New light shed on black holes
Co-evolution theory shakes up dark reputation
The colliding galaxies known as The Mice and their black holes will eventually merge into a single giant galaxy. Such mergers can generate a quasar phase of galactic evolution.
By Robert Roy Britt
Jan. 28 Black holes suffer a bad rap. Indicted by the press as gravity monsters, labeled highly secretive by astronomers, and long considered in theoretical circles as mere endpoints of cosmic evolution, these unseen objects are depicted as mysterious drains of destruction and death. So it may seem odd to reconsider them as indispensable forces of creation. Yet this is the bright new picture of black holes and their role in the evolution of the universe.
INTERVIEWS WITH MORE than a half dozen experts presently involved in rewriting the slippery history of these elusive objects reveals black holes as galactic sculptors.
In this revised view, which still contain some highly debated facts, fuzzy paragraphs and sketchy initial chapters, black holes are shown to be fundamental forces in the development and ultimate shapes of galaxies and the distribution of stars in them. The new history also shows that a black hole is almost surely a product of the galaxy in which it resides. Neither, it seems, does much without the other.
The emerging theory has a nifty, Darwinist buzzword: co-evolution.
As a thought exercise, co-evolution has been around for less than a decade, or as much as 30 years, depending on who you ask. Many theorists never took it seriously, and no one had much evidence to support it. Only in the past six years or so has it gained steam. And only during the past three years have observations provided rock-solid support and turned co-evolution into the mainstream idea among the cognoscenti in both black hole development and galaxy formation.
The emerging picture of co-evolving black holes and galaxies has turned our view of black holes on its head, says Meg Urry, an astronomer and professor of physics at Yale University. Previously, black holes were seen as the endpoints of evolution, the final resting state of most or all of the matter in the universe. Now we believe black holes also play a critical role in the birth of galaxies.
The idea is particularly pertinent to explaining how massive galaxies developed in the first billion years of the universe. And it is so new that just last week theorists got what may be the first direct evidence that galaxies actually did form around the earliest black holes.
Like archeologists, astronomers spend most of their careers looking back. They like to gather photons that have been traveling across time and space since well before Earth was born, some 4.5 billion years ago. Rogier Windhorst, an Arizona State University astronomer, has peered just about as deep into the past as anyone, to an era when the universe was roughly 5 percent of its present age.
Earlier this month, Windhorst and a colleague, Haojing Yan, released a Hubble Space Telescope image showing the most distant normal galaxies ever observed.
Though stretched and distorted by the technique used to spot them (an intervening galaxy cluster was used as a gravitational lens), the newfound galaxies, Windhorsts team assures us, resemble our own Milky Way. They are seen as they existed more than 13 billion years ago, within 1 billion years of the Big Bang.
Practically side-by-side in time, discovered in separate observations made as part of the Sloan Digital Sky Survey, are compact but bright objects known as quasars. These galaxies-to-be shine brilliantly because, researchers believe, each has a gargantuan black hole at its core, whose mass is equal to a billion suns or more, all packed into a region perhaps smaller than our solar system.
The resulting gravity pulls in nearby gas. The material is accelerated to nearly the speed of light, superheated, and swallowed. The process is not entirely efficient, and there is a byproduct: An enormous amount of energy radio waves, X-rays and regular light hyper-illuminates the whole scene.
Quasars also seem to be surrounded by halos of dark matter, a cryptic and unseen component of all galaxies. Co-existing around and amongst all this, researchers are coming to realize, is a collapsing region of stars and gas as big or larger than our galaxy.
It was no coincidence that the announcements of the two findings distant quasars and normal galaxies were made together at a meeting of the American Astronomical Society (AAS) Jan. 9. Co-evolution was on the minds of the discoverers.
Among co-evolutions significant impacts is its ability to render mostly moot a longstanding chicken-and-egg question in astronomy: Which came first, the galaxy or the black hole?
How about both? Windhorst asks. You could actually have the galaxy form simultaneously around a growing black hole.
Urry, who was not involved in either finding but was asked to analyze them, explained it this way: We believe that galaxies and quasars are very intimately connected, that in fact quasars are a phase of galaxy evolution. In our current picture, as every galaxy forms and collapses, it has a brief quasar phase.
So when a quasar goes dormant, whats left are the things we associate with a normal galaxy stars and gas swirling around a central and hidden pit of matter.
Quasars are cagey characters, however. (The term is short for quasi-stellar radio source; astronomers first mistook the objects for stars within our galaxy in the early 1960s.) When one is firing, its brightness can exceed a thousand normal galaxies. The quasar outshines its entire host galaxy so significantly that scientists have not been able to see whats really causing all the commotion. That veil is lifting as you read this, however, as telescopic vision extends ever backward in time and data is fed into powerful new computer models.
