21cm forest probes on the axion dark matter in the post-inflationary
  Peccei-Quinn symmetry breaking scenarios
  We study the future prospects of the 21cm forest observations on the
axion-like dark matter when the spontaneous breaking of the global Peccei-Quinn
(PQ) symmetry occurs after the inflation. The large isocurvature perturbations
of order unity sourced from axion-like particles can result in the enhancement
of minihalo formation, and the subsequent hierarchical structure formation can
affect the minihalo abundance whose masses can exceed ${\cal O}(10^4)
M_{\odot}$ relevant for the 21cm forest observations. We show that the 21cm
forest observations are capable of probing the axion-like particle mass in the
range $10^{-18}\lesssim m_a \lesssim 10^{-12}$ eV for the temperature
independent axion mass. For the temperature dependent axion mass, the zero
temperature axion mass scale for which the 21cm forest measurements can be
affected is extended further to as big as of order $10^{-6}$ eV.
