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The Lyman alpha forest is a set of absorption lines (primarily of hydrogen) superimposed over the spectra of some quasars, indicating the presence of hydrogen gas at intermediate redshifts between us and the quasar. These clouds indicate the presence of either faint galaxies or galaxies not yet formed at these intermediate redshifts.

If we were to observe an unobstructed quasar at a cosmological red shift of zq, we would see a set of ordinary emission and absorption lines redshifted to wavelengths of

wavelength × (1 + zq)

If we then have an absorbing cloud at a redshift zc between us and the quasar, we see additional absorption lines with wavelengths of

wavelength × (1 + zc)

Often, we may see several sets of absorption lines at different redshifts, indicating the presence of clouds at different redshifts along the line of sight.

Lyman absorption systems are classified into several subgroups. The ordinary Lyman alpha forest has hydrogen column densities of NH ~ 1014 cm-2. These systems show up as weak absorption lines against a quasar spectrum.

Lyman limit systems have higher column densities of 1017-20 cm-2 and block nearly all the light at the wavelength of the absorption line.

Damped Lyman alpha systems have such high column densities (N > 1020 cm-2) that they not only absorb light at the absorption line, but also in the wings (slightly higher and lower wavelengths) of the absorption line.

Metal line systems are those damped Lyman alpha systems which also show absorption lines from metallic elements, indicating the presence of a chemically-enriched interstellar medium in the cloud (and as a consequence, of stars).

The lyman alpha forest is used to study the structure of the universe. The redshift of the absorption lines tells us their distance, while the column density lets us estimate their mass. It's not quite as easy as studying the large scale structure with bright galaxies, but it works.

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