South African scientists make astronomy breakthrough
26 January 2015
Wits scientists have found three extremely luminous gamma-ray sources in the Large
Magellanic Cloud, a satellite dwarf galaxy of the Milky Way.
The scientists are part of a multinational team of astronomers working on the High
Energy Stereoscopic System (HESS) telescopes that have again demonstrated its
excellent capabilities in searching for high-energy gamma rays.
In the latest discovery using the HESS, the team has found the most powerful pulsar
wind nebula; the most powerful supernova remnant; and a shell of 270 light years in
diameter blown by multiple stars, and supernovae – a so-called superbubble.
The discovery is announced in the latest edition of the scientific journal,
Science, in a research paper titled: "The exceptionally powerful TeV ƴ-
ray emitters in the Large Magellanic Cloud", published on 23 January 2015.
Important discovery
"This is a very important breakthrough
for the team," says Professor Sergio
Colafrancesco, DST/NRF SKA Research Chair in the Wits School of Physics. "It paves
the way to study external galaxies with very high-E telescopes such as HESS and
then later with the planned Cherenkov Telescope Array (CTA) in Namibia.
"It will lead us to re-examine galaxy evolution and answer questions such as how
high-E particles can affect the evolution of cosmic structures in the universe,
principally galaxies, and the life cycles of matter in galaxies."
Very high-energy gamma rays are the best tracers of cosmic accelerators such as
supernova remnants and pulsar wind nebulae – end-products of massive stars. There,
charged particles are accelerated to extreme velocities. When these particles
encounter light or gas in and around the cosmic accelerators, they emit gamma rays.
Very high-energy gamma rays can be measured on Earth by observing the Cherenkov
light emitted from the particle showers produced by
incident gamma rays high up in
the atmosphere using large telescopes with fast cameras.
The Large Magellanic Cloud is a dwarf satellite galaxy of our Milky Way, located about
170.000 light years away and showing us its face. New, massive stars are formed at
a high rate in the cloud, and it harbours numerous massive stellar clusters. The
cloud's supernova rate relative to its stellar mass is five times that of our galaxy.
The youngest supernova remnant in the local group of galaxies, SN 1987A, is also a
member of the cloud. Therefore, the HESS scientists dedicated significant
observation to searching for very high-energy gamma rays from this cosmic object.
Star formation
HESS has observed the largest star-forming region within the cloud, called Tarantula
Nebula, for a total of 210 hours. For the first time in a galaxy outside the Milky Way,
individual sources of very high-energy gamma rays could be resolved: three
extremely
energetic objects of different type.
The so-called superbubble 30 Dor C is the largest known X-ray-emitting shell and
appears to have been created by several supernovae and strong stellar winds.
Superbubbles are broadly discussed as (complementary or alternative to individual
supernova remnants) factories where the galactic cosmic rays are produced.
The HESS results demonstrate that the bubble is a source of, and filled by, highly
energetic particles. The superbubble represents a new class of sources in the very
high-energy regime.
Observed at the limits of detectability, and partially overlapping with each other,
these new sources challenged the HESS scientists. The discoveries were possible
following the development of advanced methods of interpreting the Cherenkov images
captured by the telescopes.
The new HESS II 28m telescope will boost the performance of the HESS telescope
system, and in the more distant future the planned Cherenkov
Telescope Array (CTA)
will provide even deeper and higher-resolution gamma-ray images of the Large
Magellanic Cloud – in the plans for science with CTA, the satellite galaxy is already
identified as a Key Science Project deserving special attention.
International team
The HESS telescope is operated by an international collaboration of scientists with a
strong involvement by South African universities, in particular Wits and North West
universities, and the universities of the Free State and Johannesburg.
Wits physicists are particularly involved in data analysis techniques, the development
of theoretical interpretation tools of both extragalactic and galactic sources, and in
the operational shifts at the telescope location in Namibia.
Source: University of the Witwatersrand
The CT2 and CT3 telescopes of the HESS telescope array in Namibia, near Gamsberg mountain. (Image: Wikimedia Commons: HESS Collaboration)
