Probing Oscillons of Ultra-Light Axion-like Particle by 21cm Forest
  Ultra-Light Axion-like Particle (ULAP) is motivated as one of the solutions
to the small scale problems in astrophysics. When such a scalar particle
oscillates with an $\mathcal{O}(1)$ amplitude in a potential shallower than
quadratic, it can form a localized dense object, oscillon. Because of its
longevity due to the approximate conservation of the adiabatic invariant, it
can survive up to the recent universe as redshift $z \sim \mathcal{O}(10)$. The
scale affected by these oscillons is determined by the ULAP mass $m$ and
detectable by observations of 21cm line. In this paper, we examine the
possibility to detect ULAP by 21cm line and find that the oscillon can enhance
the signals of 21cm line observations when $m \lesssim 10^{-19}\ {\rm eV}$ and
the fraction of ULAP to dark matter is much larger than $10^{-2}$ depending on
the form of the potential.
