A quasar in every galaxy? For every superstar in the Hollywood firmament, thousands of pedestrian actors just try to make ends meet. Likewise, for every brilliant quasar that illuminates the universe, countless other galaxies--like our Milky Way--shine with the unspectacular light of middle age.
Theorists and observers reached this conclusion by focusing on the early epochs when ravenous black holes gobbled most of their mass. Ironically these black holes limited their own growth by unleashing torrents of energy that drove away the surrounding gas (S&T: April 2005, page 42). These waves of unrest also dictated the ebb and flow of starbirth in the host galaxies. This feedback process forged a close link between massive black holes and their surrounding stars.
This view of abrupt but dazzling mayhem in major galaxies represents a profound conceptual shift in our understanding of cosmic evolution. Long ago astronomers grew accustomed to a universe with extreme ranges of behaviors. Stately spirals like the Milky Way and Andromeda seemed normal; quasars and other types of active galaxies, such as blazars and Seyfert galaxies, were the freaks. But researchers now regard the freaks as unusual only in the time domain. It's all a matter of when in their evolution we happen to see them.
"Quasars appear rare because they are suicidal," says Abraham Loeb (Harvard-Smithsonian Center for Astrophysics). "They accrete a lot of gas, then they shut themselves off. On cosmological time scales, it's just like an explosion." The Milky Way's core certainly erupted in this way before settling down. But even at its peak vigor, our galaxy didn't hold a candle to the quasars raging in the far reaches of the universe.
Black Holes Everywhere
With luminosities that often exceed that of the billions of stars in their host galaxies, quasars are the most energetic members of a class known as active galactic nuclei, or AGN. Quasars can shine so brightly only by devouring huge quantities of gas--enough to build black holes packing hundreds of millions to billions of solar masses. Our Milky Way, by contrast, hosts a modest central black hole containing "only" about 4 million solar masses.
Direct observational evidence of these monster black holes first surfaced in the 1990s, when the Hubble Space Telescope probed the cores of 40 nearby galaxies. Hubble's spectrograph revealed pronounced Doppler shifts, clear signs that stars at the very center of almost every observed galaxy whirl at breakneck speeds toward us on one side and away from us on the other. The rapid stellar orbits indicated deep gravitational fields carved by supermassive black holes.
The results of Hubble's survey surprised even the observing team. "At the beginning I thought black holes were rare, maybe 1 galaxy in 10 or 100," says Douglas Richstone (University of Michigan, Ann Arbor). "Now we've shown they are standard equipment."
There is one key caveat, however. Only galaxies with a central bulge host a monster black hole. Bulges are the nearly spherical swarms of old stars that surround the cores of most large galaxies. But as many as 15% of spiral galaxies consist only of starry disks, with no bulge whatsoever. The face-on spiral M33 in Triangulum is a nearby example. "It has no bulge and no black hole that we can detect," says Richstone.
Still, most large galaxies follow a remarkable trend. In 2000 teams led by Karl Gebhardt (University of Texas, Austin) and Laura Ferrarese (Rutgers University) showed that the masses of black holes inside galaxy cores correlate very strongly with the velocities of stars orbiting through their host bulges. A higher bulge mass produces faster stellar orbits, on average. And the researchers found that the higher the bulge mass, the higher the mass of the central black hole.
This landmark discovery, called the M-sigma relation, * catalyzed the field. It provided compelling evidence that central black holes are intimately connected to the evolution of their host galaxies. Most astronomers had suspected that galaxies and their black holes grew simultaneously during periods when they accreted lots of gas, possibly during galaxy mergers. "To me, the M-sigma relation made the physical connection clear," says Megan Urry (Yale University). "All galaxies with supermassive black holes must have gone through this active phase."
The tightness of the M-sigma relation amazed theorists as well. "It's almost magical, because the scales are so completely different," says Nickolay Gnedin (University of Colorado, Boulder). "If you draw a galaxy on a sheet of paper, the central black hole would be the size of an atom. It's hard to understand how the black holes know about the galaxies, or how the galaxies know about them." But observers and theorists are converging on an explanation.
Active Galaxies Near and Far
To understand the feedback processes at work in galaxies, astronomers use deep surveys at various wavelengths to trace black-hole activity across billions of years. In visible light the ongoing Sloan Digital Sky Survey has played a pivotal role. Its 3.5-meter (140-inch) telescope in New Mexico has detected about 70,000 quasars. Each quasar's spectrum reveals how much its light has been redshifted by cosmic expansion, which astronomers convert into the age when the quasar was active.
