Small-scale clumps in the Galactic halo

A mass function of small-scale dark matter clumps is calculated. We take into account the tidal destruction of clumps at early stages of structure formation starting from a time of clump detachment from the Universe expansion. Only a small fraction of these clumps, ∼0.1%, in each logarithmic mass interval Δ log M ∼ 1 survives the stage of hierarchical clustering. We calculate the probability of surviving of the remnants of dark matter clumps in the Galaxy by modelling the tidal destruction of the small-scale clumps by disk and stars. It is demonstrated that a substantial fraction of clump remnants may survive through the tidal destruction during the lifetime of the Galaxy if a radius of core is rather small. The resulting mass spectrum of survived clumps is extended down to the mass of the core of the cosmologically produced clumps with a minimal mass. The survived dense remnants of tidally destructed clumps provides a large contribution to the annihilation signal in the Galaxy. We describe the anisotropy of dark matter clump distribution caused by tidal destruction of clumps in the Galactic disk. A corresponding annihilation of dark matter particles in small-scale clumps produces the anisotropic gamma-ray signal with respect to the Galactic disk.     

Dark Matter annihilation in small-scale clumps in the Galactic halo

The enhancement of the annihilation signal due to Dark Matter (DM) clumpiness in the Galactic halo, valid for arbitrary DM particles, is described. The mass spectrum of small-scale DM clumps with M≤10³ M _⊙ is calculated with tidal destruction of the clumps taken into account within the hierarchical model of clump structure. The mass distribution of the clumps has a cutoff at M _min due to diffusion of DM particles out of a fluctuation and free streaming. In the case of neutralino (considered as a pure bino) being a DM particle, M _min～10⁸ M _⊙. The evolution of the density profile in a DM clump does not result in singularity, because of formation of the core under the influence of tidal interaction. The number density of clumps as a function of their mass, radius, and distance to the Galactic center is presented.     

Dark-matter annihilation in the Galaxy

The possibility of amplification of the annihilation signal from dark matter in the Galaxy halo due to its clusterization into small-mass clumps (about the Moon’s or Earth’s mass) is discussed. Formation of dark-matter clumps is considered within the conventional cosmological model taking into account their hierarchical clustering and tidal destruction. It is shown that the amplification factor of γ rays from dark-matter annihilation due to small-mass clumps in the Galaxy may range from 10 to 1000.     

Destruction of axion miniclusters in the Galaxy

Previously, it has been established that axion dark matter (DM) is clustered to form clumps (axion miniclusters) with masses M ≈ 10^–12 M _⊙. The passages of such clumps through the Earth are very rare events occurring once in 10⁵ years. It has also been shown that the Earth’s passage through DM streams, which are the remnants of clumps destroyed by tidal gravitational forces from Galactic stars, is a much more probable event occurring once in several years. In this paper, we have performed detailed calculations of the destruction of miniclusters by taking into account their distribution in orbits in the Galactic halo. We have investigated two DM halo models, the Navarro–Frenk–White and isothermal density profiles. Apart from the Galactic disk stars, we have also taken into account the halo and bulge stars. We show that about 2–5% of the axion miniclusters are destroyed when passing near stars and transform into axion streams, while the clump destruction efficiency depends on the DM halo model. The expected detection rate of streams with an overdensity exceeding an order of magnitude is 1–2 in 20 years. The possibility of detecting streams by their tidal gravitational effect on gravitational-wave interferometers is also considered.     

Is dark matter visible by galactic gamma rays?

The EGRET excess in the diffuse galactic gamma ray data above 1 GeV shows all features expected from dark matter WIMP annihilation: (a) It is present and has the same spectrum in all sky directions, not just in the galactic plane. (b) The intensity of the excess shows the 1/r ² profile expected for a flat rotation curve outside the galactic disc with an additionally interesting substructure in the disc in the form of a doughnut-shaped ring at 14 kpc from the centre of the galaxy. At this radius a ring of stars indicates the probable infall of a dwarf galaxy, which can explain the increase in DM density. From the spectral shape of the excess the WIMP mass is estimated to be between 50 and 100 GeV, while from the intensity the halo profile is reconstructed. Given the mass and intensity of the WIMPs the mass of the ring can be calculated, which is shown to explain the peculiar change of slope in the rotation curve at about 11 kpc. These results are model-independent in the sense that only the known shapes of signal and background were fitted with free normalization factors, thus being independent of model-dependent flux calculations. The statistical significance is more than 10σ in comparison with a fit of the conventional galactic model to the EGRET data. These signals of dark matter annihilation are compatible with supersymmetry including all electroweak constraints. The statistical significance combined with all features mentioned above provide an intriguing hint that the EGRET excess is indeed a signal from dark matter annihilation.     

