View source for Astronomy:Lyman-alpha forest

{{Short description|Astronomical spectroscopic term}}
[[File:Lymanalpha.gif|thumb|A computer simulation of a possible Lyman-alpha forest configuration at ''z''&nbsp;=&nbsp;3]]
In [[Physics:Astronomical spectroscopy|astronomical spectroscopy]], the '''Lyman-alpha forest''' is a series of  absorption lines in the spectra of distant [[Astronomy:Galaxy|galaxies]] and [[Astronomy:Quasar|quasar]]s arising from the [[Physics:Lyman-alpha line|Lyman-alpha]] [[Physics:Atomic electron transition|electron transition]] of the neutral [[Chemistry:Hydrogen|hydrogen]] atom.  As the light travels through multiple gas clouds with different redshifts, multiple absorption lines are formed.

==History==
The Lyman-alpha forest was first discovered in 1970 by astronomer Roger Lynds in an observation of the [[Astronomy:Quasar|quasar]] [[Astronomy:List of quasars#Most distant quasars|4C 05.34]].<ref name="mcdonald2004">{{cite journal | title=The Lyman-α Forest Power Spectrum from the Sloan Digital Sky Survey | author1=Patrick McDonald | author2=Uros Seljak | author3=Scott Burles | author4=Schlegel | author5=Weinberg | author6=David Shih | author7=Joop Schaye | author8=Schneider | author9=Brinkmann | s2cid=118878555 | doi=10.1086/444361 | journal=Astrophys. J. Suppl. Ser. | volume=163 | issue=1 | pages=80–109 | year=2006 | arxiv=astro-ph/0405013|bibcode = 2006ApJS..163...80M  }}</ref> 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 same [[Physics:Lyman series|Lyman-alpha]] transition.<ref name="lynds_1971">{{cite journal |last=Lynds |first=Roger |date=1971-03-01 |title=The Absorption-Line Spectrum of 4C 05.34 |journal=The Astrophysical Journal |volume=164 |pages=L73–L78 |bibcode=1971ApJ...164L..73L |doi=10.1086/180695 |doi-access=free}}</ref> 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.<ref name='bahcall_1971'>{{cite journal | title = On the Absorption-Line Spectrum of 4c 05.34 | journal = The Astrophysical Journal | date = 1971-11-15 | first = John | last = Bahcall |author2=Samuel Goldsmith  | volume = 170 | pages = 17–24| doi=10.1086/151185 | bibcode = 1971ApJ...170...17B | bibcode-access=free }}</ref> 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".<ref name='burbidge_skytel_quasars'>{{cite journal | title = A catalog of quasars near and far | journal = Sky & Telescope | date = December 1994 | first = Geoffrey R. | last = Burbidge |author2=Adelaide Hewitt  | volume = 88 | issue = 6 | pages = 32|bibcode = 1994S&T....88...32B }}</ref> Jan Oort argued that the absorption features are due not to any physical interactions within the quasars themselves, but to absorption inside clouds of [[Astronomy:Outer space#Intergalactic space|intergalactic gas]] in superclusters.<ref name='oort_obit'>{{cite journal | title = Jan Hendrik Oort (1900–1992) | journal = Publications of the Astronomical Society of the Pacific | date = July 1993 | first = Adriaan | last = Blaauw |author2=Martin Schmidt  | volume = 105 | issue = 689 | pages = 681–685 | doi=10.1086/133220 | bibcode=1993PASP..105..681B| doi-access =  }}</ref>

==Physical background==
[[File:Lyman-alpha forest.gif|thumb|upright=2|A quasar spectrum with Lyman absorbers being continuously redshifted due to cosmic expansion forming a "forest" of lines.]]

