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It is well known that annihilations in the homogeneous fluid of dark matter(DM) can leave imprints in the cosmic microwave background (CMB) anisotropypower spectrum. However, the relevance of DM annihilations in halos forcosmological observables is still subject to debate, with previous worksreaching different conclusions on this point. Also, all previous studies used asingle type of parameterization for the astrophysical reionization, andincluded no astrophysical source for the heating of the intergalactic medium.In this work, we revisit these problems. When standard approaches are adopted,we find that the ionization fraction does exhibit a very particular (andpotentially constraining) pattern, but the currently measurable optical depthto reionization is left almost unchanged: In agreement with the most of theprevious literature, for plausible halo models we find that the modification ofthe signal with respect to the one coming from annihilations in the smoothbackground is tiny, below cosmic variance within currently allowed parameterspace. However, if different and probably more realistic treatments of theastrophysical sources of reionization and heating are adopted, a morepronounced effect of the DM annihilation in halos is possible. We thus concludethat within currently adopted baseline models the impact of the virialised DMstructures cannot be uncovered by CMB power spectra measurements, but a largerimpact is possible if peculiar models are invoked for the redshift evolution ofthe DM annihilation signal or different assumptions are made for theastrophysical contributions. A better understanding (both theoretical andobservational) of the reionization and temperature history of the universe,notably via the 21 cm signal, seems the most promising way for using haloformation as a tool in DM searches, improving over the sensitivity of currentcosmological probes.

Dark Matter annihilations in halos and high-redshift sources of reionization of the universe