Big, dark, and powerful, black holes are turning up in all sorts of galaxies after years of keeping a low profile
The universe is populated with all kinds of cosmic creatures, and in a galaxy 30 million light-years away, a monster rules the heart. With the gravity of a billion Suns, it controls the movements of millions of stars in its domain. Long ago, it blazed with a light that eclipsed the galaxy it lives in. Today, it is dark and quiet. If you were to look at NGC 3115 through a telescope, you'd have no idea that this ancient giant even exists.
Such monsters lurk in the centers of many galaxies, including our own Milky Way. They are almost surely black holes, an extremely dense form of matter predicted by the general theory of relativity. In 1963, soon after the discovery of quasi-stellar radio sources, or quasars, astronomers proposed that quasars' tremendous luminosities come from matter falling into giant black holes. The confirmation of this idea has been one of the exciting sagas of modern astrophysics, and one of the most esoteric. Yet the story begins with something as commonplace as radio waves.
Radio astronomy is a technical by-product of World War II and the advances of radar technology. Throughout the 1950s and '60s, the field progressed rapidly as more and more sources of radio noise in the sky were linked to visible objects. Normal stars and galaxies proved to be weak sources, but some objects in the sky emitted strong radio waves. Among these were gaseous nebulae, the remnants of exploded stars, and "radio galaxies." Other sources did not correspond to any visible object, or were close on the sky to an apparently ordinary star.
A jet races away from the core of M87.
Maarten Schmidt at the California Institute of Technology was one of the astronomers trying to understand these strange radio stars. He observed them with a spectrograph on the 200-inch (5-meter) telescope on Palomar Mountain, and he quickly found that these quasi-stellar radio sources were distinctly unlike stars. Their spectra showed only a few broad emission lines at unfamiliar wavelengths. Â
Puzzled by these unusual spectra, Schmidt tried to plot the line spacings for the brightest of the objects, 3C 273, against the wavelengths of light emitted by hydrogen gas. Much to his surprise, the wavelengths matched when he lined them up, provided that the lines were redshifted (accounting for the object's movement) by an unprecedented 16 percent. The fact that he had to make such a big adjustment meant that the quasar was moving away from Earth at a tremendous speed.
A black hole in NGC 4438 expels a bubble of hot gas (red).
This came as a huge shock. The law that describes the expansion of the universe (the Hubble law) immediately implied that 3C 273 is very far away. At 13th magnitude, however, this "star" was not faint; it can be seen with a fairly large amateur telescope. Schmidt had just discovered one of the most distant objects known, yet its light was bright enough to overexpose his first spectrographic plates. For 3C 273 to be so far away and to be still so bright, it must be enormously luminous -- trillions of times more luminous than our Sun. Â
Another radio quasar, 3C 48, was confirmed within a few hours. Others followed. Soon, some quasars were found to vary in brightness in only a few days or weeks, indicating that they are no larger than the solar system. (Since we know the speed of light, we can determine the size of the quasar by the time it takes the light to travel across it. The quasar could not vary in only a few weeks, because the back side would appear to fade while the front side would grow brighter; the effects would cancel out.)
How could such a huge luminosity come from such a small volume? Likely sources included groups of exploding stars, called supernovae; supermassive stars; giant pulsars; and supermassive black holes. For a while, one theory was pitted against the next to explain these quirky quasars. Black holes finally emerged as the winner, opening the door to some of the most fascinating theoretical and observational work in science today.
Less powerful versions of quasars are seen in some nearby galaxies. They are called Seyfert galaxies. Radio galaxies, Seyfert galaxies, and quasars collectively are known as active galactic nuclei (AGNs). All are thought to be powered by black holes devouring nearby gas and stars.
The escape velocity from the surface of an object depends on both its mass and radius. If Nature can make the radius of a given mass small enough, the escape velocity reaches the speed of light. Neither matter nor radiation can then escape from the surface of the object to the universe outside. What's more, atomic or subatomic forces cannot hold the object up against its own gravity -- the object is doomed to collapse to an infinitesimal point. The original matter is lost from view forever, and only its gravity remains.
The point of no return, where the radius gets small enough so that the escape velocity equals the speed of light, is called the Schwarzschild radius, or the horizon. This radius is proportional to the mass of the hole. If the mass is 50 million times that of the Sun, the radius is equal to the radius of Earth's orbit around the Sun. (The mass of the Sun equals one solar mass -- a convenient unit of measurement in astronomy when discussing objects as massive as black holes.)
Because no light can escape, physicist John Wheeler named such objects "black holes." Any matter that later passes inside the horizon is also lost from view, and its mass is added to the mass of the hole. It will continue to grow as long as there is enough "stellar food" to gobble up. Theory and observations indicate that black holes of a few solar masses form when very massive stars die. Astronomers find such black holes in various places in our galaxy by their X-ray emission. The black holes in galactic nuclei, however, are much bigger.
