What is producing the gamma-rays from the radio galaxy Fornax A?

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.

Publication details:

B. McKinley, R. Yang, M. López-Caniego, F. Briggs, N. Hurley-Walker, R. B. Wayth, A. R. Offringa, R. Crocker, G. Bernardi, P. Procopio, B. M. Gaensler, S. J. Tingay, M. Johnston-Hollitt, M. McDonald, M. Bell, N. D. R. Bhat, J. D. Bowman, R. J. Cappallo, B. E. Corey, A. A. Deshpande, D. Emrich, A. Ewall-Wice, L. Feng, R. Goeke, L. J. Greenhill, B. J. Hazelton, J. N. Hewitt, L. Hindson, D. Jacobs, D. L. Kaplan, J. C. Kasper, E. Kratzenberg, N. Kudryavtseva, E. Lenc, C. J. Lonsdale, M. J. Lynch, S. R. McWhirter, D. A. Mitchell, M. F. Morales, E. Morgan, D. Oberoi, S. M. Ord, B. Pindor, T. Prabu, J. Riding, A. E. E. Rogers, D. A. Roshi, N. Udaya Shankar, K. S. Srivani, R. Subrahmanyan, M. Waterson, R. L. Webster, A. R. Whitney, A. Williams, C. L. Williams in MNRAS (2014) “Modelling the Spectral Energy Distribution of Fornax A: Leptonic and Hadronic Production of High Energy Emission in the Radio Lobes”