Editing Reionization (section)

==Energy sources==
[[File:Hubble opens its eye again.tif|thumb|Astronomers hope to use observations such as this 2018 [[Hubble Space Telescope]] image to answer the question of how the Universe was reionised.<ref>{{cite web |title=Hubble opens its eye again |url=https://www.spacetelescope.org/images/potw1851a/ |website=www.spacetelescope.org |access-date=17 December 2018 |language=en}}</ref>]]
While observations have come in which narrow the window during which the epoch of reionization could have taken place, it is still uncertain which objects provided the photons that reionized the IGM. To ionize neutral hydrogen, an energy larger than 13.6 [[electronvolt|eV]] is required, which corresponds to photons with a wavelength of 91.2 [[nanometre|nm]] or shorter. This is in the [[ultraviolet]] part of the [[electromagnetic spectrum]], which means that the primary candidates are all sources which produce a significant amount of energy in the ultraviolet and above. How numerous the source is must also be considered, as well as the longevity, as protons and electrons will recombine if energy is not continuously provided to keep them apart. Altogether, the critical parameter for any source considered can be summarized as its "emission rate of hydrogen-ionizing photons per unit cosmological volume."<ref name="qso_source1">{{cite journal |author=Madau |first=Piero |display-authors=etal |date=1999 |title=Radiative Transfer in a Clumpy Universe. III. The Nature of Cosmological Ionizing Source |journal=The Astrophysical Journal |volume=514 |issue=2 |pages=648–659 |arxiv=astro-ph/9809058 |bibcode=1999ApJ...514..648M |doi=10.1086/306975 |s2cid=17932350}}</ref> With these constraints, it is expected that quasars and first generation [[star]]s and [[Galaxy|galaxies]] were the main sources of energy.<ref name="reion_his">{{cite journal | last1=Barkana | first1=R. | last2=Loeb | first2=A. | year=2001 | title=In the Beginning: The First Sources of Light and the Reionization of the Universe | journal=Physics Reports | volume=349 | issue=2 | pages=125–238 | bibcode=2001PhR...349..125B | doi=10.1016/S0370-1573(01)00019-9|arxiv = astro-ph/0010468| s2cid=119094218 | url=https://cds.cern.ch/record/471794 }}</ref>

=== Dwarf galaxies ===
[[Dwarf galaxies]] are currently considered to be the primary source of ionizing photons during the epoch of reionization.<ref name="Bouwens_LLG">{{cite journal |author=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 Letters |volume=752 |issue=1 |pages=L5 |arxiv=1105.2038 |bibcode=2012ApJ...752L...5B |doi=10.1088/2041-8205/752/1/L5 |s2cid=118856513}}</ref><ref>{{Cite journal |last1=Atek |first1=Hakim |last2=Richard |first2=Johan |last3=Jauzac |first3=Mathilde |last4=Kneib |first4=Jean-Paul |last5=Natarajan |first5=Priyamvada |last6=Limousin |first6=Marceau |last7=Schaerer |first7=Daniel |last8=Jullo |first8=Eric |last9=Ebeling |first9=Harald |last10=Egami |first10=Eiichi |last11=Clement |first11=Benjamin |date=2015-11-18 |title=Are ultra-faint galaxies at z = 6–8 responsible for cosmic reionization? Combined constraints from the Hubble frontier fields clusters and parallels |url=https://iopscience.iop.org/article/10.1088/0004-637X/814/1/69 |journal=The Astrophysical Journal |volume=814 |issue=1 |pages=69 |doi=10.1088/0004-637X/814/1/69 |arxiv=1509.06764 |bibcode=2015ApJ...814...69A |s2cid=73567045 |issn=1538-4357}}</ref> For most scenarios, this would require the log-slope of the UV galaxy [[Luminosity function (astronomy)|luminosity function]], often denoted α, to be steeper than it is today, approaching α = -2.