Demonstrations of co-evolution began to emerge in the mid-1990s when researchers found hints that the existence of a significant black hole at the center of a galaxy was related to the galaxys shape, says Martin Haehnelt of the University of Cambridge. Only galaxies with a spherical bulge-like component appear to accommodate supermassive black holes.
Our Milky Way, if it could be viewed edge on, would display a good example of one of these galactic bulges: Imagine the profile of a stereotypical flying saucer, though with a wider and flatter disk. The Milky Way is smaller than many galaxies, however, and it has a correspondingly less massive black hole roughly 2.6 million suns worth. It almost surely once had a quasar phase, astronomers say.
At any rate, in the mid-1990s no one knew for sure how prevalent black holes were. Theory and some observational data pointed to the likelihood that they were ubiquitous.
Then, in the year 2000, astronomers found solid evidence that black holes lurk deep inside many and probably all galaxies that have the classic central bulge of stars. Further, an analysis showed a direct correlation between the mass in each black hole and the shape and scope of the bulge and the overall size of the galaxy.
At an AAS meeting in June of 2000, John Kormendy of the University of Texas at Austin, presented evidence for 10 mammoth black holes whose masses were related to their galactic bulges. Kormendy worked on a large team of researchers led by University of Michigan astronomer Douglas Richstone. This along with other studies in surrounding months by other teams served as a collective turning point for co-evolution, several researchers now say, advancing it to a stable quantitative footing.
Subsequently the idea of the co-evolution of galaxies and supermassive black holes became more widely discussed and accepted, Haehnelt says.
Evidence continues to mount. In 2001, two separate teams showed that many smaller galaxies that dont have bulges also do not seem to contain significant black holes.
Over the past six months or so, other important studies have emerged, providing independent confirmation to some of the initial work. Haehnelt: It becomes more and more clear that supermassive black holes can significantly change the structure and evolution of galaxies.
The first large-scale scientific meeting devoted to co-evolution a sure sign of a theory coming into its own was held just three months ago, sponsored by the prestigious Carnegie Observatories.
There are many variations on the basic theory of co-evolution. Each version attempts to explain a vexing fact: In the blink of a cosmic eye just a half a billion years invisible spheres of matter were born, and several gained the mass of a billion or more suns and were driving the shape and texture of swirling agglomerations of newborn stars.
Co-evolution is not a done deal. Perhaps, some have suggested, a huge black hole simply collapses out of a pre-galactic cloud and serves as a ready-made engine to drive further galaxy development. Even staunch supporters of co-evolution say there are still viable theories, not yet refutable, putting the immense black hole in place first, and others that have the galaxy solely responsible for driving the formation of a black hole.
If black holes did grow incrementally, it is unclear whether cooperative construction reigned from the beginning, or if it kicked in after some certain amount of mass was gathered.
I think it is still unclear whether black holes play any role in the formation of the first galaxies, said Cambridges Sir Martin Rees, who has collaborated with Haehnelt and who long ago authored some of the first scientific papers on the question.
Indeed, Sir Martin says, there is a lot of debate about whether black holes can form in very small galaxies, and whether there is a link between the small holes that form as the endpoint of the evolution of massive stars and the holes of above a million solar masses that exist in the centers of galaxies.
ANOTHER DARK MATTER
Infusing itself into the equation is an utter unknown: dark matter. This as-yet-undetected stuff permeates all galaxies, researchers believe. A halo of it surrounds our Milky Way. Dark matter does not interact with light, but it does possess great gravitational prowess, acting as invisible glue to help hold galaxies together.
Dark matter is taken into account in the leading co-evolution models, but only in a general, overall sense. Some researchers, however, think dark matter, more than a black hole, is clearly connected to a galaxys birth and development.
Just last week, the first possible direct evidence was announced for dark matter halos around early quasars. The finding, by Rennan Barkana of Tel Aviv University and Harvard astronomer Abraham Loeb, appears to be the first glimpse at the anatomy of the most distant quasars. Importantly, it supports the fundamental ideas of co-evolution, Loeb said. But it also makes it clear that dark matter will not be denied a chapter in any book about the theory.
Laura Ferrarese, a Rutgers University physicist, analyzed the new dark matter finding. She says it shows that a supermassive black hole, the stars around it, and an all-encompassing dark matter halo are working in concert to build structure.
Taken with other evidence, Ferrarese sees dark matters role as more significant, or at least more obvious, than many theorists have considered.
