Constraints on nature of ultra light dark matter particles with 21cm
  forest
  The ultra-light scalar fields can arise ubiquitously, for instance, as a
result of the spontaneous breaking of an approximate symmetry such as the axion
and more generally the axion-like particles. In addition to the particle
physics motivations, these particles can also play a major role in cosmology by
contributing to dark matter abundance and affecting the structure formation at
sub-Mpc scales. In this paper, we propose to use the 21cm forest observations
to probe the nature of ultra-light dark matter. The 21cm forest can probe much
smaller scales than the Lyman-$\alpha$ forest, that is, $k\gtrsim
10\mathrm{Mpc}^{-1}$. We explore the range of the ultra-light dark matter mass
$m_{u}$ and $f_u$, the fraction of ultra-light dark matter with respect to the
total matter, which can be probed by the 21cm forest. We find that 21cm forest
can potentially put the dark matter mass lower bound $m_u \gtrsim 10^{-18}$ eV
for $f_u=1$, which is 3 orders of magnitude bigger mass scale than those probed
by the current Lyman-$\alpha$ forest observations.While the effects of the
ultra-light particles on the structure formation become smaller when the
dominant component of dark matter is composed of the conventional cold dark
matter, we find that the 21cm forest is still powerful enough to probe the
sub-component ultra-light dark matter mass up to the order of $10^{-19}$ eV.
The Fisher matrix analysis shows that $(m_u,f_u)\sim (10^{-20}\mathrm{eV},
0.3)$ is the most optimal parameter set which the 21cm forest can probe with
the minimal errors for a sub-component ultra-light dark matter scenario.
