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.