BULK
SOURCE OF UNIVERSE'S GAMMA RAYS IDENTIFIED, SCIENTISTS SAY Scientists
at Columbia University and Barnard College have found that the majority of the
gamma rays outside of our galaxy are likely emitted by galaxy clusters and other
massive structures. This may resolve a 30-year-old mystery as to the origin of
the Universe's gamma-ray background. The
finding may also provide new insight about how structure formed in the Universe,
as well as the nature of magnetic fields in the intergalactic medium, of which
little is currently known. Caleb
Scharf of Columbia University and Reshmi Mukherjee of Barnard College publish
this result in an upcoming issue of the Astrophysica Journal. The finding is based
on the analysis of a nine-year record of gamma rays arriving at Earth from deep
space, collected by NASA's Compton Gamma Ray Observatory throughout the 1990s.
"This
result not only resolves the question of where all these gamma rays are coming
from, but provides a new probe of the gravity-driven picture of structure formation
in the Universe," said Scharf. Gamma
rays are the highest-energy form of light. In the Milky Way galaxy, gamma rays
are largely produced when cosmic rays, atomic particles moving at near light speed,
collide with interstellar gas. Black holes and neutron stars are also sources
of gamma rays. Galaxy
clusters are the largest gravitationally bound structures in the Universe. The
clusters in this analysis contain up to several thousand Milky Way-size galaxies,
and they are still accumulating material (gas and whole galaxies) from the surrounding
space.
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The
finding announced today supports a theory of gamma-ray production
posed by Prof. Avi Loeb of Harvard University and Prof. Eli
Waxman of the Weizmann Institute in Rehovot, Israel. Not unlike
a black hole, the sheer mass of a cluster serves as a gravitational
drain, drawing in matter at speeds of up to a thousand miles
per second. Electrons in this flow are accelerated, with an
additional boost from magnetic fields, to near light speed
and collide with microwave light, the afterglow from the big
bang known as the cosmic microwave background.
These
microwave light particles, or photons, are bumped up to the gamma-ray photon energy
level. The gamma rays form a halo around the galaxy clusters. Other scientists,
however, have suggested that the bulk of the gamma-ray background is produced
not by this mechanism but by quasar-type galaxies, called blazars, each powered
by a supermassive black hole. This background was discovered by NASA's second
Small Astronomy Satellite (SAS-2) in the early 1970s.
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Scharf
and Mukherjee's new research compared a catalog of 2,469 galaxy
clusters with the Compton database. Using sophisticated statistical
techniques, they showed that the sky surrounding the most
massive clusters was systematically brighter in gamma rays
than other regions.
"The
more massive the cluster (and greater the gravitational potential), the brighter
the gamma-ray halo," said Mukherjee. "The enhancement observed was very
similar to that predicted by the Loeb-Waxman theory." The
result announced today also supports the theory of the cosmic web. Scientists
say that matter in the Universe forms a cosmic web, in which galaxies are formed
along filaments of ordinary matter and dark matter like pearls on a string. Clusters
form at the intersection of these filaments. The electrons that fuel the gamma-ray
production rush into clusters along these rivers or filaments of matter connecting
galaxies and clusters. Thus, gamma rays serve as probes to the early structure-forming
epoch of the Universe.
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Gamma
ray halos around clusters also provides a means to measure
intergalactic magnetic fields. Two of the three variables
to measure magnetic fields are known: the mass of galaxy clusters
and the distribution of the microwave background. The third
variable is electron efficiency, which can now be measured
by virtue of gamma-ray production.
The
Gamma-ray Large Area Space Telescope (GLAST), scheduled for launch in 2006, should
resolve gamma-ray haloes around galaxies with unprecedented clarity. GLAST could
measure intergalactic magnetic fields and watch the formation of structure in
the universe through its gamma-ray eyes, the scientists said. Back
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