Gunn-Peterson trough
Encyclopedia
In astronomical spectroscopy
, the Gunn-Peterson trough is a feature of the spectra
of quasars due to the presence of neutral hydrogen
in the Intergalactic Medium (IGM). The trough is characterized by suppression of electromagnetic emission
from the quasar at wavelengths less than that of the Lyman-alpha
line at the redshift
of the emitted light. This effect was originally predicted in 1965 by James E. Gunn
and Bruce Peterson.
by Robert Becker using data from the Sloan Digital Sky Survey
, that a Gunn-Peterson trough was finally observed. The paper also included quasars at redshifts of
and 
, 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 
, however, was effectively zero beyond the Lyman-alpha limit, meaning that the neutral hydrogen fraction in the IGM must have been larger than ~10−3.
quasar, and the absence of the trough in quasars detected at redshifts just below 
presented strong evidence for the hydrogen in the universe having undergone a transition from neutral to ionized around 
. After recombination
, the universe was expected to be neutral, until the first objects in the universe started emitting light and energy which would 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 spacecraft in 2003, the determination by Becker that the end of reionization occurred at
≈ 
appeared to conflict with estimates made from the WMAP measurement of the electron column density. 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.
Astronomical spectroscopy
Astronomical spectroscopy is the technique of spectroscopy used in astronomy. The object of study is the spectrum of electromagnetic radiation, including visible light, which radiates from stars and other celestial objects...
, the Gunn-Peterson trough is a feature of the spectra
Electromagnetic spectrum
The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object....
of quasars due to the presence of neutral hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
in the Intergalactic Medium (IGM). The trough is characterized by suppression of electromagnetic emission
Electromagnetic radiation
Electromagnetic radiation is a form of energy that exhibits wave-like behavior as it travels through space...
from the quasar at wavelengths less than that of the Lyman-alpha
Lyman series
In physics and chemistry, the Lyman series is the series of transitions and resulting ultraviolet emission lines of the hydrogen atom as an electron goes from n ≥ 2 to n = 1...
line at the redshift
Redshift
In physics , redshift happens when light seen coming from an object is proportionally increased in wavelength, or shifted to the red end of the spectrum...
of the emitted light. This effect was originally predicted in 1965 by James E. Gunn
James E. Gunn
James Edward Gunn is the Eugene Higgins Professor of Astronomy at Princeton University. Gunn's early theoretical work in astronomy has helped establish the current understanding of how galaxies form, and the properties of the space between galaxies...
and Bruce Peterson.
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, with the discovery of a quasar with a redshift by Robert Becker using data from the Sloan Digital Sky SurveySloan Digital Sky Survey
The Sloan Digital Sky Survey or SDSS is a major multi-filter imaging and spectroscopic redshift survey using a dedicated 2.5-m wide-angle optical telescope at Apache Point Observatory in New Mexico, United States. The project was named after the Alfred P...
, that a Gunn-Peterson trough was finally observed. The paper also included quasars at redshifts of
and , 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 , however, was effectively zero beyond the Lyman-alpha limit, meaning that the neutral hydrogen fraction in the IGM must have been larger than ~10−3.Evidence for reionization
The discovery of the trough in a quasar, and the absence of the trough in quasars detected at redshifts just below presented strong evidence for the hydrogen in the universe having undergone a transition from neutral to ionized around . After recombinationRecombination (cosmology)
In cosmology, recombination refers to the epoch at which charged electrons and protons first became bound to form electrically neutral hydrogen atoms.Note that the term recombination is a misnomer, considering that it represents the first time that electrically neutral hydrogen formed. After the...
, the universe was expected to be neutral, until the first objects in the universe started emitting light and energy which would reionize
Reionization
In Big Bang cosmology, reionization is the process that reionized the matter in the universe after the "dark ages," and is the second of two major phase changes of gas in the universe. As the majority of baryonic matter is in the form of hydrogen, reionization usually refers to the reionization of...
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
Optical depth
Optical depth, or optical thickness, is a measure of transparency. Optical depth is defined by the negative logarithm of the fraction of radiation that is not scattered or absorbed on a path...
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 spacecraft in 2003, the determination by Becker that the end of reionization occurred at
≈ appeared to conflict with estimates made from the WMAP measurement of the electron column density. 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.