If no light can escape from black holes, how can they explain the huge luminosities of quasars? The radiation comes from matter that is close to the hole but still outside the horizon. Matter that is falling into a black hole accelerates closer and closer to the speed of light as it approaches the horizon, acquiring a tremendous amount of energy. Under some circumstances, this energy can be converted to radiation. For example, the gas is expected to form an "accretion" disk around the black hole, and friction of the gas in this disk heats it up to very high temperatures. That is why it radiates so ferociously -- at efficiencies of up to 10 percent, which means 10 percent of the mass is converted to energy. In contrast, nuclear fusion reactions like those that power stars have an efficiency of less than 0.7 percent. For quasar luminosities, which range from 100 billion to 100 trillion (1011-1014) solar luminosities, a black hole would have to eat 0.02 to 20 solar masses per year.
How massive does the hole need to be? If the black hole's feeding frenzy lasts for the estimated lifetime of a quasar (about 10 million years), its mass will grow to at least 105-108 solar masses. Also, the black hole must be massive enough to pull in new fuel despite the fact that its radiation is trying to blow the fuel away. This minimum mass is 106-109 solar masses, depending on the quasar's luminosity.
The strong gravity of a spinning black hole may explain the ability of some quasars to eject "jets" of material at close to the speed of light. X-ray observations show that gas orbits the centers of some AGNs at speeds of up to one-third the speed of light -- about 62,000 miles (100,000 km) per second. This, too, supports the presence of a black hole. The location of the gas can be estimated for some AGNs by observing how their spectral lines change with time. If the observed widths of the lines result from the Doppler effect caused by the orbital motion of the gas clouds, we can calculate the mass of the central object from the laws of gravity.
Given the black hole mass and the accretion rates, we would expect the temperature of the accretion disk to resemble that of a very hot star. Consequently, the disk should emit much of its radiation at ultraviolet wavelengths. Observations of quasars generally agree with this prediction. The accretion disk should be brightest at a distance of about 10 times the black hole's radius, which is consistent with how the light from quasars varies over time.
Finding Supermassive Black Holes in Active Galaxies
A giant black hole in a galactic nucleus exerts a powerful gravitational force on nearby gas and stars, causing them to move at high speeds. This is hard to see in quasars, because they are far away and the dazzling light of the active nucleus swamps the light from the host galaxy. In a radio galaxy with a fainter nucleus, the stars and gas are more visible.
The giant elliptical galaxy Messier 87, one of the two brightest objects in the Virgo cluster of galaxies, is just such a radio galaxy, with a bright jet emerging from its nucleus. It has long been thought to contain a black hole. Recent observations of M 87 with Hubble Space Telescope (HST) reveal a disk of gas 500 light-years in diameter whose orbital speeds imply a central mass of 3 billion solar masses. The ratio of this mass to the central light output is more than 100 times the solar value. No normal population of stars has such a high mass-to-light ratio. This is consistent with the presence of a black hole, but it does not rule out some other concentration of faint matter.
A more compelling argument is possible in the Seyfert galaxy NGC 4258, where water molecules in gas clouds in the nucleus emit microwave radiation ("masers"). The location and velocity of this gas can be mapped with amazing precision by making coordinated observations with radio telescopes separated by large distances. The angular resolution (sharpness) given by this technique is 100 times better than that of HST. The measurements imply that 40 million solar masses lie within half a light-year of the center. Could this material be a cluster of dark stars? Â
Dan Maoz of Tel-Aviv University has shown that this is not the case. There are two possibilities -- failed stars or dead stars. Failed stars, or brown dwarfs, are too low in mass for their insides to heat sufficiently to ignite the nuclear reactions that power stars. Brown dwarfs are low-mass objects -- less than 0.08 solar masses -- so there would have to be many of them to explain the dark mass in NGC 4258. Plus, they would have to be very close together, which would cause most of them to collide with other brown dwarfs. But since stars that collide generally stick together, the fusion of two brown dwarfs of almost 0.08 solar masses would create a luminous star, lighting up the dark cluster. Â
The other alternative is dead stars -- white dwarf stars, neutron stars, or relatively small stellar-mass black holes. But these are more massive than brown dwarfs, so there would be fewer of them. The gravitational evolution of clusters of stars is well understood: Individual stars get ejected from the cluster, the remaining cluster contracts, and the evolution speeds up.
Calculations show that a cluster of dead stars in NGC 4258 would evaporate completely in about 100 million years. From a cosmic perspective, this is almost no time at all -- much less than the age of the galaxy. So the most plausible alternatives to a black hole can be excluded. NGC 4258 almost certainly contains a supermassive black hole.
