Astronomy at the end of the rainbow - the extreme Universe
One H.E.S.S. telescope reflected in the mirrors of another (The H.E.S.S. Collaboration).
Mr Dave Allan, Mr Anthony Brown - Durham University
A team from Durham University are using the world's most powerful gamma ray telescopes to detect black holes, the remains of dead stars and mysterious dark objects in our galaxy. The High Energy Stereoscopic System (HESS) telescopes were built in Namibia by a collaboration of European and African scientists, including the Durham team. The collaboration holds the record for the most distant sources of very high-energy gamma rays detected in the Universe and are throwing light on cosmic rays, galaxy formation and dark matter. 'HESS has tripled the known sources of gamma rays out there,' says Paula Chadwick from Durham. 'Our Galaxy appears to be studded with objects that emit gamma rays.'
The gamma rays detected by HESS are in fact used as a proxy measure for cosmic rays that continually bombard Earth from outer space. Cosmic rays are mostly charged sub-atomic particles, and large numbers of them pass through our bodies every day. The trouble is that, because they are electrically charged, they don't travel in straight lines. However, when the cosmic ray particles are accelerated to extreme energies, gamma rays are produced, and gamma rays travel in straight lines. The four telescopes in the HESS array can pinpoint the exact source of the gamma rays and therefore the source of the cosmic rays.
The most distant sources of gamma rays detected by the HESS array are active galactic nuclei (AGN), thought to be an early phase in the evolution of galaxies. When gamma rays travel from such distant sources, it provides enough time for a very rare interaction to take place, causing the loss of the gamma rays and creating a potential problem for gamma ray astronomers. In the case of AGN the gamma rays are thought to react with photons produced by the earliest stars the extragalactic background light (EBL). 'This EBL is very hard to measure because there is so much emission from much nearer objects,' explains Paula. 'So we can turn our problem into a virtue and look at how the gamma rays from AGN are absorbed in order to understand the EBL.' Results so far have been surprising. 'We see very little, if any, absorption from the EBL,' says Paula. 'This puts a question mark over the nature of the EBL and gives those interested in galaxy formation and the earliest stars something to think about.' It is good news for HESS, however, as it means the Universe is more transparent to gamma rays than previously thought.
HESS may also allow the identification of the elusive dark matter that is thought to make up 25% of the Universe. One candidate for dark matter is the WIMP weakly interacting massive particles which collect around massive objects, including the black hole at the centre of our galaxy. 'These particles are expected to annihilate, and when they do they will produce gamma rays, so we hunt for the gamma rays and bingo! We detect dark matter', explains Paula. In reality the situation is far more complex. 'So far, we don't think we have seen evidence for dark matter', says Paula. 'But we shall be looking further a field, and with HESS phase II adding a fifth telescope and greater power to the array we are hopeful'.