Probing axion dark matter with 21cm fluctuations from minihalos
  If the symmetry breaking inducing the axion occurs after the inflation, the
large axion isocurvature perturbations can arise due to a different axion
amplitude in each causally disconnected patch. This causes the enhancement of
the small-scale density fluctuations which can significantly affect the
evolution of structure formation. The epoch of the small halo formation becomes
earlier and we estimate the abundance of those minihalos which can host the
neutral hydrogen atoms to result in the 21cm fluctuation signals. We find that
the future radio telescopes, such as the SKA, can put the axion mass bound of
order $m_a \gtrsim 10^{-13}$ eV for the simple temperature-independent axion
mass model, and the bound can be extended to of order $m_a \gtrsim 10^{-8}$eV
for a temperature-dependent axion mass.
