Some things are so much on the edge they can only be referred to as extreme. There are extreme sports and extreme holidays and now there are extreme galaxies.
Only a handful of these entities have been discovered so far, but they are so unusual that astronomers are happy to refer to them as extreme. They release light at extraordinarily high energies, the highest discovered anywhere in the universe.
Scientists have long sought unusual energy sources across the universe and they pool resources and knowledge to achieve this. One such effort is the Whipple Collaboration.
"The Whipple Collaboration has spent over 20 years developing instruments and techniques to discover exotic high energy events," explained Prof David Fegan, a high-energy astrophysicist in UCD's department of experimental physics.
"There is a loose affiliation of Irish institutions funded by Enterprise Ireland to be part of the Whipple Collaboration," he said. Included in the group are NUI Galway, NUI Maynooth, Cork Institute of Technology and Galway Mayo Institute of Technology. They work with other collaboration members to find ways to explain strange high-energy sources.
Prof Fegan and his departmental co-investigator, Dr John Quinn, with research student Ms Deirdre Horan, helped in the recent discovery of two extreme galaxies, named 1H1426 and 1ES2344. They spit out gamma rays at enormous energies - almost too high to comprehend.
Light energy is measured in electron volts and a room or office light fitting pushes out photons at about one electron volt. A hospital x-ray machine might reach 1,000 electron volts and the latest cancer therapy devices have proton beams reaching two or three million electron volts (mev). These are as nothing compared to the energies reached by the extreme galaxies that fire out photons at about 10,000 mev.
Super black holes at the centre of galaxies are thought to be the energy source for these photons, Prof Fegan said. They have the mass of 300 million of our suns packed into a space no larger than our own solar system. They act "like giant cosmic vacuum cleaners" drawing in any matter in their vicinity.
When this material falls into the black hole "it is heated to enormous temperatures and radiates", Prof Fegan said. It is this radiation that reaches remarkable energy levels. which we see only because of the greatest of good luck.
There are probably many of these "active galactic nuclei", Prof Fegan said. The difficulty is spotting them. The high energy photons stream out into space and most of them just pass us by, effectively leaving them out of sight.
Sometimes however the photons are released directly towards us, allowing us to look straight into the source of the energy. This is why only a handful of extreme galaxies have been identified so far.
"I like to think of them as looking down an exhaust pipe but they are called jets," Prof Fegan said. The photons come straight at us but we can only watch their arrival indirectly when they collide with the atmosphere and produce a phenomenon known as Cerenkov radiation.
The photons smash into the upper atmosphere, knocking loose a cascade of particles and giving off a faint light. "It lasts for a few [millionths of a second] so you need very special techniques to pick it out," Prof Fegan said.
A major difficulty is the atmosphere is constantly bombarded by high-energy cosmic rays and other particles coming in from space and these cause Cerenkov radiation. "We have to pick the gamma rays out from a huge background," he said. The latest techniques allow them to screen out the background and pick up only the light caused by the gamma rays from the extreme galaxies, he added. "We have been quite successful."
This data provides valuable information about the source of the photons, he said. The extreme galaxies emit the photons intermittently and at different wave lengths. The time during which the photons are visible to us is directly related to the size of the emitting object, its "emission region", Prof Fegan explained.
An object with a huge emission region would switch on and off over a long period but the extreme galaxies can do it in just 60 to 90 minutes. This implies they are quite small in astrophysical terms, about the size of our solar system.
The change in wavelength is a function of the physical mechanisms involved in the emission, Prof Fegan said. This gives scientists an idea of how the galaxy's "engine" operates.
The Whipple Collaboration is about to move into a new era with clusters rather than single devices measuring Cerenkov Radiation. This will make measurements much more sensitive, Prof Fegan said and better able to eliminate the background interference.