What is producing the gamma-rays from the radio galaxy Fornax A?
16 June 2015
Radio galaxies are distant objects that emit tremendous amounts of energy, allowing astronomers – such as CAASTRO postdoctoral fellow Benjamin McKinley (University of Melbourne) and colleagues – to study them across the vast expanses of the Universe. Named so because they are easiest to observe in the radio part of the electromagnetic spectrum, radio galaxies such as Fornax A actually emit across a broad range of wavelengths, from microwaves to X-rays and gamma-rays. By combining data from a number of different astronomical instruments, Dr McKinley and team have studied the spectral properties of Fornax A to try and understand something that is still unknown: what mechanism is producing the high-energy gamma-rays that we observe?
The radio waves that we observe from galaxies such as Fornax A are produced by electrons being accelerated to close to the speed of light and spiraling around in the magnetic field of the galaxy. In a phenomenon that is extremely difficult to observe here on Earth, gamma-rays are generated when these high-speed electrons collide with low-energy background photons. The collisions boost the energy of the photons, producing the gamma-rays that we observe, or so it was thought. However, when you run the numbers, the observations don’t match the theory: there just aren’t enough gamma-rays resulting from this process, known as inverse-Compton scattering, to account for the observed signal.
The researchers investigated an alternative mechanism for the generation of gamma-rays in Fornax A and found that including this new process in their modelling gave a much better fit to the data. They found that most of the gamma-rays are likely the result of collisions between protons, rather than between electrons and photons. The particles produced when protons collide decay into gamma-rays and this process, likely to occur frequently in the relatively thin and dense filaments of Fornax A’s radio lobes, can account for the observed high-energy emission.