High redshift signatures in the 21 cm forest due to cosmic string wakes
  Cosmic strings induce minihalo formation in the early universe. The resultant
minihalos cluster in string wakes and create a "21 cm forest" against the
cosmic microwave background (CMB) spectrum. Such a 21 cm forest can contribute
to angular fluctuations of redshifted 21 cm signals integrated along the line
of sight. We calculate the root-mean-square amplitude of the 21 cm fluctuations
due to strings and show that these fluctuations can dominate signals from
minihalos due to primordial density fluctuations at high redshift ($z \gtrsim
10$), even if the string tension is below the current upper bound, $G \mu < 1.5
\times 10^{-7}$. Our results also predict that the Square Kilometre Array (SKA)
can potentially detect the 21 cm fluctuations due to strings with $G \mu
\approx 7.5 \times 10^{-8}$ for the single frequency band case and $4.0 \times
10^{-8}$ for the multi-frequency band case.
