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
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.
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.