Timeline of δT_b. The colour bars are in units of [mK]; ⟨δT_b⟩ represents the mean of the temperature brightness. These maps come from a 21cmFAST simulation, taken at a given depth from the model ζ55 and for each redshift used in this study. White corresponds to the absence of signal, meaning that the hydrogen is ionised at this region. For this simulation, at z = 5.5, there is no neutral hydrogen anymore. Starting at z = 15, δT_b is mainly seen in absorption (negative values in blue) until z = 11, where ⟨δT_b⟩ becomes positive and the signal is seen in emission (positive values in red). During the whole process, HII bubbles grow with time. Figure 2 corresponds to the t_reion(r) associated to these maps.