Power spectra of IGM mass field ﹩\delta _{b}﹩ (squares) and dark matter ﹩\delta _{\mathrm{DM}\,}﹩ (circles) from the hydrodynamic simulation in the 12 Mpc box. The mean and variance are calculated from the 100 realizations. The physical scale for j is ﹩\Delta v=2^{13-j}\times 4﹩ km s⁻¹. For clarity, the power spectrum of the IGM is shifted slightly.