Many quasars have redshifts of two or three, which corresponds to a time when the universe was about five billion years old, only a third of its present age. At that time, many large galaxies had luminous active nuclei. Since then, quasars have mostly died out; very few are active today. What is left is the waste mass from the quasar era -- black holes of a million to a few billion solar masses that are starving for fuel. Some of them accrete slowly; this produces very little light. Â Many are not active at all. Since quasars were once found in most large galaxies, we expect that dormant supermassive black holes should hide in most large galaxies today.
How can we find these black holes? If they are dormant, they make their presence felt only through their gravitational pull on nearby stars and gas. When they are surrounded by a disk of stars or gas, this disk spins very rapidly. In fact, the signature of a black hole is that the disk spins too rapidly for the number of stars that it contains. Without an extra dark object in its middle, the disk would fly apart. Â
In some galactic nuclei, the orbits of stars in a galactic nucleus are not all aligned in a disk but, instead, are distributed randomly. They swarm around like a cluster of bees, rather than spinning like a flying Frisbee. A spectrograph sees light from many thousands of stars at once. Some are approaching us in their orbits and others are receding. As a result, the stellar absorption lines are broadened by the Doppler shift Â (light from a star moving away from us will appear red, whereas it will appear blue if the star is moving toward Earth). Measuring the width of the lines tells us the random velocities of the stars. From the rotation velocities or the random velocities, we can determine the mass near the center.
If quasars have been known for well over 30 years, why is finding black holes difficult? For all of this time, astronomers have been inspired to confirm or disprove the theory that quasars are powered by black holes. The answer is that even a supermassive black hole dominates the motions of stars and gas in only a tiny volume near the center. At most ground-based observatories, this volume is hopelessly blurred by atmospheric turbulence. Only with the best ground-based seeing does it become possible to find black holes in favorable cases, and only with the resolution of the refurbished HST does it become routine to find black holes in most nearby galaxies.
Monster black holes live in galaxies whose total masses are more than those of a hundred billion Suns. Over most of the galaxy, the stars move in response to the gravity of the other stars; they do not know that a black hole is present at the center. Only near the nucleus is the combined mass in stars smaller than the mass of the black hole. Only here does it dominate the local gravity.
Telescopes as Measuring Tools
Even for our nearest neighbors like the Andromeda galaxy (about two million light-years away), the region of influence of the black hole has an angular size of only about one arc second -- a tiny sliver of the galaxy's real estate. Ground-based telescopes that have especially good resolution can find black holes in such objects. The telescopes at the 13,800-foot (4,200-meter) summit of Mauna Kea in Hawaii have exceptionally good resolution, because 40 percent of Earth's blurring atmosphere is below them.
John Kormendy found five black holes in the late 1980s and early 1990s using the Canada-France-Hawaii telescope (CFHT) on Mauna Kea. The first of these was in the Andromeda galaxy. Its 30-million-solar-mass black hole was discovered independently and almost simultaneously by Alan Dressler at the Carnegie Observatories and Douglas Richstone of the University of Michigan using the great telescope at Palomar Mountain. Messier 32, the small elliptical companion of the Andromeda galaxy, also has a black hole. This one weighs a "mere" three million solar masses.
NGC 3115 is a nearby galaxy, an intermediate type between spirals and ellipticals. Spectroscopic observations made in 1992 with the CFHT revealed rapid rotation from an edge-on disk and large random velocities in the surrounding stars. Kormendy and Richstone found a black hole of one billion solar masses; in 1996; observations with HST confirmed this estimate.
Another feature of NGC 3115 is a tiny star cluster in the nucleus -- exactly the sort of dense stellar structure that astronomers have long expected to find around black holes. This nucleus would fly apart in a few tens of thousands of years unless there is a dark mass equivalent to a billion Suns holding it together.
The center of our Milky Way galaxy is only about 27,000 light-years away, but its visible light is completely absorbed by intervening dust. Fortunately, infrared light penetrates the dust. Early infrared measurements suggested that we have a black hole of about a million solar masses. It is coincident with a weak radio source called Sagittarius A* (Sgr A*) that is exceptionally tiny: Radio observations show that it is about the size of Saturn's orbit around the Sun.
In a dramatic breakthrough, two groups of astronomers, led by Reinhard Genzel in Munich, Germany, and Andrea Ghez at UCLA, have measured the motions of individual stars near the galactic center as projected on the plane of the sky. They used a technique called "speckle imaging" to reduce the blurring effect of Earth's atmosphere. Stars here have such small orbits that they revolve around the galactic center in a few decades. We, however, can look forward to seeing the galactic center rotate once in our lifetime!
