Understanding the Extragalactic g -ray Background

1. Understanding the Extragalactic g-ray Background F.W. Stecker* Astrophysics Science Division, NASA/GSFC *Based on work in collaboration with T. M. Venters, arXiv:1012.3678

3. Extragalactic g-ray Background Results

4. Unresolved Sources: Unresolved Blazars Unresolved Galaxies Truly Diffuse Processes: UHE Electromagnetic Cascades Dark Matter Annihilation and Decay ?? Components of the Background

5. Efficiency of a g-ray telescope for detecting extragalactic sources determined by: • The flux of the source • The spectral index of the source • The intrinsic detector background from cosmic-ray induced events • The foreground from the Milky Way • The diffuse extragalactic background

6. Unresolved Blazars • Blazars are active galactic nuclei with jets of relativistic plasma and particles aimed directly at us. • They are by far the most abundant extragalactic sources of g-rays.

7. log N – log S for Fermi and EGRETBlazars 50% survey completeness: Fermi: > ~2x10-8 cm-2s-1 EGRET: >~8x10-8 cm-2s-1

8. log N – log S from Stecker & Salamon (1996)

9. Radio/g-ray Correlation derived from Fermidata(Ghirlandaet al. 2010)

10. log N – log S Data and Models of the Unresolved Background from Blazars

11. Source Confusion • Source confusion for faint sources is critical in determining the spectrum and flux of the background of unresolved sources. Because the angular resolution of EGRET and Fermi are comparable at 0.1 GeV, the unresolved background at that energy should be the same. At 1 GeV, Fermi should resolve out many more sources, resulting in a lower background observed by Fermi compared to EGRET(Stecker & Salamon 1999).

12. Angular Resolution for Fermi and EGRET

13. Background from Unresolved Blazars and Data

14. Unresolved Galaxies (Almost ALL Galaxies are Unresolved by Fermi Regardless of g-ray Energy)

15. Andromeda Galaxy g-rays: d = 800 kpc, 2yr data(Abdo et al. 2010) F(>100 MeV) ~ 10-8 cm-2s-1

16. g-ray Production in Star-Forming Galaxies, Assumptions: • A Galaxy’s g-ray flux is mainly from p0 production in CR-gas interactions followed by p0 decay (Stecker 1969;1977). • \g-ray flux ∝ (CR flux) x(gas density). • CR flux ∝ supernova rate (Stecker 1975, Phys. Rev. Letters 35, 188)∝ star formation rate (SFR).

17. Three Strategies for Estimating g-Ray Luminosity in Star Forming Galaxies: • Relate the galaxy γ-ray luminosity to its SFR, which, in turn, is related to an observable for which there is a redshift distribution (e.g., IR luminosity). • Relate the galaxy gas mass to its stellar mass assuming a gas fraction that evolves with redshift. • Relate the cosmic density of gas in star forming galaxies to the star formation rate density.

18. Relation Between Galaxy g-ray Luminosity and Star Formation Rate for Galaxies Observed byFermi

19. SFR Density vs. z(Ly 2010)

20. SFR Density vs. z: More Data

21. SFR Density vs. z(Le Borgne et al. 2009)

22. Models of the g-ray Background from Unresolved Star Forming Galaxies (Ψ = SFR) • Lg ∝ Ψ1.2 with modeled gas evolution • Lg ∝ Ψ1.2 , Ψ ∝(1+z)3.4, 0< z<1.3; const., 1.3<z<4.0 (Ψ determined from IR LFs: Ly 2010) • fH(z) ∝ (1+z)0.9(Papovich et al. 2010) with Ψ ∝(1+z)3.4 and a Schechter function Φ(M∗) for galaxy mass distribution • Starburst model using IR LFs for starburst galaxies and with Lg ∝ Ψ1.2 • Lg ∝ Ψ2 ∝(1+z)6.8, 0 < z < 1.3; const., 1.3 < z <4.0 ( CR ∝ SN rate ∝ Ψ and Ψ∝ NH ∴ Lg∝ ΨNH ∝ Ψ2 )

23. Background From Unresolved Galaxies and Data

24. UHE Electromagnetic Cascades • Ultrahigh energy cosmic rays interact with photons of the cosmic microwave background (CMB) radiation to produce mesons (the GZK effect). • These mesons decay into g-rays, electrons and positrons that, in turn, can interact with CMB photons, resulting in electromagnetic cascades down to the GeV energy range.

25. Maximum Background from EM Cascades of UHE Electrons from the GZK Effect (Ahlerset al. 2010)

26. Dark Matter Annihilation and Decay • Dark matter may be made up of the lightest supersymmetric particles (LSPs) if supersymmetry theory is correct. • These particles are Majorana particles that can annihilate with each other through photon, lepton and quark channels, resulting in a g-ray background from the annihilation in the halos of galaxies.

27. Spectral Features of a Background from Dark Matter Annihilation (Abdoet al. 2010)

28. Conclusions: • The EGRET and Fermi g-ray data do not rule out a scenario in which the background is dominated by g-rays from unresolved blazars. • g-rays from unresolved star forming galaxies may or may not contribute significantly to the background (cf. Fields & Pavlidou 2010; Makiyaet al. 2010). • There are no spectral features signaling dark matter annihilation seen by Fermi. • The spectrum from UHE EM cascading is too hard for it to make a dominant contribution to the background. • Starburst galaxies make a negligible contribution to the background (cf. Stecker 2007, Makiyaet al. 2010).

29. Supplemental Material

30. Source Confusion Criterion • The probability P of finding a nearest neighboring source with S ≥ Slim at a distance closer than the minimum angular separation θminis given by P(< θmin) = 1 - exp(-πNθ²min). An acceptable probability limit is Pmax = 0.1 and we take θmin= θ67%. • Source density criterion: NSDC = -ln(1-P(<θmin)) / πk2θ2min. The limiting source flux, SSDC is then determined from the log N – log S relation. • If we were to think of Fermi as an ordinary telescope with θmin = kθFWHMand 0.8≤ k ≤ 1, the source density criterion for k = 1 would correspond to 1/16.7 sources per beam. For both Fermi and EGRET θminis a strong function of energy.

31. Factors that determine the efficiency of a g-ray telescope for detecting extragalactic sources: (1) the flux of the source (2) the spectral index of the source (3) the intrinsic detector background from cosmic-ray induced events (4) the foreground from the Milky Way (5) the diffuse extragalactic background

32. Fermi Source Detection Efficiency from Monte Carlo Simulation (Abdo et al. 2010)

33. Result from Abdoet al. 2010