Editing Reionization (section)

===Quasars===
[[Quasars]], a class of [[active galactic nuclei]] (AGN), were considered a good candidate source because they are highly efficient at converting [[mass]] to [[energy]], and emit a great deal of light above the threshold for ionizing hydrogen. It is unknown, however, how many quasars existed prior to reionization. Only the brightest of quasars present during reionization can be detected, which means there is no direct information about dimmer quasars that existed. However, by looking at the more easily observed quasars in the nearby universe, and assuming that the [[Luminosity function (astronomy)|luminosity function]] (number of quasars as a function of [[luminosity]]) during reionization will be approximately the same as it is today, it is possible to make estimates of the quasar populations at earlier times. Such studies have found that quasars do not exist in high enough numbers to reionize the IGM alone,<ref name="qso_source1" /><ref name="qso_source0">{{cite journal |author=Shapiro |first1=Paul |author1-link=Paul R. Shapiro |last2=Giroux |first2=Mark |name-list-style=amp |date=1987 |title=Cosmological H II regions and the photoionization of the intergalactic medium |journal=The Astrophysical Journal |volume=321 |pages=107–112 |bibcode=1987ApJ...321L.107S |doi=10.1086/185015|doi-access=free }}</ref> saying that "only if the ionizing background is dominated by low-luminosity AGNs can the quasar luminosity function provide enough ionizing photons."<ref name="qso_source2">{{cite journal |author=Fan |first=Xiaohu |display-authors=etal |date=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>