The rapid motions show that there is an object with three million solar masses centered on Sgr A*. The fact that the mass enclosed inside a particular distance from Sgr A* stops dropping toward the center at a distance of about three light-years means that the mass in stars inside three light-years has become negligible compared with the dark mass at the center. As in NGC 4258, the implied density of matter is too high to allow a cluster of dark stars or stellar remnants. Because of this work, the most compelling example of a supermassive black hole is in our own galaxy. Â
As of June 2000, supermassive black holes have been detected in 34 galaxies. This is enough of an "inventory" for a huge improvement in what we can learn about black holes and galaxy formation.
First, the amount of mass that we find in black holes is consistent with predictions of the waste mass left behind by quasars. Also, the individual masses of the black holes are consistent with predictions from quasar energies.
Two new results demonstrate fundamental correlations between black hole masses and the properties of their host galaxies. Galaxies come in two basic types -- flat spinning disks (like Frisbees) and more nearly spherical bulges. Many galaxies, like our own and the nearby Andromeda galaxy, consist of a bulge in the middle of a disk. When a galaxy contains only a bulge and not a disk, it is called an elliptical galaxy.
Supermassive black holes have been found in elliptical galaxies and in galaxies that contain both a bulge and a disk but not in galaxies that consist only of a disk. In 1993, Kormendy found that black hole mass is roughly proportional to the luminosity of the bulge in the host galaxy. This is confirmed by the new black hole detections, implying that the mass of a black hole is normally about 0.2 percent of the mass of the bulge. We're not sure what causes this correlation, but it implies that, as galaxies form, an approximately standard fraction of the mass ends up in the black hole. The correlation contains important clues to the origin and growth of galaxies.
In the past few months, Karl Gebhardt of the University of Texas at Austin and collaborators have found a new and more fundamental correlation. "More massive black holes live in galaxies whose stars move faster," says Gebhardt. "By this, I don't mean the stars near the center that we use to find black holes, I mean the stars in the main body of the galaxies." Of course, the stars near the center must have high velocities; they are the ones that are used to find the black holes. Concluding that black holes correlate with these stars would be circular reasoning. Instead, the new correlation involves the stars in the main bodies of the galaxies. These stars do not "feel" the black holes. But they, too, move more rapidly than do stars in galaxies with less massive black holes. Â
"The scatter in the new correlation is almost zero. That is, it is almost the same as the measurement errors," says Sandra Faber, an astronomer at the University of California at Santa Cruz (and one of Gebhardt's colleagues). "Tight correlations in astronomy have always led to fundamental advances in our understanding of how things work. They tell us that there is an underlying astrophysical constraint that we didn't know about before. In the present case, we do not yet have an explanation of why the correlation is so tight, but it implies that there is something almost magically regular about the process by which black holes are fed and grown."
"The observed correlations of black holes with galaxy properties are solid," adds Richstone. "Interpretation is harder. Galaxy formation was complicated and our observations of faraway things are incomplete. But the connection between black holes and galaxy formation is now clearing up rapidly."
On the other hand, we can use the correlation without knowing why it exists. It has much to say about when, in relation to their host galaxies, black holes grew.
So far, astronomers have found a supermassive black hole in every galaxy observed that contains a bulge component.
Cosmologists believe that small galaxies formed first after the Big Bang and then merged to form larger and larger galaxies. Mergers are violent enough to turn disks into bulges. If the first fragments already had black holes, then black holes and galaxies would grow together as the galaxies merged. Alternatively, the black holes might have grown during or after the galaxies formed. Two arguments point to the latter picture.
First, some galaxies have a special kind of bulge called a "pseudo-bulge," believed to form in a bulgeless disk galaxy when gas flows toward the center. Pure disk galaxies have only small black holes or none at all. Yet Â galaxies with pseudo-bulges are observed to contain black holes with masses of about 0.2 percent of the mass of the pseudo-bulge, the standard proportion. These black holes must have grown after the disk galaxy formed and during the formation of the pseudo-bulge.
Second, a few galaxies contain anomalously big black holes, compared with the bulge mass. But the stars in these galaxies move faster, too, so that the black hole mass still has the normal relationship to the stellar velocities. The stellar velocities are high because, in these galaxies, the matter that formed the bulge collapsed to a smaller size, increasing the gravitational forces between stars. The extra contraction must also have led to more growth of the black hole. Â Evidently, black hole masses are fixed by the collapse process when the bulge forms. Astronomers believe that they have found galaxies that are now in the middle of the formation and mergerprocess. These are a rare type of galaxy that produce enormous amounts of infrared light, called "ultraluminous infrared galaxies."
Looking back on the past 15 years, we are struck by how fundamentally astronomers have changed their views of black holes. In the mid-1980s, AGN research was thriving but was disconnected from other research on galactic structure and formation. Now black holes have become a necessary ingredient in our understanding of galaxies. Continuing HST searches are finding more and more black holes. By studying them, our understanding of galaxy formation and evolution should improve rapidly in the coming years.