Inastronomical spectroscopy, theLyman-alpha forest is a series ofabsorption lines in the spectra of distantgalaxies andquasars arising from theLyman-alphaelectron transition of the neutralhydrogen atom. As the light travels through multiple gas clouds with different redshifts, multiple sets of absorption lines are formed.
The Lyman-alpha forest was first discovered in 1970 by astronomer Roger Lynds in an observation of thequasar4C 05.34.[1] 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 and suggested that most of the absorption lines were all due to the sameLyman-alpha transition.[2] Follow-up observations byJohn Bahcall and Samuel Goldsmith confirmed the presence of the unusual absorption lines, though they were less conclusive about the origin of the lines.[3] 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".[4]Jan Oort argued that the absorption features are due not to any physical interactions within the quasars themselves, but to absorption inside clouds ofintergalactic gas in superclusters.[5]
For a neutralhydrogen 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 theultraviolet portion of theelectromagnetic spectrum.[6]
The Lyman-alpha absorption lines in the quasar spectra result from intergalactic gas through which thegalaxy or quasar's light has traveled. Sinceneutral hydrogen clouds in the intergalactic medium are at different degrees ofredshift (due to their varying distance from Earth), their absorption lines are observed at a range ofwavelengths. Each individual cloud leaves itsfingerprint as anabsorption line at a different position in the observedspectrum.
The Lyman-alpha forest is an important probe of theintergalactic medium and can be used to determine the frequency and density of clouds containing neutralhydrogen, as well as their temperature. Searching for lines from other elements likehelium,carbon andsilicon (matching inredshift), the abundance of heavier elements in the clouds can also be studied. A cloud with a high column density of neutral hydrogen will show typicaldamping wings around the line and is referred to as adamped Lyman-alpha system.
For quasars at higher redshift the number of lines in the forest is higher, until at a redshift of about 6, where there is so much neutral hydrogen in the intergalactic medium that the forest turns into aGunn–Peterson trough. This shows the end of thereionization of the universe.
The Lyman-alpha forest observations can be used to constrain cosmological models.[7] They can also be used to constrain the properties ofdark matter, such as the dark matter free streaming scale, which for thermal relic dark matter models is closely related to the dark matter particle mass.