<ref name=Bouwens_LLG/> With the advent of the ''James Webb Space Telescope'' (JWST), constraints on the UV luminosity function at the Epoch of Reionization have become commonplace,<ref>{{Cite journal |last1=Harikane |first1=Yuichi |last2=Ouchi |first2=Masami |last3=Oguri |first3=Masamune |last4=Ono |first4=Yoshiaki |last5=Nakajima |first5=Kimihiko |last6=Isobe |first6=Yuki |last7=Umeda |first7=Hiroya |last8=Mawatari |first8=Ken |last9=Zhang |first9=Yechi |date=2023-03-01 |title=A Comprehensive Study of Galaxies at z ∼ 9–16 Found in the Early JWST Data: Ultraviolet Luminosity Functions and Cosmic Star Formation History at the Pre-reionization Epoch |journal=The Astrophysical Journal Supplement Series |volume=265 |issue=1 |pages=5 |doi=10.3847/1538-4365/acaaa9 |arxiv=2208.01612 |bibcode=2023ApJS..265....5H |issn=0067-0049 |doi-access=free }}</ref><ref>{{Cite journal |last1=McLeod |first1=D. J. |last2=Donnan |first2=C. T. |last3=McLure |first3=R. J. |last4=Dunlop |first4=J. S. |last5=Magee |first5=D. |last6=Begley |first6=R. |last7=Carnall |first7=A. C. |last8=Cullen |first8=F. |last9=Ellis |first9=R. S. |last10=Hamadouche |first10=M. L. |last11=Stanton |first11=T. M. |date=2023 |title=The galaxy UV luminosity function at z ≃ 11 from a suite of public JWST ERS, ERO and Cycle-1 programs |journal=Monthly Notices of the Royal Astronomical Society |volume=527 |issue=3 |page=5004 |doi=10.1093/mnras/stad3471 |doi-access=free |arxiv=2304.14469|bibcode=2024MNRAS.527.5004M }}</ref> allowing for better constraints on the faint, low-mass population of galaxies.

In 2014, two separate studies identified two [[Pea galaxy|Green Pea galaxies]] (GPs) to be likely [[Lyc photon|Lyman Continuum]] (LyC)-emitting candidates.<ref name="Jaskot_2014(b)">{{cite journal |author=Jaskot |first1=A. E. |last2=Oey |first2=M. S. |name-list-style=amp |date=2014 |title=Linking Ly-alpha and Low-Ionization Transitions at Low Optical Depth |journal=The Astrophysical Journal Letters |volume=791 |issue=2 |pages=L19 |arxiv=1406.4413 |bibcode=2014ApJ...791L..19J |doi=10.1088/2041-8205/791/2/L19 |s2cid=119294145}}</ref><ref name=Verhamme_2014>{{cite arXiv |first1=A. | last1=Verhamme |first2=I. | last2=Orlitova |first3=D. | last3=Schaerer|first4=M. | last4=Hayes |title=On the use of Lyman-alpha to detect Lyman continuum leaking galaxies| date=2014| class=astro-ph.GA |eprint=1404.2958v1 }}</ref> Compact dwarf star-forming galaxies like the GPs are considered excellent low-redshift analogs of high-redshift Lyman-alpha and LyC emitters (LAEs and LCEs, respectively).<ref>{{Cite journal |last1=Izotov |first1=Y. I. |last2=Guseva |first2=N. G. |last3=Fricke |first3=K. J. |last4=Henkel |first4=C. |last5=Schaerer |first5=D. |last6=Thuan |first6=T. X. |date=February 2021 |title=Low-redshift compact star-forming galaxies as analogues of high-redshift star-forming galaxies |url=https://www.aanda.org/10.1051/0004-6361/202039772 |journal=Astronomy & Astrophysics |volume=646 |pages=A138 |doi=10.1051/0004-6361/202039772 |arxiv=2103.01505 |bibcode=2021A&A...646A.138I |s2cid=232092358 |issn=0004-6361}}</ref> At that time, only two other LCEs were known: [[Haro 11]] and [[Tololo-1247-232]].<ref name=Jaskot_2014(b)/><ref name=Verhamme_2014/><ref name="nakajima">{{cite journal |author=Nakajima |first1=K. |last2=Ouchi |first2=M. |name-list-style=amp |date=2014 |title=Ionization state of inter-stellar medium in galaxies: evolution, SFR-M*-Z dependence, and ionizing photon escape |journal=[[Monthly Notices of the Royal Astronomical Society]] |volume=442 |issue=1 |pages=900–916 |arxiv=1309.0207 |bibcode=2014MNRAS.442..900N |doi=10.1093/mnras/stu902 |doi-access=free |s2cid=118617426}}</ref> Finding local LyC emitters has thus become crucial to the theories about the early universe and the epoch of reionization.<ref name=Jaskot_2014(b)/><ref name=Verhamme_2014/>

