Lyman-α forest

Lyman-α forest A close series of narrow absorption lines seen at shorter wavelengths than the strong Lyman-α emission line in the spectra of quasars. They are believed to be caused by absorption in cool hydrogen clouds, perhaps in the haloes of galaxies which are too faint to be seen directly, between us and the quasar. All these absorption lines are Lyman-α lines of hydrogen, but redshifted by different amounts depending on the distance of the absorbing cloud. Since the clouds are between us and the quasar, their redshifts are less than that of the Lyman-α emission from the quasar itself. See also lyman series.