Editing Reionization (section)

=== Lyman alpha emission ===
[[Lyman-alpha|Lyman alpha]] light from galaxies offers a complementary tool set to study reionization. The Lyman alpha line is the n=2 to n=1 transition of neutral hydrogen and can be produced copiously by galaxies with young stars.<ref>{{Cite journal |last1=Partridge |first1=R. B. |last2=Peebles |first2=P. J. E. |date=March 1967 |title=Are Young Galaxies Visible? |url=http://adsabs.harvard.edu/doi/10.1086/149079 |journal=The Astrophysical Journal |language=en |volume=147 |pages=868 |doi=10.1086/149079 |bibcode=1967ApJ...147..868P |issn=0004-637X}}</ref> Moreover, Lyman alpha photons interact strongly with neutral hydrogen in intergalactic gas through resonant scattering, wherein neutral atoms in the ground (n=1) state absorb Lyman alpha photons and almost immediately re-emit them in a random direction.  This obscures Lyman alpha emission from galaxies that are embedded in neutral gas.<ref>{{Cite journal |last1=Miralda-Escude |first1=Jordi |last2=Rees |first2=Martin J. |date=1998-04-10 |title=Searching for the Earliest Galaxies Using the Gunn-Peterson Trough and the Lyα Emission Line |url=https://iopscience.iop.org/article/10.1086/305458 |journal=The Astrophysical Journal |language=en |volume=497 |issue=1 |pages=21–27 |doi=10.1086/305458 |arxiv=astro-ph/9707193 |bibcode=1998ApJ...497...21M |issn=0004-637X}}</ref> Thus, experiments to find galaxies by their Lyman alpha light can indicate the ionization state of the surrounding gas. An average density of galaxies with detectable Lyman alpha emission means the surrounding gas must be ionized, while an absence of detectable Lyman alpha sources may indicate neutral regions. A closely related class of experiments measures the Lyman alpha line strength in samples of galaxies identified by other methods (primarily [[Lyman-break galaxy|Lyman break galaxy]] searches).<ref>{{Cite journal |last1=Stark |first1=Daniel P. |last2=Ellis |first2=Richard S. |last3=Chiu |first3=Kuenley |last4=Ouchi |first4=Masami |last5=Bunker |first5=Andrew |date=2010-11-01 |title=Keck spectroscopy of faint 3 < z < 7 Lyman break galaxies - I. New constraints on cosmic reionization from the luminosity and redshift-dependent fraction of Lyman α emission: The Lyα emitting fraction at high redshift |journal=Monthly Notices of the Royal Astronomical Society |language=en |volume=408 |issue=3 |pages=1628–1648 |doi=10.1111/j.1365-2966.2010.17227.x|doi-access=free |arxiv=1003.5244 }}</ref><ref>{{Cite journal |last1=Pentericci |first1=L. |last2=Fontana |first2=A. |last3=Vanzella |first3=E. |last4=Castellano |first4=M. |last5=Grazian |first5=A. |last6=Dijkstra |first6=M. |last7=Boutsia |first7=K. |last8=Cristiani |first8=S. |last9=Dickinson |first9=M. |last10=Giallongo |first10=E. |last11=Giavalisco |first11=M. |last12=Maiolino |first12=R. |last13=Moorwood |first13=A. |last14=Paris |first14=D. |last15=Santini |first15=P. |date=2011-12-20 |title=Spectroscopic confirmation of z ∼ 7 Lyman break galaxies: Probing the earliest galaxies and the epoch of reionization |url=https://iopscience.iop.org/article/10.1088/0004-637X/743/2/132 |journal=The Astrophysical Journal |volume=743 |issue=2 |pages=132 |doi=10.1088/0004-637X/743/2/132 |arxiv=1107.1376 |bibcode=2011ApJ...743..132P |issn=0004-637X}}</ref><ref>{{Cite journal |last1=Tilvi |first1=V. |last2=Papovich |first2=C. |last3=Finkelstein |first3=S. L. |last4=Long |first4=J. |last5=Song |first5=M. |last6=Dickinson |first6=M. |last7=Ferguson |first7=H. C. |last8=Koekemoer |first8=A. M. |last9=Giavalisco |first9=M. |last10=Mobasher |first10=B. |date=2014-09-17 |title=Rapid decline of Lyα emission toward the reionization era |url=https://iopscience.iop.org/article/10.1088/0004-637X/794/1/5 |journal=The Astrophysical Journal |volume=794 |issue=1 |pages=5 |doi=10.1088/0004-637X/794/1/5 |arxiv=1405.4869 |bibcode=2014ApJ...794....5T |issn=1538-4357}}</ref>

The earliest application of this method was in 2004, when the tension between late neutral gas indicated by quasar spectra and early reionization suggested by CMB results was strong. The detection of Lyman alpha galaxies at redshift z=6.5 demonstrated that the intergalactic gas was already predominantly ionized<ref>{{Cite journal |last1=Malhotra |first1=Sangeeta |last2=Rhoads |first2=James E. |date=2004-12-10 |title=Luminosity Functions of Lyα Emitters at Redshifts z = 6.5 and z = 5.7: Evidence against Reionization at z ≤ 6.5 |url=https://iopscience.iop.org/article/10.1086/427182 |journal=The Astrophysical Journal |language=en |volume=617 |issue=1 |pages=L5–L8 |doi=10.1086/427182 |arxiv=astro-ph/0407408 |bibcode=2004ApJ...617L...5M |issn=0004-637X}}</ref> at an earlier time than the quasar spectra suggested. Subsequent applications of the method suggested some residual neutral gas as recently as z=6.5,<ref>{{Cite journal |last1=Hu |first1=E. M. |last2=Cowie |first2=L. L. |last3=Barger |first3=A. J. |last4=Capak |first4=P. |last5=Kakazu |first5=Y. |last6=Trouille |first6=L. |date=2010-12-10 |title=An atlas of z = 5.7 and z = 6.5 Lyα emitters |url=https://iopscience.iop.org/article/10.1088/0004-637X/725/1/394 |journal=The Astrophysical Journal |volume=725 |issue=1 |pages=394–423 |doi=10.1088/0004-637X/725/1/394 |arxiv=1009.1144 |bibcode=2010ApJ...725..394H |issn=0004-637X}}</ref><ref>{{Cite journal |last1=Kashikawa |first1=Nobunari |last2=Shimasaku |first2=Kazuhiro |last3=Matsuda |first3=Yuichi |last4=Egami |first4=Eiichi |last5=Jiang |first5=Linhua |last6=Nagao |first6=Tohru |last7=Ouchi |first7=Masami |last8=Malkan |first8=Matthew A. |last9=Hattori |first9=Takashi |last10=Ota |first10=Kazuaki |last11=Taniguchi |first11=Yoshiaki |last12=Okamura |first12=Sadanori |last13=Ly |first13=Chun |last14=Iye |first14=Masanori |last15=Furusawa |first15=Hisanori |date=2011-06-20 |title=Completing the census of Lyα emitters at the reionization epoch $^,$ |url=https://iopscience.iop.org/article/10.1088/0004-637X/734/2/119 |journal=The Astrophysical Journal |volume=734 |issue=2 |pages=119 |doi=10.1088/0004-637X/734/2/119 |arxiv=1104.2330 |bibcode=2011ApJ...734..119K |issn=0004-637X}}</ref><ref>{{Cite journal |last1=Ouchi |first1=Masami |last2=Shimasaku |first2=Kazuhiro |last3=Furusawa |first3=Hisanori |last4=Saito |first4=Tomoki |last5=Yoshida |first5=Makiko |last6=Akiyama |first6=Masayuki |last7=Ono |first7=Yoshiaki |last8=Yamada |first8=Toru |last9=Ota |first9=Kazuaki |last10=Kashikawa |first10=Nobunari |last11=Iye |first11=Masanori |last12=Kodama |first12=Tadayuki |last13=Okamura |first13=Sadanori |last14=Simpson |first14=Chris |last15=Yoshida |first15=Michitoshi |date=2010-11-01 |title=Statistics of 207 Lyα emitters at a redshift near 7: constraints on reionization and galaxy formation models |url=https://iopscience.iop.org/article/10.1088/0004-637X/723/1/869 |journal=The Astrophysical Journal |volume=723 |issue=1 |pages=869–894 |doi=10.1088/0004-637X/723/1/869 |arxiv=1007.2961 |bibcode=2010ApJ...723..869O |issn=0004-637X}}</ref> but still indicate that a majority of intergalactic gas was ionized prior to z=7.