 Solution

The Lyman-alpha forest is the large collection of narrow Lyman-alpha absorption features in the spectrum of a distant quasar. Neutral hydrogen in foreground gas absorbs at its own redshifted $1s\to 2p$ resonance, so an absorber at $z_{\mathrm{abs}}$ places its feature atOrdinary intervening absorbers have $z_{\mathrm{abs}}<z_Q$, putting their Lyman-alpha lines blueward of the quasar's own broad Lyman-alpha emission. Many different absorbing patches along the same line of sight make the forest; they are not different transitions of one cloud. At sufficiently short wavelengths higher Lyman-series transitions can overlap the forest, so an actual analysis identifies these blends rather than treating every dip as isolated Ly-alpha.

The upper panel is an original schematic spectrum for $z_Q=3$, with absorption features at smaller cosmological redshifts and a broad emission feature near $4863$ Å. The lower panel anticipates the column-density discussion: both curves are teaching sketches, not measured spectra or fitted survey distributions. The ordinary Lyman-alpha forest mainly traces the fluctuating, photoionized intergalactic medium, rather than a collection of identical isolated, neutral clouds.