Statistical Features of 21 Centimeter Emission from the Epoch between Reionization and the Gunn‐Peterson Transparency

We investigate the 21 cm emission from the epoch between reionization z_r andGunn-Peterson transparency z_{gp}. According to the lognormal model of thethermal history around reionization, hydrogen clouds in z_r > z > z_{gp} arehot and a predominant part of baryonic gas is ionized, but still opaque toLy-alpha photons. Therefore, 21 cm emission is a distinctive characteristic ofthis epoch. We show that the 21 cm emission comes from both uncollapsed andcollapsing hydrogen clouds. The spatial distribution of the brightnesstemperature excess $\delta T_b$ is highly non-Gaussian. It consists of spikeswith high $\delta T_b$ and a low $\delta T_b$ area between the spikes. Thefield has the following statistical features: (1) the one-point distributionsof $\delta T_b$ are described approximately by power-law tailed probabilitydistribution functions; (2) the nth-order moment of $\delta T_b$ is increasingmuch faster with n than that of a Gaussian field, but slower than that of alognormal field; (3) the scale-scale correlation of the $\delta T_b$ field issignificant for all scales larger than the Jeans length of the gas. Thesefeatures would be useful for distinguishing the 21 cm emission of the earlyclustering from the noise of foreground contamination.Comment: 19 pages, 12 figure