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>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>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.