Subsequently, motivated, a series of surveys have been conducted using ''Hubble Space Telescope''<nowiki/>'s Cosmic Origins Spectrograph (''HST''/COS) to measure the LyC directly.<ref>{{Cite journal |last1=Izotov |first1=Y. I. |last2=Orlitová |first2=I. |last3=Schaerer |first3=D. |last4=Thuan |first4=T. X. |last5=Verhamme |first5=A. |last6=Guseva |first6=N. G. |last7=Worseck |first7=G. |date=2016-01-14 |title=Eight per cent leakage of Lyman continuum photons from a compact, star-forming dwarf galaxy |url=https://www.nature.com/articles/nature16456 |journal=Nature |language=en |volume=529 |issue=7585 |pages=178–180 |doi=10.1038/nature16456 |pmid=26762455 |arxiv=1601.03068 |bibcode=2016Natur.529..178I |s2cid=3033749 |issn=0028-0836}}</ref><ref>{{Cite journal |last1=Izotov |first1=Y. I. |last2=Schaerer |first2=D. |last3=Thuan |first3=T. X. |last4=Worseck |first4=G. |last5=Guseva |first5=N. G. |last6=Orlitová |first6=I. |last7=Verhamme |first7=A. |date=2016-10-01 |title=Detection of high Lyman continuum leakage from four low-redshift compact star-forming galaxies |journal=Monthly Notices of the Royal Astronomical Society |language=en |volume=461 |issue=4 |pages=3683–3701 |doi=10.1093/mnras/stw1205 |issn=0035-8711|doi-access=free |arxiv=1605.05160 }}</ref><ref>{{Cite journal |last1=Izotov |first1=Y. I. |last2=Schaerer |first2=D. |last3=Worseck |first3=G. |last4=Guseva |first4=N. G. |last5=Thuan |first5=T. X. |last6=Verhamme |first6=A. |last7=Orlitová |first7=I. |last8=Fricke |first8=K. J. |date=2018-03-11 |title=J1154+2443: a low-redshift compact star-forming galaxy with a 46 per cent leakage of Lyman continuum photons |url=http://academic.oup.com/mnras/article/474/4/4514/4683272 |journal=Monthly Notices of the Royal Astronomical Society |language=en |volume=474 |issue=4 |pages=4514–4527 |doi=10.1093/mnras/stx3115 |issn=0035-8711|doi-access=free |arxiv=1711.11449 }}</ref><ref>{{Cite journal |last1=Izotov |first1=Y. I. |last2=Worseck |first2=G. |last3=Schaerer |first3=D. |last4=Guseva |first4=N. G |last5=Thuan |first5=T. X. |last6=Fricke |last7=Verhamme |first7=A. |last8=Orlitová |first8=I. |date=2018-08-21 |title=Low-redshift Lyman continuum leaking galaxies with high [O iii]/[O ii] ratios |url=https://academic.oup.com/mnras/article/478/4/4851/5004866 |journal=Monthly Notices of the Royal Astronomical Society |language=en |volume=478 |issue=4 |pages=4851–4865 |doi=10.1093/mnras/sty1378 |issn=0035-8711|doi-access=free |arxiv=1805.09865 }}</ref><ref>{{Cite journal |last1=Wang |first1=Bingjie |last2=Heckman |first2=Timothy M. |last3=Leitherer |first3=Claus |last4=Alexandroff |first4=Rachel |last5=Borthakur |first5=Sanchayeeta |last6=Overzier |first6=Roderik A. |date=2019-10-30 |title=A New Technique for Finding Galaxies Leaking Lyman-continuum Radiation: [S ii]-deficiency |journal=The Astrophysical Journal |volume=885 |issue=1 |pages=57 |doi=10.3847/1538-4357/ab418f |arxiv=1909.01368 |bibcode=2019ApJ...885...57W |issn=1538-4357 |doi-access=free }}</ref><ref>{{Cite journal |last1=Izotov |first1=Y. I. |last2=Worseck |first2=G. |last3=Schaerer |first3=D. |last4=Guseva |first4=N. G. |last5=Chisholm |first5=J. |last6=Thuan |first6=T. X. |last7=Fricke |first7=K. J. |last8=Verhamme |first8=A. |date=2021-03-22 |title=Lyman continuum leakage from low-mass galaxies with M ⋆ &lt; 108 M⊙ |url=https://academic.oup.com/mnras/article/503/2/1734/6157755 |journal=Monthly Notices of the Royal Astronomical Society |language=en |volume=503 |issue=2 |pages=1734–1752 |doi=10.1093/mnras/stab612 |doi-access=free |issn=0035-8711|arxiv=2103.01514 }}</ref> These efforts culminated in the Low-redshift Lyman Continuum Survey,<ref name=":0">{{Cite journal |last1=Flury |first1=Sophia R. |last2=Jaskot |first2=Anne E. |last3=Ferguson |first3=Harry C. |last4=Worseck |first4=Gábor |last5=Makan |first5=Kirill |last6=Chisholm |first6=John |last7=Saldana-Lopez |first7=Alberto |last8=Schaerer |first8=Daniel |last9=McCandliss |first9=Stephan |last10=Wang |first10=Bingjie |last11=Ford |first11=N. M. |last12=Heckman |first12=Timothy |last13=Ji |first13=Zhiyuan |last14=Giavalisco |first14=Mauro |last15=Amorin |first15=Ricardo |date=2022-05-01 |title=The Low-redshift Lyman Continuum Survey. I. New, Diverse Local Lyman Continuum Emitters |journal=The Astrophysical Journal Supplement Series |volume=260 |issue=1 |pages=1 |doi=10.3847/1538-4365/ac5331 |arxiv=2201.11716 |bibcode=2022ApJS..260....1F |issn=0067-0049 |doi-access=free }}</ref> a large ''HST''/COS program which nearly tripled the number of direct measurements of the LyC from dwarf galaxies. To date, at least 50 LCEs have been confirmed using ''HST''/COS<ref name=":0" /> with LyC escape fractions anywhere from ≈ 0 to 88%. The results from the Low-redshift Lyman Continuum Survey have provided the empirical foundation necessary to identify and understand LCEs at the Epoch of Reionization.<ref>{{Cite journal |last1=Saldana-Lopez |first1=Alberto |last2=Schaerer |first2=Daniel |last3=Chisholm |first3=John |last4=Flury |first4=Sophia R. |last5=Jaskot |first5=Anne E. |last6=Worseck |first6=Gábor |last7=Makan |first7=Kirill |last8=Gazagnes |first8=Simon |last9=Mauerhofer |first9=Valentin |last10=Verhamme |first10=Anne |last11=Amorín |first11=Ricardo O. |last12=Ferguson |first12=Harry C. |last13=Giavalisco |first13=Mauro |last14=Grazian |first14=Andrea |last15=Hayes |first15=Matthew J. |date=July 2022 |title=The Low-Redshift Lyman Continuum Survey: Unveiling the ISM properties of low- z Lyman-continuum emitters |url=https://www.aanda.org/10.1051/0004-6361/202141864 |journal=Astronomy & Astrophysics |volume=663 |pages=A59 |doi=10.1051/0004-6361/202141864 |arxiv=2201.11800 |bibcode=2022A&A...663A..59S |s2cid=246411216 |issn=0004-6361}}</ref><ref>{{Cite journal |last1=Flury |first1=Sophia R. |last2=Jaskot |first2=Anne E. |last3=Ferguson |first3=Harry C. |last4=Worseck |first4=Gábor |last5=Makan |first5=Kirill |last6=Chisholm |first6=John |last7=Saldana-Lopez |first7=Alberto |last8=Schaerer |first8=Daniel |last9=McCandliss |first9=Stephan R. |last10=Xu |first10=Xinfeng |last11=Wang |first11=Bingjie |last12=Oey |first12=M. S. |last13=Ford |first13=N. M. |last14=Heckman |first14=Timothy |last15=Ji |first15=Zhiyuan |date=2022-05-01 |title=The Low-redshift Lyman Continuum Survey. II. New Insights into LyC Diagnostics |journal=The Astrophysical Journal |volume=930 |issue=2 |pages=126 |doi=10.3847/1538-4357/ac61e4 |arxiv=2203.15649 |bibcode=2022ApJ...930..126F |issn=0004-637X |doi-access=free }}</ref><ref>{{Cite journal |last1=Chisholm |first1=J. |last2=Saldana-Lopez |first2=A. |last3=Flury |first3=S. |last4=Schaerer |first4=D. |last5=Jaskot |first5=A. |last6=Amorín |first6=R. |last7=Atek |first7=H. |last8=Finkelstein |first8=S. L. |last9=Fleming |first9=B. |last10=Ferguson |first10=H. |last11=Fernández |first11=V. |last12=Giavalisco |first12=M. |last13=Hayes |first13=M. |last14=Heckman |first14=T. |last15=Henry |first15=A. |date=2022-11-09 |title=The far-ultraviolet continuum slope as a Lyman Continuum escape estimator at high redshift |url=https://academic.oup.com/mnras/article/517/4/5104/6753210 |journal=Monthly Notices of the Royal Astronomical Society |language=en |volume=517 |issue=4 |pages=5104–5120 |doi=10.1093/mnras/stac2874 |doi-access=free |issn=0035-8711|arxiv=2207.05771 }}</ref> With new observations from ''JWST'', populations of LCEs are now being studied at cosmological redshifts greater than 6, allowing for the first time a detailed and direct assessment of the origins of cosmic Reionization.<ref>{{Cite journal |last1=Mascia |first1=S. |last2=Pentericci |first2=L. |last3=Calabrò |first3=A. |last4=Treu |first4=T. |last5=Santini |first5=P. |last6=Yang |first6=L. |last7=Napolitano |first7=L. |last8=Roberts-Borsani |first8=G. |last9=Bergamini |first9=P. |last10=Grillo |first10=C. |last11=Rosati |first11=P. |last12=Vulcani |first12=B. |last13=Castellano |first13=M. |last14=Boyett |first14=K. |last15=Fontana |first15=A. |date=April 2023 |title=Closing in on the sources of cosmic reionization: First results from the GLASS-JWST program |url=https://www.aanda.org/10.1051/0004-6361/202345866 |journal=Astronomy & Astrophysics |volume=672 |pages=A155 |doi=10.1051/0004-6361/202345866 |arxiv=2301.02816 |bibcode=2023A&A...672A.155M |s2cid=255546596 |issn=0004-6361}}</ref> Combining these large samples of galaxies with new constraints on the UV luminosity function indicates that dwarf galaxies overwhelmingly contribute to Reionization.<ref>{{Cite journal |last1=Mascia |first1=S. |last2=Pentericci |first2=L. |last3=Calabrò |first3=A. |last4=Santini |first4=P. |last5=Napolitano |first5=L. |last6=Haro |first6=P. Arrabal |last7=Castellano |first7=M. |last8=Dickinson |first8=M. |last9=Ocvirk |first9=P. |last10=Lewis |first10=J. S. W. |last11=Amorín |first11=R. |last12=Bagley |first12=M. |last13=Cleri |first13=R. N. J. |last14=Costantin |first14=L. |last15=Dekel |first15=A. |date=2024 |title=New insight on the nature of cosmic reionizers from the CEERS survey |journal=Astronomy and Astrophysics |volume=685 |pages=A3 |doi=10.1051/0004-6361/202347884 |arxiv=2309.02219|bibcode=2024A&A...685A...3M }}</ref>

===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>

===Population III stars===
[[Image:NASA-WMAP-first-stars.jpg|thumb|upright=1.4|Simulated image of the first stars, 400 million years after the Big Bang.]] [[Stellar population#Population III stars|Population III stars]] were the earliest stars, which had no elements more massive than hydrogen or [[helium]]. During [[Nucleosynthesis#Big Bang nucleosynthesis|Big Bang nucleosynthesis]], the only elements that formed aside from hydrogen and helium were trace amounts of [[lithium]]. Yet quasar spectra have revealed the presence of heavy elements in the [[intergalactic medium]] at an early era. [[Supernova]] explosions produce such heavy elements, so hot, large, Population III stars which will form supernovae are a possible mechanism for reionization. While they have not been directly observed, they are consistent according to models using numerical simulation<ref name="popIII_sim">{{cite journal |author=Gnedin |first1=Nickolay |last2=Ostriker |first2=Jeremiah |name-list-style=amp |date=1997 |title=Reionization of the Universe and the Early Production of Metals |journal=Astrophysical Journal |volume=486 |issue=2 |pages=581–598 |arxiv=astro-ph/9612127 |bibcode=1997ApJ...486..581G |doi=10.1086/304548 |s2cid=5758398}}</ref> and current observations.