Constraining DM through 21 cm observations
Beyond reionization epoch cosmic hydrogen is neutral and can be directly observed through its 21 cm line signal. If dark matter (DM) decays or annihilates the corresponding energy input affects the hydrogen kinetic temperature and ionized fraction, and contributes to the Ly_alpha background. The changes induced by these processes on the 21 cm signal can then be used to constrain the proposed DM candidates, among which we select the three most popular ones: (i) 25-keV decaying sterile neutrinos, (ii) 10-MeV decaying light dark matter (LDM) and (iii) 10-MeV annihilating LDM. Although we find that the DM effects are considerably smaller than found by previous studies (due to a more physical description of the energy transfer from DM to the gas), we conclude that combined observations of the 21 cm background and of its gradient should be able to put constrains at least on LDM candidates. In fact, LDM decays (annihilations) induce differential brightness temperature variations with respect to the non decaying/annihilating DM case up to Delta_delta T_b=8 (22) mK at about 50 (15) MHz. In principle this signal could be detected both by current single dish radio telescopes and future facilities as LOFAR; however, this assumes that ionospheric, interference and foreground issues can be properly taken care of.