21 cm line bispectrum as a method to probe cosmic dawn and epoch of reionization
Redshifted 21cm signal is a promising tool to investigate the state of intergalactic medium (IGM) in the Cosmic Dawn (CD) and Epoch of Reionization(EoR) In our previous work (Shimabukuro et al 2015) we studied the variance and skewness to give a clear interpretation of 21cm power spectrum and found that skewness is a good indicator of the epoch when X ray heating becomes effective. Thus, the non Gaussian feature of the spatial distribution of the 21cm signal is expected to be useful to investigate the astrophysical effects in the CD and EoR. In this paper, in order to investigate such a non Gaussian feature in more detail, we focus on the bispectrum of the 21cm signal. It is expected that the 21cm brightness temperature bispectrum is produced by non gaussianity due to the various astrophysical effects such as Wouthysen Field (WF) effect, X ray heating and reionization. We study the various properties of 21cm bispectrum such as scale dependence, shape dependence and redshift evolution. And also we study the contribution from each component of 21cm bispectrum. We find that the contribution from each component has characteristic scale dependent feature, and it is expected that we could obtain more detailed information on the IGM in the CD and EoR by using the 21cm bispectrum in the future experiments, combined with the power spectrum and skewness.