DIFFUSE PIONIC GAMMA-RAY EMISSION FROM LARGE-SCALE STRUCTURES IN THEFERMIERA

Abstract

For more than a decade now the complete origin of the diffuse gamma-ray emission background (EGRB) has been unknown. Major components like unresolved star-forming galaxies (making < ∼ 50% of the EGRB) and blazars ( < ∼ 23%), have failed to explain the entire background observed by Fermi. Another, though subdominant, contribution is expected to come from the process of large-scale structure formation. The growth of structures is accompanied by accretion and merger shocks, which would, with at least some magnetic field present, give rise to a population of structure-formation cosmic rays (SFCRs). Though expected, this cosmic-ray population is still hypothetical and only very weak limits have been placed to their contribution to the EGRB. The most promising insight into SFCRs was expected to come from Fermi -LAT observations of clusters of galaxies, however, only upper limits and no detection have been placed. Here, we build a model of gamma-ray emission from large-scale accretion shocks implementing a source evolution calibrated with the Fermi -LAT cluster observation limits. Though our limits to the SFCR gamma-ray emission are weak (above the observed EGRB) in some case, in others, some of our models can provide a good fit to the observed EGRB. More importantly, we show that these largescale shocks could still give an important contribution to the EGRB, especially at high energies. Future detections of cluster gamma-ray emission would help place tighter constraints on our models and give us a better insight into large-scale shocks forming around them.

TOMOGRAPHY OF THE FERMI -LAT γ -RAY DIFFUSE EXTRAGALACTIC SIGNAL VIA CROSS CORRELATIONS WITH GALAXY CATALOGS

“…Observations with the Large Area Telescope (Fermi-LAT) (Atwood et al 2009) are resolving an ever growing number of sources, making possible to characterize their properties, e.g., Ajello et al (2012Ajello et al ( , 2014, and to constrain their contribution to the EGB. These constraints are further complemented by comparing the unresolved EGB with semi-analytical models of different types of sources, e.g., Stecker & Venters (2011); Makiya et al (2011); Dobardžić & Prodanović (2014); Tamborra et al (2014); Ackermann et al (2012a); Rephaeli & Persic (2013). Thanks to Fermi-LAT a sizable fraction of the EGB is starting to be resolved (Ackermann et al 2014a).…”

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

“…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).…”