Extragalactic gamma-ray background from AGN winds and star-forming galaxies in cosmological galaxy-formation models

¹ INAF–Osservatorio Astronomico di Roma, via di Frascati 33, 00078 Monte Porzio Catone, Italy
e-mail: alessandra.lamastra@oa-roma.inaf.it
² SSDC–ASI, via del Politecnico, 00133 Roma, Italy
³ School of Physics, University of the Witwatersrand, Private Bag 3, 2050 Johannesburg, South Africa
⁴ Department of Physics and Optical Engineering, ORT Braude College, 21982 Karmiel, Israel
⁵ INAF–Osservatorio Astrofisico di Torino, via Osservatorio, 20, 10025 Pino Torinese, Italy
⁶ Scuola Normale Superiore, Piazza dei Cavalieri 7, 56126 Pisa, Italy

Received: 27 June 2017
Accepted: 10 September 2017

Abstract

We derive the contribution to the extragalactic gamma-ray background (EGB) from active galactic nuclei (AGN) winds and star-forming galaxies by including a physical model for the γ-ray emission produced by relativistic protons accelerated by AGN-driven and supernova-driven shocks into a state-of-the-art semi-analytic model of galaxy formation. This is based on galaxy interactions as triggers of AGN accretion and starburst activity and on expanding blast waves as the mechanism to communicate outwards the energy injected into the interstellar medium by the active nucleus. We compare the model predictions with the latest measurement of the EGB spectrum performed by the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope (Fermi) in the range between 100 MeV and 820 GeV. We find that AGN winds can provide ~35 ± 15% of the observed EGB in the energy interval E_γ = 0.1–1 GeV, for ~73 ± 15% at E_γ = 1–10 GeV, and for ~60 ± 20% at E_γ ≳10 GeV. The AGN wind contribution to the EGB is predicted to be larger by a factor of ~3–5 than that provided by star-forming galaxies (quiescent plus starburst) in the hierarchical clustering scenario. The cumulative γ-ray emission from AGN winds and blazars can account for the amplitude and spectral shape of the EGB, assuming the standard acceleration theory, and AGN wind parameters that agree with observations. We also compare the model prediction for the cumulative neutrino background from AGN winds with the most recent IceCube data. We find that for AGN winds with accelerated proton spectral index p = 2.2–2.3, and taking into account internal absorption of γ-rays, the Fermi-LAT and IceCube data could be reproduced simultaneously.