Some of the most powerful known quasars arose earlier than a redshift of 6, which translates to 1 billion years after the Big Bang. The most distant quasar, at redshift 6.42, lived at a cosmic age of only 870 million years. "These are among the most luminous objects the universe has ever seen," says Xiaohui Fan (University of Arizona, Tucson). Theorists think these blazing objects arose in special places where matter concentrated most densely in the wake of the Big Bang. Seed black holes of a few thousand solar masses--created by an as-yet unknown process--accumulated those titanic masses by devouring gas and other black holes. But those intensely luminous quasars were rare. So far Sloan has imaged just 19 such objects with redshifts of 6 or higher.
In a universe of hundreds of billions of galaxies, the 70,000 Sloan quasars seem like a drop in the bucket. Fortunately, other eyes on the sky are finding more. Quasars spew copious X-rays as million-degree gas spirals toward the supermassive black hole. NASA's Chandra X-ray Observatory has exposed those fiery pinpricks with a narrow, deep survey in each hemisphere. Whereas the Sloan survey sees about 10 quasars per square degree, Chandra's exposures reveal 7,000 active galaxies in the same area. "At high redshifts we find active galaxies up to 100 times less luminous than the rare powerful Sloan quasars," says Niel Brandt (Penn State University). "These are the typical active galactic nuclei in the universe."
When Brandt extrapolates Chandra's detections to the rest of the sky, he calculates that the observatory can see the cores of 5% of all "decent-size" galaxies in the universe. That's a plausible statistic, says Brandt, because astronomers think the cores of major galaxies light up as X-ray-bright AGN just a small fraction of the time. Moreover, Chandra may miss galaxies so choked by thick clouds of gas and dust that X-rays cannot escape efficiently.
Fortunately, NASA's infrared Spitzer Space Telescope, launched in August 2003, can detect some of these hidden AGN. Infrared light can stream through the dense toruses thought to shroud some of the most vigorously accreting galaxy cores. In 2005 two teams of researchers reported that Spitzer finds the glowing cores of active galaxies everywhere it looks, especially between redshifts of 2 and 1 (roughly 3 billion to 6 billion years after the Big Bang)--the epoch when most galaxies assembled. "Most quasars at these redshifts and beyond are hidden behind gas and dust," says Urry. "That makes sense. As those galaxies are collapsing, they should be in the messiest, dirtiest environments."
One recent study combined optical and X-ray observations of dusty galaxies and their central black holes. David Alexander (Cambridge University, England) and his colleagues examined 20 "submillimeter" galaxies around redshift 2. These luminous galaxies were originally spotted by the 15-meter James Clerk Maxwell Telescope at Mauna Kea, Hawaii, which can detect submillimeter waves radiating from dusty stellar nurseries. An optical study led by Scott Chapman (Caltech) showed that these galaxies create about one new star every day--100 times higher than the Milky Way's rate. Chandra X-ray data revealed that 15 of the 20 submillimeter galaxies have black holes actively feeding on gas.
"These galaxies are forming a lot of stars, and at the same time you have almost continual black-hole growth," says Alexander. "We're seeing the construction of massive galaxies and their central black holes." The team claims that this is the first solid evidence of how the mysterious M-sigma relation arose. Alexander thinks the submillimeter galaxies eventually became titans like M87--a nearby giant elliptical galaxy in the Virgo Cluster whose 3-billion-solar-mass black hole propels a jet to near-light speed.
Models of a Violent Universe
This flurry of multiwavelength observations gave theorists what they needed to construct more accurate models of galaxy growth. By mid-2005 the consensus was clear: Massive black holes control the evolution of every major galaxy.
Black holes can't do it in isolation, however. A lone galaxy with a black hole at its heart will happily swirl for eons, and the hole will eat only what happens to drift nearby. To trigger rabid growth spurts, a black hole needs a major disturbance to funnel torrents of fresh gas into the core.
Collisions between gas-rich galaxies do the trick, a view supported by a model created by Philip Hopkins and Lars Hernquist (both at the Harvard-Smithsonian Center for Astrophysics) and their coworkers. Telescopes reveal galaxy mergers everywhere, but they were far more common in the early universe, when galaxies were closer together. To find out how these crashes affected the black holes and the internal dynamics of galaxies, the team created computer simulations that varied the galaxy masses, gas contents, and collision angles.
