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Register a new userGamma-rays from dark matter annihilations strongly constrain the substructure in halos
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Anders Pinzke
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Recently, it has been shown that electrons and positrons from dark matter (DM) annihilations provide an excellent fit to the Fermi, PAMELA, and HESS data. Using this DM model, which requires an enhancement of the annihilation cross section over its standard value to match the observations, we show that it immediately implies an observable level of gamma-ray emission for the Fermi telescope from nearby galaxy clusters such as Virgo and Fornax. We show that this DM model implies a peculiar feature from final state radiation that is a distinctive signature of DM. Using the EGRET upper limit on the gamma-ray emission from Virgo, we constrain the minimum mass of substructures within DM halos to be > 5x10^-3 M_sun -- about four orders of magnitudes larger than the expectation for cold dark matter. This limits the cutoff scale in the linear matter power spectrum to k < 35/kpc which can be explained by e.g., warm dark matter. Very near future Fermi observations will strongly constrain the minimum mass to be > 10^3 M_sun: if the true substructure cutoff is much smaller than this, the DM interpretation of the Fermi/PAMELA/HESS data must be wrong. To address the problem of astrophysical foregrounds, we performed high-resolution, cosmological simulations of galaxy clusters that include realistic cosmic ray (CR) physics. We compute the dominating gamma-ray emission signal resulting from hadronic CR interactions and find that it follows a universal spectrum and spatial distribution. If we neglect the anomalous enhancement factor and assume standard values for the cross section and minimum subhalo mass, the same model of DM predicts comparable levels of the gamma-ray emission from DM annihilations and CR interactions. This suggests that spectral subtraction techniques could be applied to detect the annihilation signal.
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The $gamma$-ray and neutrino emissions from dark matter (DM) annihilation in galaxy clusters are studied. After about one year operation of Fermi-LAT, several nearby clusters are reported with stringent upper limits of GeV $gamma$-ray emission. We use the Fermi-LAT upper limits of these clusters to constrain the DM model parameters. We find that the DM model distributed with substructures predicted in cold DM (CDM) scenario is strongly constrained by Fermi-LAT $gamma$-ray data. Especially for the leptonic annihilation scenario which may account for the $e^{pm}$ excesses discovered by PAMELA/Fermi-LAT/HESS, the constraint on the minimum mass of substructures is of the level $10^2-10^3$ M$_{odot}$, which is much larger than that expected in CDM picture, but is consistent with a warm DM scenario. We further investigate the sensitivity of neutrino detections of the clusters by IceCube. It is found that neutrino detection is much more difficult than $gamma$-rays. Only for very heavy DM ($sim 10$ TeV) together with a considerable branching ratio to line neutrinos the neutrino sensitivity is comparable with that of $gamma$-rays.
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Dark matter halos contain a wealth of substructure in the form of subhalos and tidal streams. Enhancements in the dark matter density of these regions leads to enhanced rates in direct detection experiments, as well as enhanced dark matter capture rates in the Sun and the Earth. Direct detection experiments probe the present-day dark matter density, while energetic neutrinos probe the past history of the dark matter density along the solar systems orbit about the Galactic center. We discuss how an elevated energetic neutrino flux can be used to probe the level of substructure present at the Galactic radius of the solar system.
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