Star-forming galaxies as the origin of diffuse high-energy backgrounds: gamma-ray and neutrino connections, and implications for starburst history

Abstract

Abstract. Star-forming galaxies have been predicted to contribute considerably to the diffuse gamma-ray background as they are guaranteed reservoirs of cosmic rays. Assuming that the hadronic interactions responsible for high-energy gamma rays also produce highenergy neutrinos and that O(100) PeV cosmic rays can be produced and confined in starburst galaxies, we here discuss the possibility that star-forming galaxies are also the main sources of the high-energy neutrinos observed by the IceCube experiment. First, we compute the diffuse gamma-ray background from star-forming galaxies, adopting the latest Herschel PEP/HerMES luminosity function and relying on the correlation between the gamma-ray and infrared luminosities reported by Fermi observations. Then we derive the expected intensity of the diffuse high-energy neutrinos from star-forming galaxies including normal and starburst galaxies. Our results indicate that starbursts, including those with active galactic nuclei and galaxy mergers, could be the main sources of the high-energy neutrinos observed by the IceCube experiment. We find that assuming a cosmic-ray spectral index of 2.1-2.2 for all starburst-like galaxies, our predictions can be consistent with both the Fermi and IceCube data, but larger indices readily fail to explain the observed diffuse neutrino flux. Taking the starburst high-energy spectral index as free parameter, and extrapolating from GeV to PeV energies, we find that the spectra harder than E −2.15 are likely to be excluded by the IceCube data, which can be more constraining than the Fermi data for this population.

Nonthermal particles and photons in starburst regions and superbubbles

“…Since the superclusters are likely to be much more frequent in starburst galaxies CRs escaped from these accelerators may support a diffuse CR population with spectral slope harder than that observed in the Milky Way and consistent with that derived Fig. 2 The gamma-ray spectra of starburst galaxies M82, NGC 253, NGC 1068 and others detected by the Large Area Telescope aboard Fermi observatory and imaging atmospheric Cherenkov telescopes adopted from Ackermann et al (2012) by Tamborra et al (2014). Other possible PeV neutrino sources were proposed recently including semirelativistic hypernova remnants by Liu et al (2014) and shock acceleration of PeV CR particles in massive galaxy mergers or collisions by Kashiyama and Mészáros (2014).…”

“…Recent modeling by Tamborra et al (2014) suggested that starbursts, including those with active galactic nuclei and galaxy mergers, could be the main sources of the high-energy neutrinos observed by the IceCube Observatory (see also Anchordoqui et al (2014)). Assuming a cosmic-ray spectral index of the diffuse CR population to be 2.1-2.2 for all starburst-like galaxies and extrapolating from GeV to PeV energies the authors obtained fluxes consistent with both the Fermi and IceCube data.…”

Modelling the flux distribution function of the extragalactic gamma-ray background from dark matter annihilation

“…In other words, the number i of high flux sources in a pixel is drawn from the Poisson distribution P (i|k ). This is reflected by the binomial expansion: for 1 (which also implies k i) we have 6) such that (reassembling all the elements of the discussion above)…”

“…Components contributing to this background include blazars [4,5], star-forming and starburst galaxies [6], and misaligned active galaxies [7]. The combination of these sources gives reasonably good fit to the spectral data [5,8], while the anisotropies are consistent with the blazar component alone [3,9].…”

“…. , θ h,n } to be given by 6) where the functionsθ h,i (M, z) give the mean values for the parameters. With this assumption, we can perform the integrals over θ h and M in Eq.…”