<ref name="qso_z">{{cite arXiv | author=Limin 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> A [[Gravitational lens|gravitationally lensed]] galaxy also provides indirect evidence of Population III stars.<ref>{{cite journal |author=Fosbury |first=R. A. E. |display-authors=etal |year=2003 |title=Massive Star Formation in a Gravitationally Lensed H II Galaxy at z = 3.357 |journal=Astrophysical Journal |volume=596 |issue=1 |pages=797–809 |arxiv=astro-ph/0307162 |bibcode=2003ApJ...596..797F |doi=10.1086/378228 |s2cid=17808828}}</ref> Even without direct observations of Population III stars, they are a compelling source. They are more efficient and effective ionizers than Population II stars, as they emit more ionizing photons,<ref name="popII_vs_popIII">{{cite journal |author=Tumlinson |first=Jason |display-authors=etal |date=2002 |title=Cosmological Reionization by the First Stars: Evolving Spectra of Population III |journal=ASP Conference Proceedings |volume=267 |pages=433–434 |bibcode=2002ASPC..267..433T}}</ref> and are capable of reionizing hydrogen on their own in some reionization models with reasonable [[initial mass function]]s.<ref>{{cite journal |author=Venkatesan |first=Apama |display-authors=etal |date=2003 |title=Evolving Spectra of Population III Stars: Consequences for Cosmological Reionization |journal=Astrophysical Journal |volume=584 |issue=2 |pages=621–632 |arxiv=astro-ph/0206390 |bibcode=2003ApJ...584..621V |doi=10.1086/345738 |s2cid=17737785}}</ref> As a consequence, Population III stars are currently considered the most likely energy source to initiate the reionization of the universe,<ref name="popIII_HII">{{cite journal |author=Alvarez |first=Marcelo |display-authors=etal |year=2006 |title=The H II Region of the First Star |journal=Astrophysical Journal |volume=639 |issue=2 |pages=621–632 |arxiv=astro-ph/0507684 |bibcode=2006ApJ...639..621A |doi=10.1086/499578 |s2cid=12753436}}</ref> though other sources are likely to have taken over and driven reionization to completion.

In June 2015, astronomers reported evidence for [[Stellar population#Population III stars|Population III stars]] in the [[Cosmos Redshift 7]] [[galaxy]] at {{math|''z'' {{=}} 6.60}}. Such stars are likely to have existed in the very early universe (i.e., at high redshift), and may have started the production of [[chemical element]]s heavier than [[hydrogen]] that are needed for the later formation of [[planet]]s and [[life]] as we know it.<ref name="AJ-20150604">{{cite journal |last1=Sobral |first1=David |last2=Matthee |first2=Jorryt |last3=Darvish |first3=Behnam |last4=Schaerer |first4=Daniel |last5=Mobasher |first5=Bahram |last6=Röttgering |first6=Huub J. A. |last7=Santos |first7=Sérgio |last8=Hemmati |first8=Shoubaneh |title=Evidence For POPIII-Like Stellar Populations In The Most Luminous LYMAN-α Emitters At The Epoch Of Re-Ionisation: Spectroscopic Confirmation |date=4 June 2015 |journal=[[The Astrophysical Journal]] |doi=10.1088/0004-637x/808/2/139 |bibcode=2015ApJ...808..139S |volume=808 |issue=2 |pages=139|arxiv = 1504.01734 |s2cid=18471887 }}</ref><ref name="NYT-20150617">{{cite news |last=Overbye |first=Dennis |author-link=Dennis Overbye |title=Astronomers Report Finding Earliest Stars That Enriched Cosmos |url=https://www.nytimes.com/2015/06/18/science/space/astronomers-report-finding-earliest-stars-that-enriched-cosmos.html |date=17 June 2015 |work=[[The New York Times]] |access-date=17 June 2015 }}</ref>