Neutralino Annihilations and the Gas Temperature in the Dark Ages

Assuming the dark matter is made entirely from neutralinos, we re-visit the role of their annihilation on the temperature of diffuse gas in the high redshift universe. We consider neutralinos of particle mass 36 GeV and 100 GeV, respectively. The former is able to produce ∼ 7 ee particles per annihilation through the fremionic channel, and the latter ∼ 53 particles assuming a purely bosonic channel. High energy ee particles up-scatter the Cosmic Microwave Background (CMB) photons into higher energies via the inverse-Compton scattering. The process produces a power-law ee energy spectrum of index −1 in the energy range of interest, independent of the initial energy distribution. The corresponding energy spectrum of the up-scattered photons is a power-law of index −1/2, if absorption by the gas is not included. The scattered photons photo-heat the gas by releasing electrons which deposit a fraction ( 14%) of their energy as heat into the ambient medium. For uniformly distributed neutralinos the heating is insignificant. The effect is greatly enhanced by the clumping of neutralinos into dense haloes. We use a time-dependent clumping model which takes into account the damping of density fluctuations on mass scales smaller than ∼ 10M⊙. With this clumping model, the heating mechanism boosts the gas temperature above that of the CMB after a redshift of z ∼ 30. By z ≈ 10 the gas temperature is nearly 100 times its temperature when no heating is invoked. Similar increase is obtained for the two neutralino masses considered.