Studying topological structure in the epoch of reionization with
  3D-Minkowski functionals of 21cm line fluctuations
  The brightness temperature of the redshifted 21cm line brings rich
information on the Inter Galactic Medium (IGM) from the Cosmic Dawn and Epoch
of Reionization (EoR). While the power spectrum is a useful tool to
statistically investigate the 21cm signal, the 21cm brightness temperature
field is highly non-Gaussian, and the power spectrum is inadequate to
characterize the non-Gaussianity. The Minkowski Functionals (MFs) are promising
tools to extract non-gaussian features of the 21cm signal and give topological
information such as morphology of ionized bubbles. In this work, we study the
21cm line signal in detail with MFs. To promote understanding of basic features
of the 21cm signal, we calculate the MFs of not only the hydrogen neutral
fraction but the matter density and spin temperature, which contribute to the
brightness temperature fluctuations. We find that the structure of the
brightness temperature depends mainly on the ionized fraction and the spin
temperature at late and early stages of the EoR, respectively. Further, we
investigate the redshift evolution of the MFs at $7 < z < 20$. Then, we find
that, after the onset of reionization, the MFs reflect mainly the ionized
bubble property. In addition, the MFs are sensitive to model parameters which
are related to the topology of ionized bubbles and we consider the possibility
of constraining the parameters by the future 21cm signal observations.
