"Hidden" Black Holes Discovered in Distant Galaxies

Victoria Jaggard in Austin, Texas
National Geographic News
January 14, 2008
Displaying the cosmic equivalent of high metabolism, some skinny galaxies seem to have huge black holes hidden in their "stomachs," astronomers announced yesterday.

Skinny galaxies are slender disks that do not have big bulges of stars at their centers.

Until now, astronomers had thought such bulges were fundamentally connected to the growth of supermassive black holes believed to lie at the centers of most galaxies.

But the first survey of 32 skinny galaxies using infrared observations from the Spitzer Space Telescope revealed that some had the signatures of monstrous black holes at their cores.

"We discovered eight hidden feeding—or what we call active—black holes in completely unexpected places: in the centers of skinny galaxies," lead study author Shobita Satyapal told reporters at a meeting of the American Astronomical Society (AAS) in Austin, Texas.

"This constitutes the best evidence yet that a bulge is not necessary for a black hole to form and grow."

Symbiotic Growth

Galaxies come in all shapes and sizes, noted Satyapal, an astronomer at George Mason University in Virginia.

Elliptical galaxies resemble fluffy balls of stars and gas that are themselves massive bulges. Spiral galaxies are disks that can exist with or without fat bulges at their centers.

Previously, scientists had noticed that almost every galaxy with a bulge harbors a supermassive black hole, and that the mass of a galaxy's black hole is roughly equal to the mass of its bulge.

This suggested that the two must develop together, like a pair of symbiotic species. The correlation meant that skinny galaxies shouldn't have central black holes.

But galaxies with little or no bulge have lots of gas and dust in their middles that obscure visible light, Satyapal said. So her team examined nearby skinny galaxies in infrared, which can penetrate the haze.

Although astronomers can't see black holes, they can detect the light emitted from hot matter as it rapidly spirals into the void.

What the team found is that the centers of several skinny galaxies have highly ionized neon—a form of gas that can be created only by high-energy light spat out from an active black hole.

Extending Theory

Satyapal's team doesn't yet have direct measurements of the masses of the skinny galaxies' black holes. But the researchers believe they must be hundreds of times as massive as the sun.

"That clearly will impact theories of galaxy formation and evolution," Satyapal said.

Lynn Cominsky is an astrophysicist at Sonoma State University and a deputy press officer for AAS.

She commented at the briefing that students often ask her which comes first: the galaxy or its black hole?

When astronomers noticed the correlation between the masses of black holes and their bulges, "I didn't have to answer that, because [the correlation suggests] they grow up together."

Now, with Satyapal and colleagues' work, "it looks like this is additional evidence that this happens even down at the lower mass ranges."

Spin Me Right 'Round

In an unrelated paper presented at the AAS meeting, Rodrigo Nemmen at Pennsylvania State University and his colleagues found that many supermassive black holes at the centers of galaxies are spinning almost at the maximum rate allowed by theory.

"We think these monster black holes are spinning close to the limit set by Einstein's theory of relativity, which means that they can drag material around them at close to the speed of light," Nemmen said.

A galaxy feeds its central black hole material equal to ten Earth masses each month, he noted.

"The black hole, in answer to that feeding of gas, produces jets that emit in one second more than 50 times the energy radiated by our sun over a year."

Observations made with the Chandra X-Ray Observatory show such jets blowing bubbles of gas inside several very massive galaxies.

To reproduce this observation in theoretical models, the black holes must be spinning extremely fast, Nemmen said, creating the energy needed to account for the jets.

"Fast spins are the turbo in the black hole engine," he said.

Understanding how this turbo works "could help us understand how massive black holes acquired their spin over their evolution."

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