Thesis defense [before December 2013]
Licentiate thesis: High resolution simulations of galaxy clusters with cosmic ray physics
by
→
Europe/Stockholm
FB42
FB42
Description
Gamma-ray astronomy has entered an exiting era as a vast number of experiments have recently emerged. The quest for the first detection of a galaxy cluster in the high energy gamma-ray regime is ongoing, and even though clusters are observed in several other wave-bands, there is still no firm detection in gamma-rays. This is because the emission spectra of clusters are sufficiently steep which results in very low gamma-ray fluxes at these high energies. The steepness of the emission spectrum depends to a large extent on the cosmic ray (CR) spectrum as CR interactions with ambient matter hadronically produce pions which decay into gamma-rays. In this thesis, we use high-resolution simulations of galaxy clusters that include self-similar CR physics to complement those observational efforts. We aim at theoretically understanding the connection between the observable gamma-ray emission and the generating relativistic components. This knowledge ultimately will enable us to study the underlying fundamental plasma astrophysics and important cosmological questions. Our result is based on 14 galaxy cluster simulations performed with the cosmological state-of-the-art code Gadget3. We find an universal concave shaped CR spectrum independent of the simulated galaxy cluster with a break at ~106 GeV. The secondary emission spectra, dominating the central cluster emission, naturally correlate well with the parent CR spectrum. At the cluster periphery, there is a transition to the inverse Compton emission from relativistic electrons that are accelerated at structure formation shocks. Due to physics related to shock acceleration, this emission component shows a steeper spectrum in the energy regimes accessible to the Fermi space telescope and imaging air Cherenkov telescopes. Furthermore, based on newly derived flux and luminosity functions, we predict a detection of 2-13 galaxy clusters above 100 MeV with Fermi after two years, with Ophiuchus and Coma being the brightest clusters.