NASA'S Swift Survey Finds 'Smoking Gun' Of Black Hole Activation

WASHINGTON -- Data from an ongoing survey by NASA's Swift satellite
have helped astronomers solve a decades-long mystery about why a
small percentage of black holes emit vast amounts of energy.

Only about one percent of supermassive black holes exhibit this
behavior. The new findings confirm that black holes "light up" when
galaxies collide, and the data may offer insight into the future
behavior of the black hole in our own Milky Way galaxy. The study
will appear in the June 20 issue of The Astrophysical Journal
Letters.

The intense emission from galaxy centers, or nuclei, arises near a
supermassive black hole containing between a million and a billion
times the sun's mass. Giving off as much as 10 billion times the
sun's energy, some of these active galactic nuclei (AGN) are the most
luminous objects in the universe. They include quasars and blazars.

"Theorists have shown that the violence in galaxy mergers can feed a
galaxy's central black hole," said Michael Koss, the study's lead
author and a graduate student at the University of Maryland in
College Park. "The study elegantly explains how the black holes
switched on."

Until Swift's hard X-ray survey, astronomers never could be sure they
had counted the majority of the AGN. Thick clouds of dust and gas
surround the black hole in an active galaxy, which can block
ultraviolet, optical and low-energy, or soft X-ray, light. Infrared
radiation from warm dust near the black hole can pass through the
material, but it can be confused with emissions from the galaxy's
star-forming regions. Hard X-rays can help scientists directly detect
the energetic black hole. Since 2004, the Burst Alert Telescope (BAT)
aboard Swift has been mapping the sky using hard X-rays.

"Building up its exposure year after year, the Swift BAT Hard X-ray
Survey is the largest, most sensitive and complete census of the sky
at these energies," said Neil Gehrels, Swift's principal investigator
at NASA's Goddard Space Flight Center in Greenbelt, Md.

The survey, which is sensitive to AGN as far as 650 million
light-years away, uncovered dozens of previously unrecognized
systems.

"The Swift BAT survey is giving us a very different picture of AGN,"
Koss said. The team finds that about a quarter of the BAT galaxies
are in mergers or close pairs. "Perhaps 60 percent of these galaxies
will completely merge in the next billion years. We think we have the
'smoking gun' for merger-triggered AGN that theorists have
predicted."

Swift, launched in November 2004, is managed by Goddard. It was built
and is being operated in collaboration with Penn State, the Los
Alamos National Laboratory in New Mexico, and General Dynamics in
Falls Church, Va.; the University of Leicester and Mullard Space
Sciences Laboratory in the United Kingdom; Brera Observatory and the
Italian Space Agency in Italy; plus additional partners in Germany
and Japan.

Other members of the study team include Richard Mushotzky and Sylvain
Veilleux at the University of Maryland and Lisa Winter at the Center
for Astrophysics and Space Astronomy at the University of Colorado in
Boulder.

For more information, images and animations, visit:

http://www.nasa.gov/swift

Source: NASA