Anti-deuterons from heavy Dark Matter

We study the fluxes of anti-deuterons that could be produced by annihilations in the galactic halo of Dark Matter particles with multi-TeV mass and a large annihilation cross section, as indicated by the recent PAMELA results. The model of Minimal Dark Matter (MDM) is an example in this category. We find that the fluxes are well within the reach of planned experiments for DM candidates that annihilate mainly into quark pairs, and also extend into the multi-GeV range above the expected astrophysical background. They are instead less promising if the main annihilation channel is into gauge bosons.     

MeV dark matter: has it been detected?

We discuss the possibility that the recent detection of 511 keV gamma rays from the galactic bulge, as observed by INTEGRAL, is a consequence of low mass (1-100 MeV) particle dark matter annihilations. We discuss the type of halo profile favored by the observations as well as the size of the annihilation cross section needed to account for the signal. We find that such a scenario is consistent with the observed dark matter relic density and other constraints from astrophysics and particle physics.     

Estimation of the Density of the Self-Interacting Dark-Matter Component within a Clump with Allowance for Recombination

The model of dark matter featuring a component in the form of free particles and antiparticles (a, ā ) possessing self-interaction of the Coulomb type is considered. Darkmattermay form small-scale clumps where the annihilation of particles a and ā is enhanced. This annihilation may lead to observable effects (in cosmic rays, for example) and/or to the destruction of these clumps. However, there is an ambiguity in describing the annihilation (via recombination) of very slow particles, which may include a and ā in clumps. The effect of annihilation (in terms of the residual number of free particles a and ā in clumps) is estimated within two approaches (simplified quantum-mechanical and classical) at chosen parameter values.     

Search for the Galactic Disk and Halo Components in the Arrival Directions of High-Energy Astrophysical Neutrinos

The arrival directions of 40 neutrino events with energies ?100 TeV, observed by the IceCube experiment, are studied. Their distribution in the Galactic latitude and in the angular distance to the Galactic Center allow searching for the Milky-Way disk and halo-related components, respectively. No statistically significant evidence for the disk component is found, though even 100% disk origin of the flux is allowed at the 90% confidence level. Contrary, the Galactic Center–Anticenter dipole anisotropy, specific for dark-matter decays (annihilation) or for interactions of cosmic rays with the extended halo of the circumgalactic gas, is clearly favored over the isotropic distribution (the probability of fluctuation of the isotropic signal is ~2%).     

Possible galactic sources of ultrahigh-energy cosmic rays and a strategy for their detection via gravitational lensing

If decays of superheavy relic particles in the galactic halo are responsible for ultrahigh-energy cosmic rays, these particles must be clustered to account for small-scale anisotropy in the AGASA data. We show that the masses of such clusters are large enough for them to gravitationally lens stars and galaxies in the background. We propose a general strategy that can be used to detect such clusters via gravitational lensing, or to rule out the hypothesis of decaying relic particles as the origin of highest energy cosmic rays.     

Explosive dark matter annihilation

In this Letter we study pair annihilation processes of dark matter (DM) in the Universe, in the case that the DM is an electroweak gauge nonsinglet. In the current Universe, in which the DM is highly nonrelativistic, the nonperturbative effect may enhance the DM annihilation cross sections, especially for that to two photons, by several orders of magnitude. We also discuss sensitivities in future searches for anomalous gamma rays from the galactic center, which originate from DM annihilation.     

Dark matter annihilation in the milky way galaxy: effects of baryonic compression

If the dark matter (DM), which is considered to constitute most of the mass of galaxies, is made of supersymmetric particles, the central region of our Galaxy should emit gamma rays produced by their annihilation. We use detailed models of the Milky Way to make accurate estimates of continuum gamma-ray fluxes. We argue that the most important effect, which was previously neglected, is the compression of the dark matter due to the infall of baryons to the galactic center: it boosts the expected signal by a factor 1000. To illustrate this effect, we computed the expected gamma fluxes in the minimal supergravity scenario. Our models predict that the signal could be detected at high confidence levels by imaging atmospheric C erenkov telescopes assuming that neutralinos make up most of the DM in the Universe.     

The dearth of halo dwarf galaxies: is there power on short scales?

