Statistical features of 21-cm emission from the epoch between reionization and Gunn-Peterson transparency

Abstract

We investigate the 21 cm emission from the epoch between reionization z_r and Gunn-Peterson transparency z_{gp}. According to the lognormal model of the thermal history around reionization, hydrogen clouds in z_r > z > z_{gp} are hot and a predominant part of baryonic gas is ionized, but still opaque to Ly-alpha photons. Therefore, 21 cm emission is a distinctive characteristic of this epoch. We show that the 21 cm emission comes from both uncollapsed and collapsing hydrogen clouds. The spatial distribution of the brightness temperature excess $\delta T_b$ is highly non-Gaussian. It consists of spikes with high $\delta T_b$ and a low $\delta T_b$ area between the spikes. The field has the following statistical features: (1) the one-point distributions of $\delta T_b$ are described approximately by power-law tailed probability distribution functions; (2) the nth-order moment of $\delta T_b$ is increasing much faster with n than that of a Gaussian field, but slower than that of a lognormal field; (3) the scale-scale correlation of the $\delta T_b$ field is significant for all scales larger than the Jeans length of the gas. These features would be useful for distinguishing the 21 cm emission of the early clustering from the noise of foreground contamination.