The 21 cm Forest as a Probe of the Reionization and the Temperature of
  the Intergalactic Medium
  Using high redshift radio sources as background, the 21cm forest observations
probe the neutral hydrogen absorption signatures of early structures along the
lines of sight. Directly sensitive to the spin temperature, it complements the
21cm tomography observations, and provides information on the temperature as
well as the ionization state of the intergalactic medium (IGM). We use a
radiative transfer simulation to investigate the 21cm forest signals during the
epoch of reionization. We first confirmed that the optical depth and equivalent
width (EW) are good representations of the ionization and thermal state of the
IGM. The features selected by their relative optical depth are excellent
tracers of ionization fields, while those selected by absolute optical depth
are very sensitive to the IGM temperature, so the IGM temperature information
could potentially be extracted, thus breaking a degeneracy in 21cm tomographic
observation. From the distributions of EWs and the number evolution of
absorbers and leakers with different EWs, we see clearly the cosmological
evolution of ionization state of the IGM. The number density of potentially
observable features decreases rapidly with increasing gas temperature. The
sensitivity of the proposed EW statistic to the IGM temperature makes it a
unique and potentially powerful probe of reionization. Missing small-scale
structures, such as small filaments and minihalos that are unresolved in our
current simulation, and lack of an accurate calculation of the IGM temperature,
however, likely have rendered the presented signals quantitatively inaccurate.
Finally we discuss the requirements of the background radio sources for such
observations, and find that signals with equivalent widths larger than 1kHz are
hopeful to be detected.(Abridged)
