Hidden black holes located in gravity wells

Hidden black holes finally located in many undetected gravity wells
By Robert Roy Britt, Senior Science Writer, posted: 07 August 2005,
07:24 a.m. ET


A host of hidden black holes have been revealed in a narrow region of
the sky, confirming astronomers' suspicions that the universe is loaded
with many undetected gravity wells.

Black holes cannot be seen directly, because they trap light and
anything else that gets too close. But astronomers infer their presence
by noting the behavior of material nearby: gas is superheated and
accelerated to a significant fraction of light-speed just before it is
consumed.
The activity releases X-rays that escape the black hole's clutches and
reveal its presence.

The most active black holes eat so voraciously that they create a
colossal cloud of gas and dust around them, through which astronomers
cannot peer. That sometimes prevents observations of the region nearest
the black hole, making it impossible to verify what's actually there.

These hyperactive black holes are called quasars. They can consume the
mass of a thousand stars a year and are thought to be precursers to
large, normal galaxies. The exist primarily at great distances, seen as
they existed when the universe was young.

A few quasars have been identified, but many more are thought to await
discovery, based on the total number of X-rays detected in broad sky
surveys.
"From past studies using X-rays, we expected there were a lot of hidden
quasars, but we couldn't find them," said study leader Alejo
Martínez-Sansigre of the University of Oxford, England.

New observations with NASA's Spitzer Space Telescope cut through dust to
spot quasars blocked by their own clouds, as well as other quasars
hidden inside galactic dust. Spitzer records infrared light, which
penetrates dust. It found 21 quasars in a small patch of sky.

"If you extrapolate our 21 quasars out to the rest of the sky, you get a
whole lot of quasars," said study team member Mark Lacy of the Spitzer
Science Center at the California Institute of Technology. "This means
that, as suspected, most super-massive black hole growth is hidden by
dust."

The results are detailed in the Aug. 4 issue of the journal Nature.
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