Studying topological structure of 21-cm line fluctuations with 3D Minkowski functionals before reionization

Abstract

The brightness temperature of the redshifted 21-cm line brings rich information about the intergalactic medium (IGM) from the cosmic dawn and epoch of reionization (EoR). While the power spectrum is a useful tool to investigate the 21-cm signal statistically, the 21-cm brightness temperature field is highly non-Gaussian and the power spectrum is inadequate to characterize the non-Gaussianity. Minkowski functionals (MFs) are promising tools to extract non-Gaussian features of the 21-cm signal and give topological information, such as morphology of ionized bubbles. In this work, we study the 21-cm line signal in detail with MFs. To promote understanding of basic features of the 21-cm signal, we calculate the MFs of not only the hydrogen neutral fraction but also the matter density and spin temperature, which contribute to 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 MFs at 7 < z < 20. We find that, after the onset of reionization, MFs mainly reflect the ionized bubble property. In addition, MFs are sensitive to model parameters related to the topology of ionized bubbles and we consider the possibility of constraining the parameters using future 21-cm signal observations.