On dark matter - dark radiation interaction and cosmic reionization
  An intriguing possibility for the dark sector of our universe is that the
dark matter particle could interact with a dark radiation component. If the
non-gravitational interactions of the dark matter and dark radiation species
with Standard Model particles are highly suppressed, then astrophysics and
cosmology could be our only windows into probing the dynamics of such a dark
sector. It is well known that such dark sectors would lead to suppression of
small scale structure, which would be constrained by measurements of the
Lyman-$\alpha$ forest. In this work we consider the cosmological signatures of
such dark sectors on the reionization history of our universe. Working within
the "ETHOS" (effective theory of structure formation) framework, we show that
if such a dark sector exists in our universe, the suppression of low mass dark
matter halos would also reduce the total number of ionizing photons, thus
affecting the reionization history of our universe. We place constraints on the
interaction strengths within such dark sectors by using the measured value of
the optical depth from the Planck satellite, as well as from demanding a
successful reionization history. We compare and contrast such scenarios with
warm dark matter scenarios which also suppress structure formation on small
scales. In a model where dark matter interacts with a sterile neutrino, we find
a bound on the ETHOS parameter $a_4\lesssim 1.2\times10^6\textrm{ Mpc}^{-1}$.
For warm dark matter models, we constrain the mass $m_{\textrm{WDM}}\gtrsim
0.7\textrm{ keV}$, which is comparable to bounds obtained from Lyman-$\alpha$
measurements. Future 21-cm experiments will measure the global history of
reionization and the neutral hydrogen power spectrum, which could either lead
to stronger constraints or discovery of secret dark sector interactions.
