The Hole Story: Black Holes May Wield an Influence Far beyond Their Gravitational Reach

By Cowen, Ron | Science News, January 22, 2005 | Go to article overview

The Hole Story: Black Holes May Wield an Influence Far beyond Their Gravitational Reach


Cowen, Ron, Science News


Four years ago, astronomer Karl Gebhardt, then a postdoc at the University of California, Santa Cruz, went for a job interview at Harvard University. Although he didn't get the faculty position he sought, he may have gotten something better: a clue that led him to uncover what may be one of the most telling relationships between supermassive black holes and the galaxies in which they reside.

During the Harvard visit, Gebhardt chatted with astronomy professor Avi Loeb about the biggest black holes in the universe--gravitational monsters that lie at the center of galaxies and cram the equivalent of millions to billions of suns into a volume smaller than the solar system. Gebhardt had used the Hubble Space Telescope to determine the mass of several supermassive black holes. Now, he was trying to ascertain whether there was a link between those masses and some more-global property of their home galaxies.

Loeb suggested that Gebhardt compare the mass of each black hole with the average velocities of the billion-or-so stars that surround each hole out to a distance of several thousand light-years. This swarm of stars--a major component of a galaxy--is known as the bulge, and the stellar velocities provide a measure of the bulge's mass.

When Gebhardt made the comparison, he was stunned. Regardless of their size, the bulges always turned out to be 500 times as massive as the giant black holes at the hub of their galaxies. What could be behind this ratio?

Gebhardt wasn't alone in his perplexing finding. Laura Ferrarese, who also studies black holes, had interviewed for the same Harvard job the week before Gebhardt did, and Loeb made the same research suggestion to her Loeb had also discussed the idea with veteran astronomer David Merritt of Rutgers University in Piscataway, N.J.

Merritt teamed up with Ferrarese, who is now at the Dominion Astrophysical Observatory near Victoria, Canada. They found the surprising correlation between black holes and galactic bulges at about the same time that Gebhardt did.

To understand how puzzling this numeric relationship is, Ferrarese notes, consider that a supermassive black hole can only suck in matter that resides less than a light-year from its own location at the center of a galaxy. Most stars in the bulge, which can lie as far as 20,000 light-years away from the center, aren't in the least affected by the black hole's gravity.

Yet that fixed ratio between the mass of the bulge and the mass of the black hole shows up over and over again in the universe.

"The black hole and the bulge should really not know about each other because they're on completely different scales," notes Ferrarese. "Somehow, something at the very center of the galaxy knows about the overall structure of the galaxy.

"When I started out as a graduate student, supermassive black holes were considered a curiosity ... relegated to a part of astrophysics that was not connected to anything else," says Ferrarese. Now, they've stolen the spotlight.

LOCKSTEP MODEL The black hole correlation has spurred some 30 theoretical models over the past 4 years, says Gebhardt, now at the University of Texas at Austin. Recent observations, he notes, are winnowing down those theories, giving astronomers a new understanding of how black holes influence the growth and evolution of galaxies.

The correlation suggests that galaxies and black holes have grown in lockstep--at least for the last few billion years of cosmic history. One model of this coevolution harks back to work described in 1998 by Martin Rees of the University of Cambridge in England, and Joe Silk, now at the University of Oxford in England.

A galaxy starts out as a big cloud of gas. As gas sinks toward the center of a fledgling galaxy, notes Silk, "the natural thing is that a big black hole forms and it grows." It feeds on the rain of material falling into it.

Supermassive black holes could have a profound influence on their galaxies, Silk and Rees surmised, because they spew vast amounts of energy, in the form of radio jets, quasar beams, and intense winds. …

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