Small-scale clumps in the Galactic halo
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ELEMENTARY PARTICLES AND FIELDS Theory
Small-Scale Clumps in the Galactic Halo* V. S. Berezinsky1), 2) , V. I. Dokuchaev2)** , and Yu. N. Eroshenko2) Received October 24, 2008
Abstract—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. DOI: 10.1134/S1063778810010199
1. INTRODUCTION A primordial power-law spectrum of density fluctuations in the Dark Matter (DM) ranges from the largest scales above the scales of superclusters of galaxies to the smallest sub-stellar scales according to prediction of inflation models. This permits to predict the properties of smallest DM structures from the known CMB fluctuations at large scales. The cosmological formation and evolution of small-scale DM clumps have been studied in numerous works [1–12]. The minimum mass of clumps (the cutoff of the mass spectrum), Mmin is determined by the collision and collisionless damping processes (see, e.g., [5] and references therein). Additionally, the cutoff of mass spectrum is influenced by the acoustic absorption [13] at the time of kinetic decoupling of DM particles [14] and also by the horizon-scale perturbation modes [15]. The low-mass cut-off of the clump mass spectrum accompanies the process of decoupling. It starts when DM particles couple strongly with surrounding plasma in the growing density fluctuations. The smearing of the smallscale fluctuations is due to the collision damping ∗
The text was submitted by the authors in English. INFN, Laboratori Nazionali del Gran Sasso, Assergi (AQ), Italy. 2) Institute for Nuclear Research, Russian Academy of Sciences, Moscow. ** E-mail: [email protected] 1)
occurring just before decoupling, in analogy with the Silk damping [16]. It occurs due to diffusion of DM particles from a growing fluctuation, and only the small-scale fluctuations can be destroyed by this diff is process. The corresponding diffusive cu
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