Contribution of Galaxies and Galaxy Clusters to the Diffuse Gamma-ray Background

The aim of this thesis was to link isotropic diffuse gamma-ray background with the neutrino observations and measured lithium abundances outside of the Milky Way. The connection was made using the fact that gamma-rays, neutrinos, and lithium can be produced by interactions of cosmic rays with the interstellar medium. These links were then applied to accretion shocks as sources of cosmic rays in clusters of galaxies and cosmic rays accelerated in supernova remnants inside small galaxies, in order to learn more about these two types of objects and cosmic rays that can be accelerated in them.

We first examine accretion of new gas onto already virialized structures (for example in galaxy clusters). For the first time, we include the change of gamma-ray production with time, which reflects the evolution of accretion shocks that appear during large-scale structure formation. The models developed in this thesis are more realistic compared with those that use single redshift approximation for the gamma-ray origin (Prodanović & Fields 2005). Models are used to derive the gamma-ray flux of all unresolved galaxy clusters (Dobardžić & Prodanović 2014), which is than compared with the isotropic diffuse gamma-ray background measured by telescope Fermi-LAT. We find that these cosmic rays can have non-negligible contribution to the isotropic diffuse gamma-ray background (depending on the normalization). Gamma-ray models are also normalized using high-energy neutrinos detected by the IceCube detector, from which we conclude that if the accretion shocks are predominantly strong, neutrino background is more limiting to the possible gamma-ray emissivity of these objects, compared with the gamma-ray background we first used (Dobardžić & Prodanović 2015).

One part of the thesis deals with the production of cosmic rays in supernova remnants, in particular, the case of the Small Magellanic Cloud, which was detected in gamma-rays. In this galaxy we also have the first measurements of the lithium abundances in the interstellar gas outside of the milky way. We show that galactic cosmic rays (considered to be the dominant cosmic-ray population in the Small Magellanic Cloud) can only explain a very small part of the observed abundance of lithium, if we assume that the entire present gamma-ray emissivity that we observe also originates from the interaction of galactic cosmic rays with gas within the galaxy (Ćiprijanović 2016). This leaves room for the possible existence of other sources of lithium in the Small Magellanic Cloud. We also estimate how much irregular dwarf galaxies can contribute to the isotropic diffuse gamma-ray background.

The study of different products of cosmic-ray interactions with the interstellar medium (gamma-rays, neutrinos and lithium) on smaller scales (within the galaxy), as well as on the largest scales (galaxy clusters), show that in addition to the galactic cosmic rays accelerated in supernova remnants, other still hypothetical cosmic rays (produced, for example, during accretion of gas on the largest scales, or tidal interactions of galaxies) can have a non-negligible contribution to the measurements.