Gunn-Peterson trough
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2008-06-22T09:35:08Z
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In [[astronomical spectroscopy]], the '''Gunn-Peterson trough''' is a feature of the [[electromagnetic spectrum|spectra]] of [[quasars]] due to the presence of neutral hydrogen in the [[Intergalactic space|Intergalactic Medium (IGM)]]. The trough is characterized by suppression of electromagnetic emission from the quasar at wavelengths less than that of the [[Lyman series|Lyman Alpha]] line at the [[Redshift|redshift]] of the emitted light. This effect was originally predicted in [[1965|1965]] by [[James E. Gunn|James E. Gunn]] and Bruce Peterson<ref>{{cite journal | last = Gunn | first = J.E. | coauthors = Peterson, B.A. | title=On the Density of Neutral Hydrogen in Intergalactic Space | journal=Astrophysical Journal | year=1965 | volume=142 | pages=1633–1641 | url=http://adsabs.harvard.edu/abs/1965ApJ...142.1633G | doi=10.1086/148444}}</ref>.
==First Detection==
For over three decades after the prediction, no objects had been found distant enough to show the Gunn-Peterson trough. It was not until [[2001|2001]], with the discovery of a quasar with a redshift <math> (z) =6.28</math> by Robert Becker<ref>{{cite journal | last=Becker | first=R. H. | coauthors=''et al.'' | title=Evidence For Reionization at z ~ 6: Detection of a Gunn-Peterson Trough In A z=6.28 Quasar | year=2001 | journal=Astronomical Journal | volume=122 | pages=2850–2857 | url=http://adsabs.harvard.edu/abs/2001AJ....122.2850B | doi=10.1086/324231}}</ref> using data from the [[Sloan Digital Sky Survey]], that a Gunn-Peterson trough was finally observed. The paper also included quasars at redshifts of <math>z=5.82</math> and <math>z=5.99</math>, and, while each of these exhibited absorption at wavelengths on the blue side of the Lyman Alpha transition, there were numerous spikes in flux as well. The flux of the quasar at <math>z=6.28</math>, however, was effectively zero beyond the Lyman Alpha limit, meaning the IGM must have still been neutral when the light from the quasar was emitted.
==Evidence for Reionization==
The discovery of the trough in a <math>z=6.28</math> quasar, and the absence of the trough in quasars detected at redshifts just below <math>z=6</math> presented strong evidence for the hydrogen in the universe having undergone a transition from neutral to ionized around <math>z=6</math>. After [[recombination|recombination]], the universe was expected to be neutral, until the first objects in the universe started emitting light and energy which would [[reionization|reionize]] the surrounding IGM. However, as the scattering cross section of photons with energies near that of the Lyman Alpha limit with neutral hydrogen is very high, even a small fraction of neutral hydrogen will make the [[optical depth]] of the IGM high enough to cause the suppression of emission observed. Despite the fact that the ratio of neutral hydrogen to ionized hydrogen may not have been particularly high, the low flux observed past the Lyman Alpha limit indicates that the universe was in the final stages of reionization.
Following the first release of data from the [[WMAP]] satellite in [[2003]], the determination by Becker that the end of reionization occurred at <math>z \approx 6</math> appeared to conflict with estimates made from the WMAP measurement of the electron column density.<ref>{{cite journal | last=Kogut |first=A. | coauthors=''et al.'' | title=First-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Temperature-Polarization Correlation | year=2003 | journal=Astrophysical Journal Supplement Series | volume=148 | issue=1 | pages = 161–173 | url=http://adsabs.harvard.edu/abs/2003ApJS..148..161K | doi=10.1086/377219}}</ref> However, the WMAP III data released in [[2006]] now seems to be in much better agreement with the limits on reionization placed by observation of the Gunn-Peterson trough.<ref>{{cite journal | last=Page | first=L. | coauthors=''et al.'' | title=Three-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Polarization Analysis | year=2007 | journal=Astrophysical Journal Supplement Series | volume=170 | issue=2 | pages=335–376 | url=http://adsabs.harvard.edu/abs/2007ApJS..170..335P | doi=10.1086/513699}}</ref>
==References==
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[[fr:Effet Gunn-Peterson]]