There is an observational correlation between the mass of the black hole and the mass of the dark matter halo, not necessarily the mass of the galaxy itself, she said.
Through this haze of fuzzy information and diverse thinking, theorists must work to explain a stark and staggering fact: Somewhere between 300 million and 800 million years after the Big Bang, the first black holes were born and managed to each gulp down a mass of more than 1 billion suns.
Now before you ponder how these Sumo wrestlers of the early universe must have thrown their weight around in any evolutionary wrestling match, consider this: A black hole typically holds much less than 1 percent of the overall mass of the galaxy it anchors.
SHINING LIGHT ON THE DARK AGES
The early history of black holes what went on in the 500 million years leading up to objects observable with current technology is tied back to the development of the very first stars. Speculating about it requires first rewinding to the very beginning.
When the universe was born, there was nothing but hydrogen, helium and a little lithium. All this raced outward for about 300,000 years before anything significant happened. The gas was too compacted and therefore too hot to be stable. Gradually, the stuff of space expanded and cooled enough for gas to recombine and stabilize to neutral states, as scientists put it.
The hydrogen was still too hot to form stars, so more expansion was needed. A long stretch of boring darkness ensued, during which some ripples began to ruffle the otherwise smooth fabric of space.
For 300 million years, nothing happened, explains Windhorst, the Arizona State University astronomer. The universe is just sitting there. Then all of a sudden the first stars began to shine.
The exact timing for first light is not known. But the ensuing 500 million years are the so-called dark ages of cosmology. Or more precisely, they represent the illuminations of the universe and the elimination of the dark ages.
The tail end of that is what were seeing, Windhorst says of the latest Hubble and Sloan survey observations.
THE FIRST BLACK HOLES
Scientists once imagined galaxies forming by a sort of monolithic collapse, in which a giant cloud of gas suddenly fell inward. The modern view is one of hierarchical merging, in which bits and pieces build up over time. A rough outline of how it all went down is fairly well agreed upon.
The initial ripples in space drew together into knots and filaments, locally and over broader scales. Individual clumps of gas collapsed, and stars were born.
The first stars must have been massive, perhaps 200 times the weight of our Sun or more. They would have been almost pure hydrogen the primary ingredient of thermonuclear fusion, which makes a star shine.
Massive stars are known to die young. Some survive just 10 million years (the Sun is 4.6 billion years old and just reaching middle-age). A colossal explosion occurs, sending newly forged, heavier elements into space. Remaining material collapses. A mass equal to many stars might end up in a ball no larger than a city. The result: a stellar black hole. These object are so dense that nothing, not even light, escapes once inside a sphere of influence known as an event horizon.
Stellar gravity wells can weigh as little as a few suns. But the inaugural versions might have been 100 times as massive as the Sun or more.
During all these tens and hundreds of millions of years, more stars are being born from the detritus of the first stars. Locally denser regions of gas contract. Stars form groups of perhaps a few dozen, which might be attracted to other star clusters. Eventually, clusters of many thousands of stars develop and began to look and behave like something that could be called a sub-galaxy. Some probably harbored growing black holes near their centers.
Here, theory struggles. Intuition might suggest that many of these huge stellar black holes simply merged until one central object attained enough mass to drive the shape and future development of its galaxy.
If that intuition is right, however, which black hole became the center?
It may be a question of being in the right place at the right time, says Roger Blandford, a theoretical astrophysicist at Caltech. It could be accidental.
In fact, nobody knows for sure if the first super-sized black holes developed from a series of mergers several dozen solar masses becomes 200, then 1,000, then 10,000, and so on or if they collapsed from the condensing gas cloud. Do they start from 100 solar masses or a million solar masses? Thats a good question, Blandford said. My personal guess is that they start from a few hundred solar masses, but thats a much more speculative business.
Galaxy birth and development is a never-ending process, and clues to early black hole evolution are spread throughout our own galaxy and around the universe. Astronomers therefore examine modern-day cosmic creatures for clues to their ancestral roots.
Black holes are everywhere, for one thing. Millions of the stellar sort could litter our galaxy alone, based on early discoveries of a few.
If the mightiest black holes indeed developed out of the garden variety, then there ought to be some evidence lying around our cosmic backyard in the form of middleweight versions, one line of thinking goes.
A handful of astronomers are convinced they have found a couple of these missing links, and in fact are arguing their case this week at a conference in California. But the case of the middleweights is among the most controversial in all of astronomy.
The existence of middleweight black holes is one of the big unanswered questions in this field, said Cambridges Haehnelt. The recent claimed detections are still very controversial.
