The Loneliest Black Holes in the Universe (Forwarded)

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For Immediate Release: June 1, 2007

The Loneliest Black Holes in the Universe -- a New Study

HONOLULU -- Actively growing supermassive black holes in centers of
galaxies are common even in cosmic voids, the most rarefied and empty
regions of the universe.

In a study of more than 1,000 void galaxies, using data from the Sloan
Digital Sky Survey (SDSS-II), astronomers from Drexel and Widener
Universities announced that the growth of these monster black holes --
with masses millions to hundreds of millions times that of our sun -- are
found where galaxies are sparse and interact very little with each other.
The researchers also found that the accretion of matter onto these void
black holes is slower than in denser galactic environments.

These findings shed light on the black hole formation and evolution
process by showing that the environment does affect how quickly galaxies
proceed through their evolutionary cycle.

The simple presence of growing supermassive black holes in the rural
outposts of the universe challenges the current theoretical models of
galaxy and structure formation and evolution, explained Anca Constantin of
Drexel University, lead author of the paper delivered today at the
American Astronomical Society meeting in Honolulu.

"Interestingly, we see actively accreting galactic black holes in all
phases of evolution in these sparse regions," said Constantin. "This means
that the black hole growth process is quite similar in what could be
compared to the most reclusive countrysides and in the crowded urban
regions of the universe."

The void regions, nearly empty, three-dimensional fields hundreds of
millions of light-years across, fill half of the universe. Only five
percent of all galaxies live in these bubble-like regions. The other 95
percent of galaxies live together in communities, crowded into clusters,
filaments, and walls: the cities and suburbs of the universe.

Studying a 700-million light year wide 'slice' of the universe, the
researchers found that spectra of the centers of void galaxies show hot
gases ionized by light emitted from matter swirling around supermassive
black holes. Constantin adds that, "the more isolated accreting black
holes are however not as active as the ones in more populous environs, and
the fuel seems less available for accretion in voids than in 'urban'
galaxies."

Astronomer Fiona Hoyle, a member of the discovery team from Widener
University added: "This is strange given that these reclusive galaxies are
forming stars at higher rates than their counterparts in denser regions;
this means there is plenty of fuel, but it is not efficiently channeled
toward the central engine."

Star formation requires the presence of large amounts of gas and so there
must be more than enough gas in the void galaxies if their star forming
rates are high, explained Hoyle. The smaller accretion rate observed in
void galaxies means that this gas is just not getting down to the nuclear
region where accretion happens. Interactions with other galaxies are
thought to disturb the gravitational potential, which drives some gas into
the nuclear region. "These interactions are not as frequent in voids, so
the 'feeding' of the black hole is slower."

These rugged individuals in voids do not need to compete with their
neighbors for fuel, and their life cycle is rarely bothered, noted
Constantin. In contrast, life is more hectic in crowded regions where
galaxy interactions are frequent. As a consequence, galaxies are either
stripped of their gas or more material is funneled toward the central
engine. This means that there are many more chances the accretion onto
black holes is enhanced or turned off in more 'urban environments.'

"On the other hand, the void galaxy black holes might take longer to reach
the mature, low accretion rate phase, which might explain why the most
massive, lazy black holes are less frequent in voids," she noted. The data
studied by Constantin may also show that active black holes appear to be
more common in voids but only among small (less massive) galaxies, while
less common among massive galaxies. This is also a clue that the life
cycle of black hole growth in voids is delayed or slower compared to that
in denser regions.

Discovery team member Michael Vogeley of Drexel said that it's
particularly puzzling that the few most massive and sluggishly accreting
void systems live within the most secluded sub-regions, while their
"urban" counterparts are found in the most populated neighborhoods.

"Perhaps because massive objects are prone to accreting material around
them, such a 'cleaning' process would contribute to emptying the already
rarefied neighboring space in voids," Vogeley noted. "This would leave
little or insufficient material for future formation of other nearby
massive, bright galaxies." In contrast, within galaxy clusters where there
is plenty of stuff around, accretion of surrounding material would make a
small difference.

These results have been possible only because of the sheer number of void
regions and void galaxies found in the SDSS-II data, the most ambitious
survey of the universe ever undertaken, the researchers said. The sample
used in the analysis announced today comprises more than 1,000 void
galaxies. Previously, the black hole accretion in centers of void galaxies
had been studied in only a handful of objects contained in only one void
region, the Bootes Void.

The results are described in the paper "Active Galactic Nuclei in Void
Regions" submitted for publication in The Astrophysical Journal.

Authors:

Anca Constantin
Department of Physics, Drexel University
3141 Chestnut Street, Philadelphia, PA 19104

Fiona Hoyle
Department of Physics and Astronomy, Widener University
One University Place, Chester, PA 19013

Michael S. Vogeley
Department of Physics, Drexel University

Support for this work was provided by NASA through grant NAG5-12243 and by
the National Science Foundation through grant AST-0507647.

Color art is available at
http://www.physics.drexel.edu/~constant/agn_voids.gif [1.5MB]

Caption:

This is a "slice" of about 700 million light-years across in the
distribution of galaxies in the universe. Voids are marked as blue
circles, and galaxies within as red dots. The image shows a void galaxy
that harbors an actively accreting black hole in its center; such activity
is revealed by its optical spectrum exhibiting emission spikes,
characteristic of hot gases ionized by light emitted from matter swirling
around a supermassive black hole. Other objects in the image are either in
the foreground or background.

CREDIT: John Parejko, Danny Pan and Anca Constantin of Drexel University