The results, published in March 2006, produced a new insight. "The evolution of quasars is more complicated than people assumed," says Hernquist. "Their activity is sporadic, and they are visible as intense sources only for very short periods." The simulations suggest that a black hole accumulates most of its mass in extreme feeding episodes--triggered by mergers--that switch on a bright quasar for just 1% of a galaxy's existence. For the rest of its lifetime, the nucleus is mostly dormant.
Observations support this scenario. A team led by Charles Steidel (Caltech) reports that ordinary galaxies at redshifts of 2 to 3.5 are 50 times more common than quasars during that era--just as one would expect for a population of mostly quiescent black holes.
In this new picture, black holes and galaxy bulges form together in a sudden, violent "blowout phase" triggered by a merger. The impact on the surrounding galaxy is profound. Energy released by matter plunging toward the black hole ignites the quasar and heats gas throughout the galaxy's core. But the gas can only take so much heat before it all escapes, like water from a boiling pot. Bigger galaxies have enough gravity to hold onto hotter gas, so the black hole grows even more massive.
Astrophysicists think this explains why small galaxy bulges host small black holes, while big bulges host big black holes. "There is a critical point in the growth of black holes when they drive away gas," says Hernquist. The outpouring of energy stunts both the black hole's growth and star formation in the galaxy's bulge, in lockstep--leading to the M-sigma relation. Leftover gas then settles down into a disk of stars that resembles today's spirals.
This furious feedback was a crucial component of the Millennium Run, a simulation of the largest virtual volume of space ever attempted. A team led by Volker Springel (Max Planck Institute for Astrophysics, Germany) simulated the way in which matter clumped together inside a cube that is more than 2 billion light-years on a side. The team started with the tiny fluctuations in the distribution of matter encoded in the subtle temperature fluctuations of the cosmic microwave background. When gravity and dark energy acted on those fluctuations for the simulated age of the universe, the result was a delicate network of galaxy clusters--the familiar cosmic web (see page 24).
The Millennium Run incorporated feedback within and among galaxies, such as supernova winds and powerful jets of radio-emitting gas from supermassive black holes. The resulting shock waves dictated the fate of every galaxy. The simulation produced giant early galaxies with Sloan-type quasars, but outflows from the black holes soon blasted away the surrounding gas. These gargantuan black holes now reside within the "red and dead" giant elliptical galaxies (like M87) at the centers of today's richest clusters.
On the low-mass end, supernovae in simulated dwarf galaxies disrupted star formation, transforming many of these objects into barely visible nuggets of mostly dark matter like certain dwarfs seen in our Local Group (S&T: July 2005, page 16). These galaxies lack organized cores and most likely don't contain central black holes. No quasars or active nuclei ever illuminated these small, quiet islands, which greatly outnumber major galaxies like ours.
Our Bright Destiny
Compared to brilliant galaxies elsewhere, the Milky Way's core is anemic. Our galaxy has a spheroidal bulge of old stars, indicating that it went through at least one major merger. But its modest black hole suggests that even during that merger, the core never blazed like the quasar beacons seen by the Sloan survey.
According to radio and X-ray data, our galaxy's supermassive black hole currently swallows just 1/100,000 of the gas around it. The gas's emitted energy is so feeble that telescopes can detect it only because the black hole resides in our cosmic backyard. Simulations by Fulvio Melia (University of Arizona, Tucson) and others show that this situation won't change anytime soon. "It would take something really dramatic to churn the gas where it leads to a hyper-inflow," he says.
But something dramatic looms in the distant future. The Milky Way and the relatively nearby Andromeda Galaxy (M31) are on a collision course, with a merger inevitable in several billion years (see page 108). The interaction will funnel streams of new gas into the core of the combined supergalaxy--where the two existing black holes will coalesce. (M31 boasts a central black hole roughly 10 times as massive as ours.) Calculations by Hernquist and Melia suggest that the revitalized black hole could accrete 1 to 10 solar masses of gas per year, eventually growing to at least 100 million solar masses.
In a darkening universe of galaxies accelerating ever farther apart, the crash will light up our neighborhood. "It will appear as a quasar from the outside," Melia says. But the display will carry a cost. Both M31 and the Milky Way of our descendants will forfeit their stately spiral configurations to create an undistinguished elliptical blob.
Freelance journalist Robert Irion lives in Santa Cruz, California, and covers astrophysics for Science. He writes his articles during periods of intense activity, separated by disturbingly long intervals of quiescence.
* M is the mass of the central black hole, and sigma ([sigma]) is the velocity dispersion of the bulge, a measure of how fast its stars are moving.
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|Publication:||Sky & Telescope|
|Date:||Jul 1, 2006|
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