Components of the Extragalactic Gamma-Ray Background

Abstract

We present new theoretical estimates of the relative contributions of unresolved blazars and star-forming galaxies to the extragalactic γ -ray background (EGB) and discuss constraints on the contributions from alternative mechanisms such as dark matter annihilation and truly diffuse γ -ray production. We find that the Fermi source count data do not rule out a scenario in which the EGB is dominated by emission from unresolved blazars, though unresolved star-forming galaxies may also contribute significantly to the background, within order-of-magnitude uncertainties. In addition, we find that the spectrum of the unresolved star-forming galaxy contribution cannot explain the EGB spectrum found by EGRET at energies between 50 and 200 MeV, whereas the spectrum of unresolved flat spectrum radio quasars, when accounting for the energy-dependent effects of source confusion, could be consistent with the combined spectrum of the low-energy EGRET EGB measurements and the Fermi-Large Area Telescope EGB measurements.

Gamma-Ray Luminosity and Photon Index Evolution of FSRQ Blazars and Contribution to the Gamma-Ray Background

“…Some authors have suggested that blazars could be the primary source of the EGB (e.g., Stecker & Venters 2011;Abazajian et al 2010), and the results of BP1 did not rule this out, while the results presented in Ajello et al (2012), Abdo et al (2010c), and Malyshev & Hogg (2011) favor blazars being one of several important classes of sources. Other possible source populations for the EGB include starforming galaxies, which have been proposed as a possible significant contributor to the EGB by, e.g., Stecker & Venters (2011), Fields et al (2010, and Lacki et al (2011), although this is countered by Makiya et al (2011), radio galaxies (e.g., Inoue 2011), and non-blazar AGNs (e.g., Inoue & Totani 2009, 2011.…”

“…Other possible source populations for the EGB include starforming galaxies, which have been proposed as a possible significant contributor to the EGB by, e.g., Stecker & Venters (2011), Fields et al (2010, and Lacki et al (2011), although this is countered by Makiya et al (2011), radio galaxies (e.g., Inoue 2011), and non-blazar AGNs (e.g., Inoue & Totani 2009, 2011.…”

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

“…There are several studies that derive the contribution to the EGB from star-forming galaxies (e.g. Fields et al 2010;Stecker & Venters 2011;Makiya et al 2011;Ackermann et al 2012;Chakraborty & Fields 2013;Lacki et al 2014;Tamborra et al 2014). The studies that analyze quiescent and starburst galaxies separately find that the starburst contribution is always comparatively minor and the total (quiescent plus starburst) γ-ray emission is between 10%-50% of the EGB intensity.…”

Diffuse gamma ray background from annihilating dark matter in density spikes around supermassive black holes

“…While it is tempting to invoke dark matter annihilations for the explanation of the extragalactic gamma-ray background, the background might also be due to a variety of astrophysical classes of sources. Among these, flat spectrum radio quasars [38] are the most studied, but there are also significant contributions from star-forming galaxies, radio galaxies and Bl Lacertae objects. In the same figure, we plot the spectrum of star-forming galaxies for the case when the spectrum is determined from the IR luminosity function model (dashed line), and the integrated flux of flat spectrum radio quasars (dotted line) [38].…”

“…Among these, flat spectrum radio quasars [38] are the most studied, but there are also significant contributions from star-forming galaxies, radio galaxies and Bl Lacertae objects. In the same figure, we plot the spectrum of star-forming galaxies for the case when the spectrum is determined from the IR luminosity function model (dashed line), and the integrated flux of flat spectrum radio quasars (dotted line) [38]. The predicted spectrum for star-forming galaxies strongly depends on the assumptions of the model and the magnitude accordingly varies by an order of magnitude.…”