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Concept
Lyman-alpha forest
Summary
In astronomical spectroscopy, the Lyman-alpha forest is a series of absorption lines in the spectra of distant galaxies and quasars arising from the Lyman-alpha electron transition of the neutral hydrogen atom. As the light travels through multiple gas clouds with different redshifts, multiple absorption lines are formed.
The Lyman-alpha forest was first discovered in 1970 by astronomer Roger Lynds in an observation of the quasar 4C 05.34. Quasar 4C 05.34 was the farthest object observed to that date, and Lynds noted an unusually large number of absorption lines in its spectrum. He suggested that most of the absorption lines were all due to the same Lyman-alpha transition. Follow-up observations by John Bahcall and Samuel Goldsmith confirmed the presence of the unusual absorption lines, though they were less conclusive about the origin of the lines. Subsequently, the spectra of many other high-redshift quasars were observed to have the same system of narrow absorption lines. Lynds was the first to describe them as the "Lyman-alpha forest". Jan Oort argued that the absorption features are due not to any physical interactions within the quasars themselves, but to absorption inside clouds of intergalactic gas in superclusters.
For a neutral hydrogen atom, spectral lines are formed when an electron transitions between energy levels. The Lyman series of spectral lines are produced by electrons transitioning between the ground state and higher energy levels (excited states). The Lyman-alpha transition corresponds to an electron transitioning between the ground state (n = 1) and the first excited state (n = 2). The Lyman-alpha spectral line has a laboratory wavelength (or rest wavelength) of 1216 Å, which is in the ultraviolet portion of the electromagnetic spectrum.
The Lyman-alpha absorption lines in the quasar spectra result from intergalactic gas through which the galaxy or quasar's light has traveled. Since neutral hydrogen clouds in the intergalactic medium are at different degrees of redshift (due to their varying distance from Earth), their absorption lines are observed at a range of wavelengths.
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