GALAXY HEART EMITS GAMMA RAYS

RedOrbit
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3 October 2009, 11:13 CDT

Quite a few distant galaxies turn out to be cosmic delivery
rooms. Large numbers of massive stars are born in the hearts of
these starburst galaxies, and later explode as supernovae. In the
remnants they leave behind, particles are accelerated to very high
energies. Astrophysicists have now used the H.E.S.S. telescopes
to make detailed measurements of the gamma rays from the NGC 253
galaxy. As predicted, these high-energy rays originate from the region
of maximum supernova activity close to the center. (Science Express,
September 2009)

At a distance of some twelve million light years away, NGC 253 is one
our closest spiral galaxies outside the so-called local group of our
Milky Way and its companions. Observations in the visible light as
well as in the infrared and radio frequency ranges had already shown
there was a small region at the center of NGC 253 which gave birth to
a very high number of stars. This region exhibits a very high density
of interstellar dust and gas.

The high-mass stars born in this region use up their nuclear fuel
relatively quickly and stagger into an energy crisis at the end of
their life. The nucleus collapses while the star destroys itself in
one final explosion. Such a supernova suddenly flares up a million
or even a billion times brighter than before. The charged particles
accelerated to very high energies in the remnants of such explosions
react with the surrounding medium or with electromagnetic fields to
generate extremely high-energy gamma quanta.

Between 2005 and 2008, astrophysicists used the H.E.S.S. telescope
system in Namibia over a total observation period of 119 hours to
detect the expected gamma rays at energies exceeding 220 GeV (billion
electronvolts). The source of these rays lies precisely at the optical
center of NGC 253 and appears as a point to H.E.S.S. This makes it
the weakest source discovered to date in the very high-energy gamma
radiation range.

The flux of radiation f of NGC 253 measured by H.E.S.S. implies an
enormous cosmic ray density – more than 1,000 times higher than at
the center of the Milky Way. Moreover, the high gas density makes
the conversion of cosmic rays into gamma rays around one order of
magnitude more efficient. Accordingly, the central region of NGC 253
shines around five times as brightly in the light of gamma rays as
all the rest of the galaxy together.

The four H.E.S.S. telescopes, each with a mirror area of 108 square
meters, observe weak bluish and extremely short flashes of light. This
so called Cherenkov radiation is emitted by showers of particles
created when high-energy gamma quanta collide with molecules in Earth’s
atmosphere. H.E.S.S. stands for High Energy Stereoscopic System and
has been in operation since the beginning of 2004. Since this time it
has made many important discoveries, such as the first astronomical
image of a supernova remnant in the high-energy gamma radiation range,
or the detection of galaxies with active nuclei in the light of
gamma rays. The fifth, much larger telescope that is currently under
construction will significantly improve the sensitivity of the system
and extend the observable energy range. The H.E.S.S. collaboration
under the overall lead management of the Max Planck Institute for
Nuclear Physics involves more than 150 researchers from Germany,
France, Great Britain, Poland, Czech Republic, Ireland, Austria,
Sweden, Armenia, South Africa and Namibia.