’s not something you can separate out easily.”
A single, curious observation has driven much of the research in this area: in nearly every galaxy where a supermassive black hole has been observed, astronomers have seen a tight correlation between the mass of the black hole and the mass of the galaxy. The bigger the galaxy, the more massive the black hole at its center.
This is likely not a coincidence. A supermassive black hole’s extraordinary energy output would almost certainly have far-reaching effects on the matter surrounding it. Many quasars have been observed shooting powerful, focused jets of plasma from their centers that reach hundreds of light years outside their home galaxies. Some of these jets are so luminous they can be detected billions of light years away, but it is not entirely clear whether and how the jets might affect the galaxy itself.
Focused jets are not the only powerful energy blasts produced by quasars. In 2011, Veilleux and David Rupke, Ph.D. ’04, physics, were the first to describe winds that carry huge loads of gas and dust out beyond the edges of a galaxy. The study, published in The Astrophysical Journal Letters, showed that these winds act like a cosmic leaf blower, clearing out large amounts of star-forming gas and thus shutting down star formation in the best-studied case to date, the galaxy Markarian 231.
“We have been finding that these winds are common. They’re very important and carry a lot of mass,” Veilleux explains. “In some cases, up to 1,000 solar masses worth of material blows out every year in these winds.”
It was immediately clear that these winds controlled Markarian 231’s evolution and development, and it seemed obvious that the supermassive black hole at the galaxy’s center was responsible. But the researchers wanted confirmation.
In 2015, Veilleux, Reynolds and several UMD colleagues published a paper in the journal Nature that presented powerful evidence to explain this link. The supermassive black hole at the center of a galaxy named IRAS F11119+3257 emits small-scale radiation-driven winds from its accretion disk. These winds, in turn, directly power the galaxy-scale winds that clear out the galaxy’s supply of star-forming gas.
“This is the first galaxy in which we can see both the wind from the active galactic nucleus and the large-scale outflow of molecular gas at the same time,” says the paper’s lead author Francesco Tombesi, an assistant research scientist in UMD’s Department of Astronomy with a joint appointment at NASA’s Goddard Space Flight Center.
“It’s the first opportunity to see the connection between these two phenomena,” Tombesi continues. “It’s also among the first observational evidence to confirm that supermassive black holes can influence the entire galaxy, including the stars and the interstellar medium.”
But this mode of constant, highly energetic activity is not the only way supermassive black holes are believed to control their host galaxies. Theory suggests that black holes might also become intermittently active in short bursts, thus controlling the rate of star formation at a finer scale. These pulses of galactic wind would clear out just enough star-making gas to bring the mass balance of the galaxy back into line before shutting down once again. Astronomers call this process “feedback.”
Much like a thermostat will turn on your home’s air conditioner only when it receives a signal that the rooms are too warm, the black hole would need a signal to sense when it’s time to clear out excess gas and thus turn off the star-forming process. Conversely, the black hole also needs a signal to let it know when the job is done and balance has been achieved once again.
“When a black hole is in maintenance mode, how does the feedback actually work? We don’t yet have a good explanation for how the black hole knows how much energy to put out,” Reynolds explains. “How does the feedback loop get the message to shut down? This is an important question.”
The study of supermassive black holes continues to make huge leaps, and UMD astronomers are helping lead the way. A major technological advance will be the launch of ASTRO-H, a next-generation X-ray satellite telescope built by the Japan Aerospace Exploration Agency (JAXA) in collaboration with NASA. Mushotzky and Reynolds are members of the scientific oversight group for ASTRO-H, which is set to go into orbit before April 2016. ASTRO-H will give astronomers an unmatched view of X-ray-emitting objects throughout the universe, and active galactic nuclei are at the top of the priority list.
“ASTRO-H will have better spectroscopy than ever before, giving us better glasses with which to see,” says Mushotzky. “It will tell us much more about what is happening very near the black hole and how the accretion disk and black hole influence one another. It’s a very exciting time.”
In 2022, the Large Synoptic Survey Telescope is expected to come online in northern Chile. This instrument will survey a huge swath of the night sky and could potentially pinpoint the locations of millions of undiscovered quasars, thousands of which could be powered by binary supermassive black holes.
Further into the future, better instruments could also reveal more about the weirder, more theoretical aspects of black hole physics. Stephen Hawking has proposed the idea that black holes could “leak” radiation and strange, as-yet unknown particles. The very idea flies in the face of the classic notion that nothing can escape a black hole’s event horizon.
Finding evidence for such a phenomenon could provide a long-sought bridge between standard-model physics and quantum physics. The most powerful particle accelerator on Earth, the Large Hadron Collider (LHC), is currently the best tool available to address such questions. It recently began its second run with the goal of producing particles of dark matter. But one day, with the right technology, perhaps supermassive black holes could help expand physicists’ knowledge of matter and energy.
“A supermassive black hole can spit out matter at nearly the speed of light. Black holes might be nature’s most efficient particle accelerators,” Reynolds says. “We’re very proud of the LHC, but nature can accelerate particles at way higher energy than anything we can produce here on Earth.”
Further down the line, Sagittarius A* might yet have another moment in the spotlight. The Milky Way is on a collision course with the nearby Andromeda galaxy. One day, the two supermassive black holes could form a binary much like the one that Liu and Gezari described in early 2015.
“But don’t worry,” Gezari says reassuringly. “We have about 4 or 5 billion years to go before that happens.”
Writer: Matthew Wright
- Agents of Exploration: A sidebar highlighting alumni who are studying black holes
This article was published in the Summer 2015 issue of Odyssey magazine. To read other stories from that issue, please visit go.umd.edu/odyssey.