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Register a new userThe Origin of the Extragalactic Gamma-Ray Background and Implications for Dark-Matter Annihilation
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The origin of the extragalactic $gamma$-ray background (EGB) has been debated for some time. { The EGB comprises the $gamma$-ray emission from resolved and unresolved extragalactic sources, such as blazars, star-forming galaxies and radio galaxies, as well as radiation from truly diffuse processes.} This letter focuses on the blazar source class, the most numerous detected population, and presents an updated luminosity function and spectral energy distribution model consistent with the blazar observations performed by the {it Fermi} Large Area Telescope (LAT). We show that blazars account for 50$^{+12}_{-11}$,% of the EGB photons ($>$0.1,GeV), and that {it Fermi}-LAT has already resolved $sim$70,% of this contribution. Blazars, and in particular low-luminosity hard-spectrum nearby sources like BL Lacs, are responsible for most of the EGB emission above 100,GeV. We find that the extragalactic background light, which attenuates blazars high-energy emission, is responsible for the high-energy cut-off observed in the EGB spectrum. Finally, we show that blazars, star-forming galaxies and radio galaxies can naturally account for the amplitude and spectral shape of the background in the 0.1--820,GeV range, leaving only modest room for other contributions. This allows us to set competitive constraints on the dark-matter annihilation cross section.
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We explore two possible scenarios to explain the observed gamma-ray emission associated with the atypical globular cluster Omega-Centauri: emission from millisecond pulsars (MSP) and dark matter (DM) annihilation. In the first case the total number of MSPs needed to produce the gamma-ray flux is compatible with the known (but not confirmed) MSP candidates observed in X-rays. A DM interpretation is motivated by the possibility of Omega-Centauri being the remnant core of an ancient dwarf galaxy hosting a surviving DM component. At least two annihilation channels, light quarks and muons, can plausibly produce the observed gama-ray spectrum. We outline constraints on the parameter space of DM mass versus the product of the pair-annihilation cross section and integrated squared DM density (the so-called J-factor). We translate upper limits on the dark matter content of Omega-Centauri into lower limits on the annihilation cross section. This shows s-wave annihilation into muons to be inconsistent with CMB observations, while a small window for annihilation into light quarks is allowed. Further analysis of Omega-Centauris internal kinematics, and/or additional information on the resident MSP population will yield much stronger constraints and shed light about the origin of this otherwise mysterious gamma-ray source.
We compare the measured angular cross-correlation between the Fermi-LAT gamma-ray sky and catalogues of extra-galactic objects with the expected signal induced by weakly interacting massive particle (WIMP) dark matter (DM). We include a detailed description of the contribution of astrophysical gamma-ray emitters such as blazars, misaligned AGN and star forming galaxies, and perform a global fit to the measured cross-correlation. Five catalogues are considered: SDSS-DR6 quasars, 2MASS galaxies, NVSS radio galaxies, SDSS-DR8 Luminous Red Galaxies and SDSS-DR8 main galaxy sample. To model the cross-correlation signal we use the halo occupation distribution formalism to estimate the number of galaxies of a given catalogue in DM halos and their spatial correlation properties. We discuss uncertainties in the predicted cross-correlation signal arising from the DM clustering and WIMP microscopic properties, which set the DM gamma-ray emission. The use of different catalogues probing objects at different redshifts reduces significantly, though not completely, the degeneracy among the different gamma-ray components. We find that the presence of a significant WIMP DM signal is allowed by the data but not significantly preferred by the fit, although this is mainly due to a degeneracy with the misaligned AGN component. With modest substructure boost, the sensitivity of this method excludes thermal annihilation cross sections at 95% C.L. for WIMP masses up to few tens of GeV. Constraining the low-redshift properties of astrophysical populations with future data will further improve the sensitivity to DM.
We re-evaluate the extragalactic gamma-ray flux prediction from dark matter annihilation in the approach of integrating over the nonlinear matter power spectrum, extrapolated to the free-streaming scale. We provide an estimate of the uncertainty based entirely on available N-body simulation results and minimal theoretical assumptions. We illustrate how an improvement in the simulation resolution, exemplified by the comparison between the Millennium and Millennium II simulations, affects our estimate of the flux uncertainty and we provide a best guess value for the flux multiplier, based on the assumption of stable clustering for the dark matter perturbations described as a collision-less fluid. We achieve results comparable to traditional Halo Model calculations, but with a much simpler procedure and a more general approach, as it relies only on one, directly measurable quantity. In addition we discuss the extension of our calculation to include baryonic effects as modeled in hydrodynamical cosmological simulations and other possible sources of uncertainty that would in turn affect indirect dark matter signals. Upper limit on the integrated power spectrum from supernovae lensing magnification are also derived and compared with theoretical expectations.
If the dark matter is unstable, the decay of these particles throughout the universe and in the halo of the Milky Way could contribute significantly to the isotropic gamma-ray background (IGRB) as measured by Fermi. In this article, we calculate the high-latitude gamma-ray flux resulting from dark matter decay for a wide range of channels and masses, including all contributions from inverse Compton scattering and accounting for the production and full evolution of cosmological electromagnetic cascades. We also make use of recent multi-wavelength analyses that constrain the astrophysical contributions to the IGRB, enabling us to more strongly restrict the presence any component arising from decaying dark matter. Over a wide range of decay channels and masses (from GeV to EeV and above), we derive stringent lower limits on the dark matters lifetime, generally in the range of $tau sim (1-5)times 10^{28}$ s.
Fermi has been instrumental in constraining the luminosity function and redshift evolution of gamma-ray bright blazars. This includes limits upon the spectrum and anisotropy of the extragalactic gamma-ray background (EGRB), redshift distribution of nearby Fermi active galactic nuclei (AGN), and the construction of a log(N)-log(S) relation. Based upon these, it has been argued that the evolution of the gamma-ray bright blazar population must be much less dramatic than that of other AGN. However, critical to such claims is the assumption that inverse Compton cascades reprocess emission above a TeV into the Fermi energy range, substantially enhancing the strength of the observed limits. Here we demonstrate that in the absence of such a process, due, e.g., to the presence of virulent plasma beam instabilities that preempt the cascade, a population of TeV-bright blazars that evolve similarly to quasars is consistent with the population of hard gamma-ray blazars observed by Fermi. Specifically, we show that a simple model for the properties and luminosity function is simultaneously able to reproduce their log(N)-log(S) relation, local redshift distribution, and contribution to the EGRB and its anisotropy without any free parameters. Insofar the naturalness of a picture in which the hard gamma-ray blazar population exhibits the strong redshift evolution observed in other tracers of the cosmological history of accretion onto halos is desirable, this lends support for the absence of the inverse Compton cascades and the existence of the beam plasma instabilities.
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