It is well known that annihilations in the homogeneous fluid of dark matter
(DM) can leave imprints in the cosmic microwave background (CMB) anisotropy
power spectrum. However, the relevance of DM annihilations in halos for
cosmological observables is still subject to debate, with previous works
reaching different conclusions on this point. Also, all previous studies used a
single type of parameterization for the astrophysical reionization, and
included 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 (and
potentially constraining) pattern, but the currently measurable optical depth
to reionization is left almost unchanged: In agreement with the most of the
previous literature, for plausible halo models we find that the modification of
the signal with respect to the one coming from annihilations in the smooth
background is tiny, below cosmic variance within currently allowed parameter
space. However, if different and probably more realistic treatments of the
astrophysical sources of reionization and heating are adopted, a more
pronounced effect of the DM annihilation in halos is possible. We thus conclude
that within currently adopted baseline models the impact of the virialised DM
structures cannot be uncovered by CMB power spectra measurements, but a larger
impact is possible if peculiar models are invoked for the redshift evolution of
the DM annihilation signal or different assumptions are made for the
astrophysical contributions. A better understanding (both theoretical and
observational) of the reionization and temperature history of the universe,
notably via the 21 cm signal, seems the most promising way for using halo
formation as a tool in DM searches, improving over the sensitivity of current
cosmological probes.Comment: 30 pages, 11 figures. v2: extended version (notably astrophysical
source effects significantly expanded), references added, main conclusions
unchanged. Matches version accepted by JCA
