Impact of dark matter decays and annihilations on reionization

One of the possible methods to distinguish among various dark matter (DM) candidates is to study the effects of DM decays. We consider four different DM candidates [light dark matter (LDM), gravitinos, neutralinos and sterile neutrinos], for each of them deriving the decaying/annihilation rate, the influence on reionization, matter temperature and cosmic microwave background (CMB) spectra. We find that LDM particles (1–10 MeV) and sterile neutrinos (2–8 keV) can be sources of partial early reionization (z ~< 100). However, their integrated contribution to Thomson optical depth is small (~< 0.01) with respect to the 3-yr WMAP results (τe = 0.09 ± 0.03). Finally, they can significantly affect the behaviour of matter temperature. On the contrary, effects of heavy DM candidates (gravitinos and neutralinos) on reionization and heating are minimal. All the considered DM particles have completely negligible effects on the CMB spectra.