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.