Constraints on Dark Matter annihilations from reionization and heating
  of the intergalactic gas
  Dark Matter annihilations after recombination and during the epoch of
structure formation deposit energy in the primordial intergalactic medium,
producing reionization and heating. We investigate the constraints that are
imposed by the observed optical depth of the Universe and the measured
temperature of the intergalactic gas. We find that the bounds are significant,
and have the power to rule out large portions of the `DM mass/cross section'
parameter space. The optical depth bound is generally stronger and does not
depend significantly on the history of structure formation. The temperature
bound can be competitive in some cases for small masses or the hadronic
annihilation channels (and is affected somewhat by the details of structure
formation). We find in particular that DM particles with a large annihilation
cross section into leptons and a few TeV mass, such as those needed to explain
the PAMELA and FERMI+HESS cosmic ray excesses in terms of Dark Matter, are
ruled out as they produce too many free electrons. We also find that low mass
particles (<~ 10 GeV) tend to heat too much the gas and are therefore
disfavored.
