Probing axion dark matter with 21 cm fluctuations from minihalos

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