Anisotropy probe of galactic and extra-galactic Dark Matter annihilations

📝 Abstract

We study the flux and the angular power spectrum of gamma-rays produced by Dark Matter (DM) annihilations in the Milky Way (MW) and in extra-galactic halos. The annihilation signal receives contributions from: a) the smooth MW halo, b) resolved and unresolved substructures in the MW, c) external DM halos at all redshifts, including d) their substructures. Adopting a self-consistent description of local and extra-galactic substructures, we show that the annihilation flux from substructures in the MW dominates over all the other components for angles larger than O(1) degrees from the Galactic Center, unless an extreme prescription is adopted for the substructures concentration. We also compute the angular power spectrum of gamma-ray anisotropies and find that, for an optimistic choice of the particle physics parameters, an interesting signature of DM annihilations could soon be discovered by the Fermi LAT satellite at low multipoles, l<100, where the dominant contribution comes from MW substructures with mass M>10^4 solar masses. For the substructures models we have adopted, we find that the contribution of extra-galactic annihilations is instead negligible at all scales.

💡 Analysis

We study the flux and the angular power spectrum of gamma-rays produced by Dark Matter (DM) annihilations in the Milky Way (MW) and in extra-galactic halos. The annihilation signal receives contributions from: a) the smooth MW halo, b) resolved and unresolved substructures in the MW, c) external DM halos at all redshifts, including d) their substructures. Adopting a self-consistent description of local and extra-galactic substructures, we show that the annihilation flux from substructures in the MW dominates over all the other components for angles larger than O(1) degrees from the Galactic Center, unless an extreme prescription is adopted for the substructures concentration. We also compute the angular power spectrum of gamma-ray anisotropies and find that, for an optimistic choice of the particle physics parameters, an interesting signature of DM annihilations could soon be discovered by the Fermi LAT satellite at low multipoles, l<100, where the dominant contribution comes from MW substructures with mass M>10^4 solar masses. For the substructures models we have adopted, we find that the contribution of extra-galactic annihilations is instead negligible at all scales.

📄 Content

arXiv:0901.2921v1 [astro-ph.CO] 19 Jan 2009 Anisotropy probe of galactic and extra-galatic Dark Matter annihilations Mattia Fornasa∗ University of Padova & INFN sezione di Padova, via Marzolo 8, 35131 Padova, Italy and Institut d’Astrophysique de Paris, boulevard Arago 89bis, 75014 Paris, France Lidia Pieri† and Gianfranco Bertone‡ Institut d’Astrophysique de Paris, UMR 7095-CNRS, Universit´e Pierre et Marie Curie, boulevard Arago 89bis, 75014 Paris, France Enzo Branchini§ Department of Physics, Universit`a di Roma Tre, via della Vasca Navale 84, 00146, Rome, Italy We study the flux and the angular power spectrum of gamma-rays produced by Dark Matter annihilations in the Milky Way (MW) and in extra-galactic halos. The annihilation signal receives contributions from: a) the smooth MW halo, b) resolved and unresolved substructures in the MW, c) external DM halos at all redshifts, including d) their substructures. Adopting a self-consistent description of local and extra-galactic substructures, we show that the annihilation flux from sub- structures in the MW dominates over all the other components for angles larger than O(1) degrees from the Galactic Center, unless an extreme prescription is adopted for the substructures concen- tration. We also compute the angular power spectrum of gamma-ray anisotropies and find that, for an optimistic choice of the particle physics parameters, an interesting signature of DM annihilations could soon be discovered by the Fermi LAT satellite at low multipoles, ℓ≲100, where the dominant contribution comes from MW substructures with mass M ≳104M⊙. For the substructures models we have adopted, we find that the contribution of extra-galactic annihilations is instead negligible at all scales. PACS numbers: I. INTRODUCTION Despite the compelling evidence for Dark Matter (DM), we know very little about its nature. It is com- monly assumed that DM is composed of Weakly Interact- ing Massive Particles (WIMPs), that are kept in thermal and kinetic equilibrium with baryons in the early uni- verse through their weak coupling with ordinary matter, and that subsequently decouple from them when the self- annihilation rate drops below the expansion rate of the universe, thus naturally achieving the appropriate relic density. Being proportional to the square of the DM number density, the self-annihilation rate is today very small, but it can still lead to detectable signals in regions with very high DM density [1, 2]. Indirect DM searches are based on the detection of par- ticles originating from DM annihilation or decay. If one focuses on the most widely discussed candidates, i.e. the supersymmetric neutralino and the so-called B(1) in the- ories with Universal Extra Dimensions, the mass of the DM particles should be approximately in the 100 GeV – 1 TeV range, and the characteristic energy of the pro- duced particles, roughly an order of magnitude smaller. At these energies, photons are among the best messen- ∗Electronic address: mfornasa@pd.infn.it †Electronic address: pieri@iap.fr ‡Electronic address: bertone@iap.fr §Electronic address: branchin@fis.uniroma3.it gers, since gamma-rays travel in the local universe along geodesics without significant energy losses. The most obvious target for indirect DM searches is the center of our Galaxy, where the large concentration of or- dinary matter is believed to be associated to a large DM density enhancement. However, the search for gamma- rays from DM annihilations in the Galactic Center (GC) is complicated by the presence of a strong signal of astro- physical origin, due to the point sources detected by the EGRET satellite [3, 4, 5] and by the H.E.S.S. telescope [6]. A further degree of complication is constituted by theoretical uncertainties on the DM profile in the inner- most regions of the MW halo which is inevitably affected by the presence of a SuperMassive Black Hole and by the surrounding distribution of stars [7, 8]. To avoid such difficulties, different authors have sug- gested alternative strategies to unambiguously detect the signature of DM interpretation. One possibility is to look for enhancements in the gamma-ray flux with no obvious astrophysical origin. Moreover, the presence of possible distinctive features in the gamma-ray energy spectrum (as lines [9, 10] or “bumps” [11]) may help in ruling out alternative, more conventional, interpretations. Ando et al. [12, 13] have recently pointed out that unique an- nihilation features may also be detected in the angular correlation properties of the Extragalactic Gamma-ray Background (EGB). From an observational point of view the EGB is obtained by subtracting the galactic gamma- ray flux produced by cosmic-rays interacting with the interstellar medium at high galactic latitude from the to- 2 tal background measured by EGRET [14]. The resulting EGB is isotropic and its energy spectrum is rather un- certain, especially at energies above 10 GeV [15, 16, 17]. Unresolved extra-galactic gamma-ray sour

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