<ref>{{Cite journal |last1=Wold |first1=Isak G. B. |last2=Malhotra |first2=Sangeeta |last3=Rhoads |first3=James |last4=Wang |first4=Junxian |last5=Hu |first5=Weida |last6=Perez |first6=Lucia A. |last7=Zheng |first7=Zhen-Ya |last8=Khostovan |first8=Ali Ahmad |last9=Walker |first9=Alistair R. |last10=Barrientos |first10=L. Felipe |last11=González-López |first11=Jorge |last12=Harish |first12=Santosh |last13=Infante |first13=Leopoldo |last14=Jiang |first14=Chunyan |last15=Pharo |first15=John |date=2022-03-01 |title=LAGER Lyα Luminosity Function at z ∼ 7: Implications for Reionization |journal=The Astrophysical Journal |volume=927 |issue=1 |pages=36 |doi=10.3847/1538-4357/ac4997 |doi-access=free |arxiv=2105.12191 |bibcode=2022ApJ...927...36W |issn=0004-637X}}</ref>

Lyman alpha emission can be used in other ways to probe reionization further. Theory suggests that reionization was patchy, meaning that the clustering of Lyman alpha selected samples should be strongly enhanced during the middle phases of reionization.<ref>{{Cite journal |last1=McQuinn |first1=Matthew |last2=Hernquist |first2=Lars |last3=Zaldarriaga |first3=Matias |last4=Dutta |first4=Suvendra |date=October 2007 |title=Studying reionization with Lyα emitters |journal=Monthly Notices of the Royal Astronomical Society |language=en |volume=381 |issue=1 |pages=75–96 |doi=10.1111/j.1365-2966.2007.12085.x |doi-access=free |arxiv=0704.2239 |bibcode=2007MNRAS.381...75M |issn=0035-8711}}</ref> Moreover, specific ionized regions can be pinpointed by identifying groups of Lyman alpha emitters.<ref>{{Cite journal |last1=Tilvi |first1=V. |last2=Malhotra |first2=S. |last3=Rhoads |first3=J. E. |last4=Coughlin |first4=A. |last5=Zheng |first5=Z. |last6=Finkelstein |first6=S. L. |last7=Veilleux |first7=S. |last8=Mobasher |first8=B. |last9=Wang |first9=J. |last10=Probst |first10=R. |last11=Swaters |first11=R. |last12=Hibon |first12=P. |last13=Joshi |first13=B. |last14=Zabl |first14=J. |last15=Jiang |first15=T. |date=2020-03-01 |title=Onset of Cosmic Reionization: Evidence of an Ionized Bubble Merely 680 Myr after the Big Bang |journal=The Astrophysical Journal Letters |volume=891 |issue=1 |pages=L10 |doi=10.3847/2041-8213/ab75ec |doi-access=free |arxiv=2001.00873 |bibcode=2020ApJ...891L..10T |issn=2041-8205}}</ref><ref>{{Cite journal |last1=Hu |first1=Weida |last2=Wang |first2=Junxian |last3=Infante |first3=Leopoldo |last4=Rhoads |first4=James E. |last5=Zheng |first5=Zhen-Ya |last6=Yang |first6=Huan |last7=Malhotra |first7=Sangeeta |last8=Barrientos |first8=L. Felipe |last9=Jiang |first9=Chunyan |last10=González-López |first10=Jorge |last11=Prieto |first11=Gonzalo |last12=Perez |first12=Lucia A. |last13=Hibon |first13=Pascale |last14=Galaz |first14=Gaspar |last15=Coughlin |first15=Alicia |date=2021-01-25 |title=A Lyman-α protocluster at redshift 6.9 |url=https://www.nature.com/articles/s41550-020-01291-y |journal=Nature Astronomy |language=en |volume=5 |issue=5 |pages=485–490 |doi=10.1038/s41550-020-01291-y |arxiv=2101.10204 |bibcode=2021NatAs...5..485H |issn=2397-3366}}</ref>