Regardless, most experts agree middleweights would represent, at best, pocket change to the fully grown black hole, something like Microsofts initial millions in annual revenue compared to the billions that poured into its coffers during the tech boom.
Researchers on both sides of the middleweight argument mostly agree that the bulk of a jumbo black hole doesnt come through early mergers. Once a critical mass is achieved and this appears to coincide with a point in time prior to what astronomers can see today a black hole seems to gain most of its mass by swallowing gas from its environment.
Amid all the squabbling over middleweights looms the likelihood of much larger merger candidates.
Galaxy merging is almost a given. It is thought to have contributed significantly to the past growth of the Milky Way, for example. The early universe, having not yet expanded much, was incredibly crowded. Like racked billiard balls, nascent galaxies were more likely to collide.
If two galaxies merge, so should their black holes. Recent computer modeling speculates the event would be violent, unleashing tremendous light as gas is trapped between the two black holes and then rushes toward the more massive one.
Galactic mergers take millions of years, so they cant readily be observed in progress.
A recent peek into a nearby galaxy provided evidence for the scenario, however. At the heart of galaxy NGC 6240 astronomers found not one but two black holes, roughly 3,000 light-years apart and closing on an apparent merger course. The Chandra X-ray Observatory observations show that NGC 6240 is actually two galaxies that started joining forces about 30 million years ago.
Other indications of mega-mergers come from relatively nearby quasars.
Richard Larson, a Yale astronomer who studies star formation in galactic nuclei, says galaxies can go through several quasar phases during their lives. In studying quasars at more reasonable distances (which also means not so far in the past), he consistently sees signs of recent galaxy mergers or other large-scale interactions that served as triggers.
Interactions and mergers are an excellent way to dump a lot of gas into the center of a galaxy, Larson explains. The first thing this gas does is suddenly form huge numbers of stars.
Bursts of intense star formation seem to last about 10 million to 20 million years around a typical quasar.
Some of the gas that does not go into generating stars falls on in to the black hole. This violent phase of consumption is the one that is readily observed, because the castoff energy turns the incoming gas and dust into a glowing cloud. Eventually, the chaos settles and the new stars become visible. Later, the quasar itself is left naked. Finally, it goes dormant.
Larson figures this scenario for black hole feeding probably applies to the most distant quasars, too. And it supports the notion that black holes do in fact gain most of their bulk by accreting gas.
To sort out the specifics of co-evolution, astronomers will need to see more of the universe and inspect it in greater detail. The prospects are good, especially toward the end of this decade.
A project called LISA (Laser Interferometer Space Antenna) would search for gravitational waves kicked up in the aftermath of black hole mergers, perhaps proving that such colossal collisions do occur. The NASA satellite is tentatively slated for launch in 2008.
A vastly improved understanding of dark matter is also needed. Several telescopes should contribute to this effort, but since no one knows what the stuff is, forecasting any sort of resolution is highly speculative.
And the specific mechanics of black holes must be investigated fully. For now, theorists dont even know exactly how matter is shuttled inward and consumed. Much of this work can be done by observing the nearby universe.
Roger Blandford, the Caltech theoretician, has suggested a novel way to prove that early mergers were not serious contributors to black hole growth. Blandford says two primary parameters characterize black holes. Mass is the most obvious. A more subtle measurement is spin.
Yes, black holes seem to spin. The idea only emerged from theory to relatively firm observations in May of 2001, and it remains unproven.
But if spin can be proved a universal aspect of black holes, then the rate of spin can be used to infer something very important about a black holes history.
If black holes grow by merging, by combinations of black holes, they should spin down quite quickly, Blandford explains. This then becomes a fairly good argument that, if you can show that black holes really are spinning rapidly, they probably didnt grow by merging, but would have grown by accreting gas.
Most important, vision simply must be extended further back in time, beyond the quasars that are now being studied, says Karl Gebhardt, a University of Texas astronomer and a member of Richstones team.
Theyre essentially the tip of the iceberg, Gebhardt says of the objects so far observed. We are projecting from what we see in a very special number of objects to the whole sample. That is part of the problem of the uncertainty now.
Hubble may extend current vision a bit, but the next boon in deep-space discovery will likely have to wait for the James Web Space Telescope, planned for launch in 2010. Billed as the first-light machine, the JWST will be Hubble on steroids, and it should muscle its way to a better view of a good portion of the cosmic dark ages.
It is ironic to think that when JWST goes up, many astronomers and cosmologists will be banking on black holes to light the way to a scientific account of the earliest epoch of the visible universe, an obscure time they have long dreamed about and can now, almost, see.
© 2003 Space.com. All rights reserved.