Thanks to RDT for the link.

Reposted from:
http://www.washingtonpost.com/wp-dyn/content/article/2007/10/30/AR2007103002073.html?wpisrc=newsletter

For years, astronomers speculated that a giant, mysterious force lay at the center of the Milky Way, but it wasn't until four years ago that UCLA astronomer Andrea Ghez definitively showed what it was.

Using new techniques for peering into the dusty heart of the galaxy, Ghez's observations proved that scores of stars were rapidly orbiting what could only be a black hole. But it wasn't the kind of garden-variety black hole created when a star explodes and dies; it was hundreds of thousands of times as powerful -- a "supermassive" black hole, as they are now known.

Her discoveries, along with the work of scientists studying other galaxies, have in a short time led researchers to the surprising conclusion that most, if not all, of the universe's hundreds of billions of galaxies have supermassive black holes at their core. Even more striking, the astronomers have found that the black holes' mass and nature are closely related to the size and makeup of the surrounding galaxies.

It also appears that these cosmic monsters -- which can "eat" stars whole -- are key to understanding how galaxies were formed and are still being formed today.

"Many of these discoveries were unexpected," said Ghez, a self-described "telescope junkie" and rising astronomy star who does much of her galaxy-gazing at the W.M. Keck Observatory in Hawaii, the world's largest optical telescope. "There's tremendous interest in this field now because of the potential that it can tell us so much about the dynamics of very basic galaxy creation."

Black holes appear, for instance, to be both creators and destroyers -- swallowing stars or gases that come too close while also spewing out jets of super-high-energy particles and radiation generated by this violent feeding process. The jets, which can be millions of light-years in length, are believed to seed galaxies with the mass and energy that will, in time, become new stars and perhaps even planets.

With many promising areas to research, the supermassives are drawing astronomers and astrophysicists back into black hole research. In 1915, based on purely theoretical calculations, Albert Einstein laid the groundwork for the existence of these bizarre phenomena, which have such strong gravitational pull that not even light can escape them. But research on them languished for decades because there was no way to observe them directly.

The Hubble Space Telescope provided the first real evidence of the existence of supermassive black holes -- revealing in 1994 that something was orbiting rapidly around the nuclei of some distant galaxies, suggesting the presence of a huge mass contained in a very small area.

Since then, the Hubble, NASA's orbiting Chandra X-Ray Observatory, the Keck and other very large, high-resolution ground telescopes have begun to unravel more about these central black holes -- which can be as large as the distance from the sun to well past Mars, and as small as New Jersey.

Because nobody knows what happens after a star or gas is swallowed by a black hole, astrophysicists have focused instead on learning and theorizing more about its outer structure. They believe that black holes have an "event horizon" -- the point where anything will be inexorably captured by the gravitational pull -- and that they have "accretion disks," a vast, swirling region where matter is funneled into the hole. The process creates intense friction and heat, and as a result energy and matter can get supercharged and shot out in jets.

As supermassive black holes go, the one at the center of the Milky Way (about 27,000 light-years, or 158 trillion miles, away from our exurbanite sun) is dormant and small. It is believed to have the mass of almost 4 million suns and does not appear to be sending out jets of radiation. Some of the larger supermassives are hundreds of millions to many billions times as massive as our sun. (A typical stellar black hole has five to 10 times the mass of the sun, although researchers yesterday reported discovering an exploded star that was a record two to three times as massive as that.)

To the enormous surprise of those who study the universe, the size of a supermassive black hole appears to have a direct and unusual correlation to the galaxy around it. Researchers calculated a decade ago that the mass of a supermassive black hole appeared to have a constant relation to the mass of the central part of its galaxy, known as its bulge. This relationship supports the notion that the evolution and structure of a galaxy is closely tied to the scale of its black hole.

"Something very profound is going on here, and the formation of black holes and galaxies is related in some way," said Juna Kollmeier, an astrophysicist and fellow with the Observatories of the Carnegie Institution. She focuses on the theoretical side of how the structure of the universe came to be.

"This is an exciting new wrinkle on the old concept of black holes, and that's why so many researchers are drawn to it," she said.

And mysteries do abound. Many researchers have offered theories of how supermassive black holes might have formed, but there is no consensus. Were they created in the early universe when exploding stars were larger, or are they created by the merger of galaxies with smaller black holes at their center?

There is also the puzzling question of why some are active -- feeding regularly on stars around them -- while others are nearly dormant. Ghez added to the mystery recently by reporting that some of the stars orbiting the Milky Way's central black hole are quite young, even though the galaxy is mature and its nucleus has long been dormant.

But dormancy can be temporary, as Suvi Gezari of the California Institute of Technology documented recently. Using the telescope on NASA's orbiting Galaxy Evolution Explorer -- which measures ultraviolet light from the early universe -- her team detected a distinctive flare from a distant galaxy and watched it diminish over time. They concluded that they had seen an unfortunate star stray too close to its galaxy's central black hole, where the star was torn apart and then swallowed by the force of its gravity. The bright flare -- a rarely witnessed event -- was the result of this "feeding."

"Most of these are sleeping cosmic beasts, just sitting there," Gezari said. "So we have to scan the skies to see those very rare times that there's a burst of radiation as the black hole feeds." Like Ghez and Kollmeier, she is young and female -- still somewhat unusual in astrophysics.

Her team's goal is to measure the mass of central black holes and to correlate that with the mass of the galaxies that surround them. Might the activity or dormancy of a central black hole be tied to that surprising relationship between the mass of central black holes and their surrounding galaxies, she wonders, leading a black hole to begin "feeding" when it begins to get out of balance?

David Thompson of the NASA Goddard Space Flight Center, a key astrophysicist in the agency's Gamma Ray Large Area Space Telescope (GLAST) mission -- expected to launch next year -- is studying how that connection between a black hole and its surrounding galaxy is maintained. He believes that the high-powered jets created during feedings are probably involved.

"These black holes at the center of galaxies are relatively small but are extremely massive and have a strong influence on a huge amount of space surrounding them," he said. "It looks likely that the jets have to be playing a big part in that process."

Because the jets from supermassive black holes contain substantial amounts of gamma-ray radiation, the most highly energized form, the GLAST mission is expected to answer more questions about what's happening at the center of galaxies. Thompson will be studying "blazars" in particular -- jets aimed directly toward Earth.

The fast pace of the recent discoveries is largely the result of the newer orbiting observatories and extremely powerful ground telescopes that have begun operating in the past decade. NASA's "Beyond Einstein" initiative was approved by Congress several years ago to speed development of space-based observatories and probes designed to tell us more about the early universe and the forces that ruled it.

But tight agency budgets have slowed the initiative, and most of the Beyond Einstein probes won't be launched for years, if at all. The proposed orbiting X-ray observatory Constellation-X would be especially valuable in learning more about the structure and dynamics of supermassive black holes -- measuring, for instance, the powerful spin of the accretion disk -- but that project has been put on hold.

This has black hole researchers concerned, especially since the field has turned so productive.

"Con-X is absolutely essential, or central, to our making major advances in supermassive black hole research," said Harvey Tananbaum of the Harvard-Smithsonian Astrophysical Observatory, who chairs the science team for the Constellation project. But he said he is optimistic the project will ultimately be funded, because "the science just gets stronger and stronger."