Editing Quasar (section)

=== Spectral lines, reionization, and the early universe===
{{main|Reionization|Chronology of the Universe}}
[[File:Fingerprint of the early Universe.jpg|thumb|Spectrum from quasar HE&nbsp;0940-1050 after it has travelled through [[intergalactic medium]]]]
[[File:QuasarStarburst.jpg|thumb|right|This view, taken with infrared light, is a false-color image of a quasar-starburst tandem with the most luminous [[starburst (astronomy)|starburst]] ever seen in such a combination.]]

Quasars also provide some clues as to the end of the [[Big Bang]]'s [[reionization]]. The oldest known quasars ([[redshift|''z'']]&nbsp;=&nbsp;6){{needs update|date=January 2021}} display a [[Gunn–Peterson trough]] and have absorption regions in front of them indicating that the [[intergalactic medium]] at that time was [[neutral gas]]. More recent quasars show no absorption region, but rather their spectra contain a spiky area known as the [[Lyman-alpha forest]]; this indicates that the intergalactic medium has undergone reionization into [[plasma (physics)|plasma]], and that neutral gas exists only in small clouds.

The intense production of [[ionization|ionizing]] [[ultraviolet]] radiation is also significant, as it would provide a mechanism for reionization to occur as galaxies form. Despite this, current theories suggest that quasars were not the primary source of reionization; the primary causes of reionization were probably the earliest generations of [[star]]s, known as [[Population III]] stars (possibly 70%), and [[dwarf galaxies]] (very early small high-energy galaxies) (possibly 30%).<ref name="popIII_sim">{{Cite journal |last1=Gnedin |first1=Nickolay Y. |last2=Ostriker |first2=Jeremiah P. |date=1997 |title=Reionization of the Universe and the Early Production of Metals |journal=The Astrophysical Journal |language=en |volume=486 |issue=2 |pages=581–598 |arxiv=astro-ph/9612127 |bibcode=1997ApJ...486..581G |doi=10.1086/304548 |issn=0004-637X |s2cid=5758398}}</ref><ref name="qso_z">{{cite arXiv |first=Limin |last=Lu |date=1998 |title=The Metal Contents of Very Low Column Density Lyman-alpha Clouds: Implications for the Origin of Heavy Elements in the Intergalactic Medium |eprint=astro-ph/9802189 |display-authors=etal}}</ref><ref name="Bouwens_LLG">{{Cite journal |last=Bouwens |first=R. J. |display-authors=etal |date=2012 |title=Lower-luminosity Galaxies Could Reionize the Universe: Very Steep Faint-end Slopes to the UV Luminosity Functions at ''z'' ⩾ 5–8 from the HUDF09 WFC3/IR Observations |journal=The Astrophysical Journal |volume=752 |issue=1 |pages=L5 |arxiv=1105.2038 |bibcode=2012ApJ...752L...5B |doi=10.1088/2041-8205/752/1/L5 |issn=2041-8205 |s2cid=118856513}}</ref><ref name="qso_source1">{{Cite journal |last=Madau |first=Piero |display-authors=etal |date=April 1999 |title=Radiative Transfer in a Clumpy Universe. III. The Nature of Cosmological Ionizing Sources |journal=The Astrophysical Journal |language=en |volume=514 |issue=2 |pages=648–659 |arxiv=astro-ph/9809058 |bibcode=1999ApJ...514..648M |doi=10.1086/306975 |issn=0004-637X |s2cid=17932350}}</ref><ref name="qso_source0">{{Cite journal |last1=Shapiro |first1=Paul R. |author-link=Paul R. Shapiro |last2=Giroux |first2=Mark L. |date=October 1987 |title=Cosmological H II regions and the photoionization of the intergalactic medium |journal=The Astrophysical Journal |language=en |volume=321 |pages=L107 |bibcode=1987ApJ...321L.107S |doi=10.1086/185015 |issn=0004-637X |doi-access=free}}</ref><ref name="qso_source2">{{Cite journal |last=Fan |first=Xiaohui |display-authors=etal |date=December 2001 |title=A Survey of ''z'' > 5.8 Quasars in the Sloan Digital Sky Survey. I. Discovery of Three New Quasars and the Spatial Density of Luminous Quasars at ''z'' ~ 6 |journal=The Astronomical Journal |volume=122 |issue=6 |pages=2833–2849 |arxiv=astro-ph/0108063 |bibcode=2001AJ....122.2833F |doi=10.1086/324111 |s2cid=119339804}}</ref>

Quasars show evidence of elements heavier than [[helium]], indicating that galaxies underwent a massive phase of [[star formation]], creating [[population III stars]] between the time of the [[Big Bang]] and the first observed quasars. Light from these stars may have been observed in 2005 using [[NASA]]'s [[Spitzer Space Telescope]],<ref>{{cite web |url=http://www.nasa.gov/centers/goddard/news/topstory/2005/universe_objects.html |title=NASA Goddard Space Flight Center: News of light that may be from population III stars |publisher=Nasa.gov |access-date=2011-07-01 |archive-date=2011-04-16 |archive-url=https://web.archive.org/web/20110416191522/http://www.nasa.gov/centers/goddard/news/topstory/2005/universe_objects.html |url-status=dead }}</ref> although this observation remains to be confirmed.