  The absorption features in spectra of high-redshift background radio sources,
caused by hyperfine structure lines of hydrogen atoms in the intervening
structures, are known collectively as the 21-cm forest. They provide a unique
probe of small-scale structures during the epoch of reionization, and can be
used to constrain the properties of the dark matter (DM) thought to govern
small-scale structure formation. However, the signals are easily suppressed by
heating processes that are degenerate with a warm DM model. Here we propose a
probe of both the DM particle mass and the heating history of the Universe,
using the one-dimensional power spectrum of the 21-cm forest. The
one-dimensional power spectrum measurement not only breaks the DM model
degeneracy but also increases the sensitivity, making the probe actually
feasible. Making 21-cm forest observations with the upcoming Square Kilometre
Array has the potential to simultaneously determine both the DM particle mass
and the heating level in the early Universe, shedding light on the nature of DM
and the first galaxies.
