The Sky is Full of Black Holes

http://www.universetoday.com/am/publish/glimpse_at_universe.html?2022006

Mon, 20 Feb 2006 - Astronomers have been puzzling over a mystery for a few
years now, and they think they might have an answer. Everywhere you look,
there's a diffuse glow of background X-ray radiation - but where's it
coming from? Astronomers pointed the Chandra X-Ray Observatory at a patch
of sky for a total of 23 days over a 2-year period, and resolved 600
separate point sources. This isn't a background glow at all, but the X-ray
radiation from hundreds of millions of supermassive black holes, like the
one at the heart of the Milky Way.

Data from X-ray observatory surveys show that black holes are much more
numerous and evolved differently than researchers would have expected,
according to a Penn State astronomer.

"We wanted a census of all the black holes and we wanted to know what they
are like," said Niel Brandt, professor of astronomy and astrophysics. "We
also wanted to measure how black holes have grown over the history of the
Universe."

Brandt and other researchers have done just that by looking at a patch of
sky in the Northern hemisphere called the Chandra Deep Field-North, using
NASA's Chandra X-ray Observatory and a similar patch in the Southern
hemisphere called the Extended Chandra Deep Field-South. Surveys also are
being carried out in other parts of the sky using both Chandra and the
European Space Agency's X-ray Multi-Mirror Mission-Newton.

The researchers looked at X-ray emissions because areas around black holes
emit X-rays as well as visible light. The penetrating nature of X-rays
provides a direct way to identify the black holes. Using X-rays also
enables astronomers to pinpoint the black holes at the centers of galaxies
without their signal being washed out by the visible light coming from a
galaxy's stars, Brandt told attendees at the annual meeting of the American
Association for the Advancement of Science in St. Louis, Mo. Feb. 17. The
black holes they studied were those that reside at the centers of galaxies
and are actively emitting X-rays, therefore they are called active galactic
nuclei.

"We find active super massive black holes at the centers of massive
galaxies," said Brandt. "Our galaxy also has its own black hole at its
center measuring 2.6 million solar masses. Our black hole is not active
today, but we presume it was active in the past."

These deep, extragalactic X-ray surveys looked at carefully chosen patches
of sky, that are largely free of anything that might interfere with
obtaining the X-ray data. Chandra looked at the Chandra Deep Field-North --
an area of sky two thirds the size of the full Moon -- for the time span of
23 days over a two-year period. The researchers detected about 600 X-ray
sources. After comparing the X-ray images with optical images of exactly
the same slice of sky taken by the Hubble Space Telescope, nearly all 600
point sources corresponded to optical galaxies, suggesting that the black
holes that were sources for the X-ray signature were in the centers of
galaxies.

"X-ray astronomers are doing better than anyone else by about a factor of
ten, in identifying these active galactic nuclei" said Brandt. "With more
time we could do even better, going even deeper."

What the researchers found was that super massive black holes are more
numerous than we might have expected. They also found that black holes
evolved differently than astronomers expected prior to the Chandra work.
Extrapolating from the 600 black holes found by Chandra, Brandt suggests
that there are about 300 million super massive black holes in the whole
sky.

The existence of so many black holes, confirmed that what was once thought
to be a truly diffuse cosmic X-ray background radiation, actually comes
from point sources.

In the 1960s, astronomers discovered quasars, very distant, highly luminous
black holes, in galactic centers. Quasars, initially called quasi-stellar
radio sources, were studied intensely. Researchers soon realized that only
some of these objects were radio emitters and that they formed early in the
history of the Universe.

"While quasars are spectacular, they are not representative of typical
active galactic nuclei," said Brandt. "Now, using Chandra and other X-ray
observatories, we can find and study the moderate-luminosity, typical
active galactic nuclei in the distant, high-redshift Universe."

Quasars and moderate-luminosity active galactic nuclei also evolved
differently. Quasars are a phenomenon of young galaxies, while
moderate-luminosity, active galactic nuclei peaked later in cosmic time.

"We would like to know if active galactic nuclei change over cosmic time,"
said Brandt. "Do black holes feed and grow in the same way over the history
of the Universe?"

Researchers looked at the relative amount of power coming out in X-rays
compared to other wavelengths and found that this ratio does not change
over 13 billion years of time. They looked at the X-ray spectra and found
that these also did not change through time.

"Despite the enormous changes in the space density of back holes, the
individual engines powering active galactic nuclei are remarkably stable,"
Brandt said.

Brandt believes that Chandra could observe the Chandra Deep Field-North for
a longer period of time and obtain more sensitive, deeper data. This would
bring to light galaxies that are currently obscured. It would also gather
more X-rays allowing better X-ray spectral and variability analyses. With
more sensitive probing, the researchers are also detecting an increasing
number of non-active galaxies like our own.

"Chandra has worked well for six years now," said Brandt. "There is no
reason why Chandra and Newton cannot continue to observe for another 10 or
more years."


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