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 contains 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.
Chicken-and-egg question
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, Windhorst's 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-evolution's 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, what's 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 what's 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.
Evolving idea
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 galaxy's 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 don't 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.
The New History of Black Holes: 'Co-evolution' Dramatically Alters Dark Reputation
