Wednesday, June 13, 2012

The mystery of outer space: What makes black holes to grow in the centre of galaxies

 New evidence from NASA's Chandra X-ray Observatory
challenges prevailing ideas about how black holes grow in the centers
of galaxies. Astronomers long have thought that a supermassive black
hole and the bulge of stars at the center of its host galaxy grow at
the same rate -- the bigger the bulge, the bigger the black hole.
However, a new study of Chandra data has revealed two nearby galaxies
with supermassive black holes that are growing faster than the
galaxies themselves.

The mass of a giant black hole at the center of a galaxy typically is
a tiny fraction -- about 0.2 percent -- of the mass contained in the
bulge, or region of densely packed stars, surrounding it. The targets
of the latest Chandra study, galaxies NGC 4342 and NGC 4291, have
black holes 10 times to 35 times more massive than they should be
compared to their bulges. The new observations with Chandra show the
halos, or massive envelopes of dark matter in which these galaxies
reside, also are overweight.

This study suggests the two supermassive black holes and their
evolution are tied to their dark matter halos and did not grow in
tandem with the galactic bulges. In this view, the black holes and
dark matter halos are not overweight, but the total mass in the
galaxies is too low.

"This gives us more evidence of a link between two of the most
mysterious and darkest phenomena in astrophysics -- black holes and
dark matter -- in these galaxies," said Akos Bogdan of the
Harvard-Smithsonian Center for Astrophysics (CfA) in Cambridge,
Mass., who led the new study.

NGC 4342 and NGC 4291 are close to Earth in cosmic terms, at distances
of 75 million and 85 million light years. Astronomers had known from
previous observations that these galaxies host black holes with
relatively large masses, but are not certain what is responsible for
the disparity. Based on the new Chandra observations, however, they
are able to rule out a phenomenon known as tidal stripping.

Tidal stripping occurs when some of a galaxy's stars are stripped away
by gravity during a close encounter with another galaxy. If such
tidal stripping had taken place, the halos mostly would have been
missing. Because dark matter extends farther away from the galaxies,
it is more loosely tied to them than the stars and more likely to be
pulled away.

To rule out tidal stripping, astronomers used Chandra to look for
evidence of hot, X-ray-emitting gas around the two galaxies. Because
the pressure of hot gas -- estimated from X-ray images -- balances
the gravitational pull of all the matter in the galaxy, the new
Chandra data can provide information about the dark matter halos. The
hot gas was found to be distributed widely around NGC 4342 and NGC
4291, implying that each galaxy has an unusually massive dark matter
halo and that tidal stripping is unlikely.

"This is the clearest evidence we have, in the nearby universe, for
black holes growing faster than their host galaxy," said co-author
Bill Forman, also of CfA. "It's not that the galaxies have been
compromised by close encounters, but instead they had some sort of
arrested development."

How can the mass of a black hole grow faster than the stellar mass of
its host galaxy? The study's authors suggest a large concentration of
gas spinning slowly in the galactic center is what the black hole
consumes very early in its history. It grows quickly, and as it
grows, the amount of gas it can accrete, or swallow, increases along
with the energy output from the accretion. After the black hole
reaches a critical mass, outbursts powered by the continued
consumption of gas prevent cooling and limit the production of new

"It's possible that the supermassive black hole reached a hefty size
before there were many stars at all in the galaxy," said Bogdan.
"That is a significant change in our way of thinking about how
galaxies and black holes evolve together."

The results were presented June 11 at the 220th meeting of the
American Astronomical Society in Anchorage, Alaska. The study also
has been accepted for publication in The Astrophysical Journal.

NASA's Marshall Space Flight Center in Huntsville, Ala., manages the
Chandra program for the agency's NASA's Science Mission Directorate
in Washington. The Smithsonian Astrophysical Observatory in
Cambridge, Mass., controls Chandra's science and flight operations.

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