FERMI investigates the origin of extragalactic gamma rays
Only a third of all emissions come from the jets of active galaxies. Dark matter and particle acceleration are further likely sources
02 Mar 2010
The FERMI satellite has made another new important finding. Data collected during the first year of observations made by the Large Area Telescope (LAT), FERMI’s "super eye", have revealed that less than a third of extragalactic gamma emissions come from what astronomers had until now considered to be the number one suspects: jets coming from the black holes of active galaxies.
After discovering the "corkscrew" magnetic fields of galaxy 3C27, the NASA probe, which was designed and built with a significant Italian contribution and is dedicated to studying high energies, is providing important data on gamma ray sources outside the Milky Way.
Active galaxies have a compact region at their centre; a black hole with a mass that ranges from a million to a billion times that of the Sun. When matter falls towards the black hole, some of it is redirected as jets of particles that travel at nearly the speed of light. These particles can produce gamma rays in two different ways. Firstly, if a flash hits a visible or infrared photon, the photon can gain energy and become a gamma ray. Or, if one of the particles from the jet hits an atom of gas, the collision can create a particle called a pion that decays rapidly generating two gamma rays.
Research conducted by the international team of FERMI shows that active galaxies, also known as active galactic nuclei (AGN), account for less than 30% of the extragalactic gamma radiation observed by the probe. This means that over 70% of the radiation detected by FERMI is of unidentified origin.
To arrive at these conclusions the LAT team’s first step was to isolate and eliminate the emissions coming from our galaxy. The extragalactic background is very weak and so it is easily confused with luminous emissions coming from the Milky Way. By separating the two energetic components it was possible to determine the level of absolute background. Then the emissions from the active galaxies identified by FERMI were compared with the number of galaxies necessary to produce the extragalactic background observed. The result was that in the high energies between 0.1 and 100 billion electron volts the active galaxies appear to have only a secondary role.
But if the jets from black holes of active galaxies do not produce most of the gamma rays in the extragalactic background, where does the energy detected by FERMI come from? Scientists have already proposed some initial theories.
Markus Ackermann of the FERMI LAT team, who led the measurement studies, says that an ideal gamma source candidate could be the particle acceleration that occurs when normal stellar galaxies form or the particle acceleration in the final assembly phase of a large structure, such as that observed during the fusion of galaxies.
Then there is dark matter, the mysterious substance that does not produce or absorb light but has gravity that affects ordinary matter. Marco Ajello, astrophysicist from the Kavli Institute for Particle Astrophysics and Cosmology (KIPAC), says that dark matter could be a type of still unknown subatomic particle and that if this is true the particles of dark matter could interact with each other in a way that produces gamma rays.
Ajello presented the research results at the meeting of the American Astronomical Society's High-Energy Astrophysics Division at Waikoloa, Hawaii. The results are published in Physical Review Letters.