Constraining warm dark matter with 21 cm line fluctuations due to minihalos

Warm dark matter (WDM) with mass m(WDM) = O(1) keV has long been discussed as a promising solution for discrepancies between cosmic structures observed at small scales and predications of the concordance CDM model. Though several cosmological observations such as the Lyman-alpha forest have already begun to constrain the range of m(WDM), WDM is yet to be fully excluded as a solution for these so-called small-scale problems. In this paper, we study 21 cm line fluctuations from minihalos in a WDM model and evaluate constraints on m(WDM) for future cosmological 21 cm surveys, such as SKA and FFTT. We show that, since WDM with mass m(WDM) greater than or similar to 10 keV decreases the abundance of minihalos by suppressing the matter power spectrum on small scales via free-streaming, such WDM can significantly affect the resultant 21 cm line fluctuations from minihalos. We find that if the 21 cm signal from minihalos can be observed above z >= 5, SKA and FFTT can give lower bounds m(WDM) greater than or similar to 24 keV and 31 keV, respectively, which are tighter than the current constraint. These future 21 cm surveys might be able to rule out a WDM model as a solution of small-scale problems.