Warm dark matter and the ionization history of the Universe
  In warm dark matter scenarios structure formation is suppressed on small
scales with respect to the cold dark matter case, reducing the number of
low-mass halos and the fraction of ionized gas at high redshifts and thus,
delaying reionization. This has an impact on the ionization history of the
Universe and measurements of the optical depth to reionization, of the
evolution of the global fraction of ionized gas and of the thermal history of
the intergalactic medium, can be used to set constraints on the mass of the
dark matter particle. However, the suppression of the fraction of ionized
medium in these scenarios can be partly compensated by varying other
parameters, as the ionization efficiency or the minimum mass for which halos
can host star-forming galaxies. Here we use different data sets regarding the
ionization and thermal histories of the Universe and, taking into account the
degeneracies from several astrophysical parameters, we obtain a lower bound on
the mass of thermal warm dark matter candidates of $m_X > 1.3$ keV, or $m_s >
5.5$ keV for the case of sterile neutrinos non-resonantly produced in the early
Universe, both at 90\% confidence level.