N-body simulations of structure formation with scale-invariant primordial perturbations show significantly more virialized objects of dwarf-galaxy mass in a typical galactic halo than are observed around the Milky Way. We show that the dearth of observed dwarf galaxies could be explained by a dramatic downturn in the power spectrum at small distance scales. This suppression of small-scale power might also help mitigate the disagreement between cuspy simulated halos and smooth observed halos, while remaining consistent with Lyman-alpha-forest constraints on small-scale power. Such a spectrum could arise in inflationary models with broken-scale invariance.     

Analysis of a possible explanation of the positron anomaly in terms of dark matter

The possibility of explaining the positron anomaly on the basis of models involving the darkmatter annihilation or decay is being widely discussed at the present time. However, such models are severely constrained by data on cosmic gamma radiation. Two different procedures that rely on the χ₂ criterion and which permit taking this constraint into account are considered in the present study. In one case, the use of positron data alone in searches for a minimum value of χ² via varying model parameters is followed by a comparison with gamma-radiation data. In the second case, the χ² functional is minimized by simultaneously employing positron and gamma-radiation data, whereby a more lenient (more “flexible”) constraint is obtained. Nevertheless, either procedure rules out the possibility of explaining the positron anomaly in terms of unstable dark matter distributed over the whole halo. The assumption that the dark-matter component undergoing annihilation (decay) is concentrated within the galactic disk makes it possible to remove the constraint in either case.     

Dark matter and the first stars: a new phase of stellar evolution

A mechanism is identified whereby dark matter (DM) in protostellar halos dramatically alters the current theoretical framework for the formation of the first stars. Heat from neutralino DM annihilation is shown to overwhelm any cooling mechanism, consequently impeding the star formation process and possibly leading to a new stellar phase. A "dark star" may result: a giant ( greater, similar 1 AU) hydrogen-helium star powered by DM annihilation instead of nuclear fusion. Observational consequences are discussed.     

Can dark matter annihilation dominate the extragalactic gamma-ray background?

Annihilating dark matter (DM) has been discussed as a possible source of gamma rays from the galactic center and as a contribution to the extragalactic gamma-ray background. Assuming universality of the density profile of DM halos, we show that it is quite unlikely that DM annihilation is a main constituent of extragalactic gamma-ray background, without exceeding the observed gamma-ray flux from the galactic center. This argument becomes stronger when we include enhancement of the density profiles by supermassive black holes or baryon cooling. The presence of a substructure may loosen the constraint, but only if a very large cross section as well as the rather flat profile are realized.     

Origin of the annihilation radiation from the galactic center region

Observations show that there are two sources of positron annihilation radiation from the region of the Galactic Center: a variable, compact source at or near the Galactic Center and a steady, diffuse source in the galactic disk. We suggest that the annihilating positrons responsible for the variable, annihilation line radiation, observed from 1977 through 1979, result from photon-photon pair production, most likely around an accreting black hole, and that the annihilating, interstellar positrons, responsible for the steady, diffuse galactic annihilation radiation, result from the decay of radionuclei produced by thermonuclear burning in supernovae.     

Dark matter spikes and annihilation radiation from the galactic center

The annihilation rate of weakly interacting cold dark matter particles at the galactic center could be greatly enhanced by the growth of a density spike around the central supermassive black hole (SBH). Here we discuss the effects of hierarchical mergers on the central spike. Mergers between halos containing SBHs lead to the formation of SBH binaries which transfer energy to the dark matter particles, lowering their density. The predicted flux of annihilation photons from the galactic center is several orders of magnitude smaller than in models that ignore the effects of SBHs and mergers. Measurement of the annihilation radiation could in principle be used to constrain the merger history of the galaxy.     

Particle dark matter in the galactic halo

The presence of Dark Matter (DM) particles in the galactic halo can be unambiguously pointed out in a model-independent way by exploiting the DM annual modulation signature with an apparatus of specific features placed deep underground. At present DAMA/LIBRA is running at the Gran Sasso National Laboratory of the I.N.F.N. and has presented so far the results obtained with the data collected in the first six annual cycles (exposure 0.87 ton × yr). When including the exposure of the former DAMA/NaI experiment (0.29 ton × yr), the total exposure is 1.17 ton × yr for 13 annual cycles, a value order of magnitude larger than those typically released in the field. These DAMA/LIBRA data have further confirmed the model-independent evidence of the presence of Dark Matter particles in the galactic halo on the basis of the exploited DM signature (8.9 σ C.L. for the cumulative exposure).