21cm Forest with the SKA
  An alternative to both the tomography technique and the power spectrum
approach is to search for the 21cm forest, that is the 21cm absorption features
against high-z radio loud sources caused by the intervening cold neutral
intergalactic medium (IGM) and collapsed structures. Although the existence of
high-z radio loud sources has not been confirmed yet, SKA-low would be the
instrument of choice to find such sources as they are expected to have spectra
steeper than their lower-z counterparts. Since the strongest absorption
features arise from small scale structures (few tens of physical kpc, or even
lower), the 21cm forest can probe the HI density power spectrum on small scales
not amenable to measurements by any other means. Also, it can be a unique probe
of the heating process and the thermal history of the early universe, as the
signal is strongly dependent on the IGM temperature. Here we show what SKA1-low
could do in terms of detecting the 21cm forest in the redshift range z =
7.5-15.
