usually known as “dirty” image) of the signal by performing a two-dimensional inverse Fourier transform of the visibilities. We note that the [H [i ]{}]{} signal in the “dirty” image is hardly distinguishable from the input [H [i ]{}]{} signal. This is due to the fact that $uv$ space is nearly filled and there are not many empty $uv$ grids at small baselines where the [H [i ]{}]{} signal is strong.

![image](p4spectrum.eps)

The left-hand panel of Figure \[image\_p4spectrum\] shows the ${\delta T_{\rm b}}$ distribution around the three types of sources considered in this work. One can easily identify that there exist four separate regions radially outward from the centre of the source (see the right-hand panel of Figure \[image\_p4spectrum\]). These are (1) [H [ii ]{}]{} [*region*]{}: the signal is zero at the medium just adjacent the source as ${x_{\rm HI}}\sim 0$. (2) [*emission region*]{}: the [H [ii ]{}]{}  region is followed by an emission region where ${T_{\rm S}}>{T_{\gamma}}$. (3) [*absorption region*]{}: the emission region is followed by a strong absorption region where ${T_{\rm S}}<{T_{\gamma}}$ and (4) ${\rm {Ly{\alpha}}}$ [*deficient region*]{}: the signal vanishes at far away region as ${\rm {Ly{\alpha}}}$ coupling is not strong enough and thus ${T_{\rm S}}= {T_{\gamma}}$. 