Probing axion dark matter with 21 cm 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 21 cm fluctuation signals. We find that the future radio telescopes, such as the Square Kilometer Array, can put the axion mass bound of order ${m}_{a}\ensuremath{\gtrsim}{10}^{\ensuremath{-}13}\text{ }\text{ }\mathrm{eV}$ for the simple temperature-independent axion mass model, and the bound can be extended to of order ${m}_{a}\ensuremath{\gtrsim}{10}^{\ensuremath{-}8}\text{ }\text{ }\mathrm{eV}$ for a temperature-dependent axion mass.