For a neutral [[Chemistry:Hydrogen|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''&nbsp;=&nbsp;1) and the first excited state (''n''&nbsp;=&nbsp;2). The Lyman-alpha spectral line has a laboratory wavelength (or rest wavelength) of 1216&nbsp;[[Angstrom|Å]], which is in the [[Physics:Ultraviolet|ultraviolet]] portion of the [[Physics:Electromagnetic spectrum|electromagnetic spectrum]].<ref name='modern_astrophysics'>{{cite book | last1 = Carroll | first1 = Bradley W. | last2 = Ostlie | first2 = Dale A. | title = An Introduction to Modern Astrophysics | chapter = The Interaction of Light and Matter | publisher = Addison-Wesley Publishing Company, Inc. | date = 1996 | location = New York, New York | pages = 134–142 | isbn = 978-0-201-54730-6}}</ref>

The Lyman-alpha absorption lines in the quasar spectra result from intergalactic gas through which the [[Astronomy:Galaxy|galaxy]] or quasar's light has traveled. Since [[Astronomy:H I region|neutral hydrogen clouds]] in the intergalactic medium are at different degrees of [[Astronomy:Redshift|redshift]] (due to their varying distance from Earth), their absorption lines are observed at a range of [[Wavelength|wavelength]]s. Each individual cloud leaves its [[Biology:Fingerprint|fingerprint]] as an absorption line at a different position in the observed [[Physics:Absorption spectrum|spectrum]].

==Use as a tool in astrophysics==

The Lyman-alpha forest is an important probe of the [[Astronomy:Outer space#Intergalactic space|intergalactic medium]] and can be used to determine the frequency and density of clouds containing neutral [[Chemistry:Hydrogen|hydrogen]], as well as their temperature. Searching for lines from other elements like [[Chemistry:Helium|helium]], [[Chemistry:Carbon|carbon]] and [[Chemistry:Silicon|silicon]] (matching in [[Astronomy:Redshift|redshift]]), the abundance of heavier elements in the clouds can also be studied. A cloud with a high column density of neutral hydrogen will show typical damping wings around the line and is referred to as a [[Astronomy:Damped Lyman-alpha system|damped 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 a [[Astronomy:Gunn–Peterson trough|Gunn–Peterson trough]]. This shows the end of the [[Astronomy:Reionization|reionization]] of the universe.

The Lyman-alpha forest observations can be used to constrain cosmological models.<ref>{{cite conference
 | author = Weinberg, D. H.
 | s2cid = 118868536
 | display-authors = etal
 | title = The Lyman-α Forest as a Cosmological Tool
 | book-title = The Emergence of Cosmic Structure: Thirteenth Astrophysics Conference
 |date=May 2003
 | arxiv=astro-ph/0301186
 | series = AIP Conference Series
 | volume = 666
 | editor = S. H. Holt
 | editor2 = C. S. Reynolds
 | pages = 157–169
 | doi = 10.1063/1.1581786
|isbn=0-7354-0128-4
 | bibcode = 2003AIPC..666..157W
 }}</ref> They can also be used to constrain the properties of dark 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.

==See also==
* [[Astronomy:Lyman-alpha blob|Lyman-alpha blob]]
* Lyman break galaxy
* Lyman alpha emitter
*[[Physics:Lyman continuum photons|Lyman continuum photons]]

==References==
{{reflist}}

==External links==
*{{cite web| first=Michael| last=Rauch| title=The Lyman Alpha Forest in the Spectra of QSOS| url=http://nedwww.ipac.caltech.edu/level5/Sept01/Rauch/frames.html| access-date=2009-03-30}}
*{{cite journal | first=J| last=Liske|author2=Webb, J. K |author3=Carswell, R. F | s2cid=119377250| title=Large-scale structure in the Lyman-α forest: a new technique | journal=[[Organization:Monthly Notices of the Royal Astronomical Society|Monthly Notices of the Royal Astronomical Society]] | volume=301 | issue=3 | pages=787–796 | date=1998 | doi=10.1046/j.1365-8711.1998.02048.x| bibcode=1998MNRAS.301..787L |arxiv = astro-ph/9808082 }}
*{{cite web| title=Lyman alpha systems and cosmology| url=http://astro.berkeley.edu/~jcohn/lya.html| access-date=2009-03-30}}

[[Category:Physical cosmology]]
[[Category:Astronomical spectroscopy]]

{{Sourceattribution|Lyman-alpha forest}}