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Detection methods

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Looking back so far in the history of the universe presents some observational challenges. There are, however, a few observational methods for studying reionization.

Quasars and the Gunn-Peterson trough

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One means of studying reionization uses the spectra of distant quasars. Quasars release an extraordinary amount of energy, being among the brightest objects in the universe. As a result, some quasars are detectable from as long ago as the epoch of reionization. Quasars also happen to have relatively uniform spectral features, regardless of their position in the sky or distance from the Earth. Thus it can be inferred that any major differences between quasar spectra will be caused by the interaction of their emission with atoms along the line of sight. For wavelengths of light at the energies of one of the Lyman transitions of hydrogen, the scattering cross-section is large, meaning that even for low levels of neutral hydrogen in the intergalactic medium (IGM), absorption at those wavelengths is highly likely.

For nearby objects in the universe, spectral absorption lines are very sharp, as only photons with energies just sufficient to cause an atomic transition can cause that transition. However, the distances between quasars and the telescopes which detect them are large, which means that the expansion of the universe causes light to undergo noticeable redshifting. This means that as light from the quasar travels through the IGM and is redshifted, wavelengths which had been below the Lyman Alpha limit are stretched, and will in effect begin to fill in the Lyman absorption band. This means that instead of showing sharp spectral absorption lines, a quasar's light which has traveled through a large, spread out region of neutral hydrogen will show a Gunn-Peterson trough.[1]

The redshifting for a particular quasar provides temporal information about reionization. Since an object's redshift corresponds to the time at which it emitted the light, it is possible to determine when reionization ended. Quasars below a certain redshift (closer in space and time) do not show the Gunn-Peterson trough (though they may show the Lyman-alpha forest), while quasars emitting light prior to reionization will feature a Gunn-Peterson trough. In 2001, four quasars were detected by the Sloan Digital Sky Survey with redshifts ranging from z = 5.82 to z = 6.28. While the quasars above z = 6 showed a Gunn-Peterson trough, indicating that the IGM was still at least partly neutral, the ones below did not, meaning the hydrogen was ionized. As reionization is expected to occur over relatively short timescales, the results suggest that the universe was approaching the end of reionization at z = 6.[2] This, in turn, suggests that the universe must still have been almost entirely neutral at z > 10. On the other hand, long absorption troughs persisting down to z < 5.5 in the Lyman-alpha and Lyman-beta forests suggest that reionization potentially extends later than z = 6.[3][4]

CMB anisotropy and polarization

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The anisotropy of the cosmic microwave background on different angular scales can also be used to study reionization. Photons undergo scattering when there are free electrons present, in a process known as Thomson scattering. However, as the universe expands, the density of free electrons will decrease, and scattering will occur less frequently. In the period during and after reionization, but before significant expansion had occurred to sufficiently lower the electron density, the light that composes the CMB will experience observable Thomson scattering. This scattering will leave its mark on the CMB anisotropy map, introducing secondary anisotropies (anisotropies introduced after recombination).[5] The overall effect is to erase anisotropies that occur on smaller scales. While anisotropies on small scales are erased, polarization anisotropies are actually introduced because of reionization.[6] By looking at the CMB anisotropies observed, and comparing with what they would look like had reionization not taken place, the electron column density at the time of reionization can be determined. With this, the age of the universe when reionization occurred can then be calculated.

The Wilkinson Microwave Anisotropy Probe allowed that comparison to be made. The initial observations, released in 2003, suggested that reionization took place from 30 > z > 11.[7] This redshift range was in clear disagreement with the results from studying quasar spectra. However, the three year WMAP data returned a different result, with reionization beginning at z = 11 and the universe ionized by z = 7.[8] This is in much better agreement with the quasar data.

Results in 2018 from Planck mission, yield an instantaneous reionization redshift of z = 7.68 ± 0.79.[9]

The parameter usually quoted here is τ, the "optical depth to reionization," or alternatively, zre, the redshift of reionization, assuming it was an instantaneous event. While this is unlikely to be physical, since reionization was very likely not instantaneous, zre provides an estimate of the mean redshift of reionization.

Lyman alpha emission

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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.[10] 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.[11] Thus, experiments to find galaxies by their Lyman alpha light can indicate the ionization state of the surrounding gas.  If Lyman alpha galaxies are found in normal numbers, the surrounding gas must be ionized; while an absence of detectable Lyman alpha emission 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 searches).[12][13][14]

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[15] 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[16][17][18], but still indicate that a majority of intergalactic gas was ionized prior to z=7[19].

Lyman alpha emission can be used in other ways to further probe reionization. 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.[20] Moreover, specific ionized regions can be pinpointed by identifying groups of Lyman alpha emitters.[21][22]

21-cm line

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Even with the quasar data roughly in agreement with the CMB anisotropy data, there are still a number of questions, especially concerning the energy sources of reionization and the effects on, and role of, structure formation during reionization. The 21-cm line in hydrogen is potentially a means of studying this period, as well as the "dark ages" that preceded reionization. The 21-cm line occurs in neutral hydrogen, due to differences in energy between the spin triplet and spin singlet states of the electron and proton. This transition is forbidden, meaning it occurs extremely rarely. The transition is also highly temperature dependent, meaning that as objects form in the "dark ages" and emit Lyman-alpha photons that are absorbed and re-emitted by surrounding neutral hydrogen, it will produce a 21-cm line signal in that hydrogen through Wouthuysen-Field coupling.[23][24] By studying 21-cm line emission, it will be possible to learn more about the early structures that formed. Observations from the Experiment to Detect the Global Epoch of Reionization Signature (EDGES) points to a signal from this era, although follow-up observations will be needed to confirm it.[25] Several other projects hope to make headway in this area in the near future, such as the Precision Array for Probing the Epoch of Reionization (PAPER), Low Frequency Array (LOFAR), Murchison Widefield Array (MWA), Giant Metrewave Radio Telescope (GMRT), Mapper of the IGM Spin Temperature (MIST), the Dark Ages Radio Explorer (DARE) mission, and the Large-Aperture Experiment to Detect the Dark Ages (LEDA).

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  2. ^ Becker, R. H.; et al. (2001). "Evidence For Reionization at z ~ 6: Detection of a Gunn-Peterson Trough In A z=6.28 Quasar". Astronomical Journal. 122 (6): 2850–2857. arXiv:astro-ph/0108097. Bibcode:2001AJ....122.2850B. doi:10.1086/324231. S2CID 14117521.
  3. ^ Becker, George D.; Bolton, James S.; Madau, Piero; Pettini, Max; Ryan-Weber, Emma V.; Venemans, Bram P. (2015-03-11). "Evidence of patchy hydrogen reionization from an extreme Lyα trough below redshift six". Monthly Notices of the Royal Astronomical Society. 447 (4): 3402–3419. arXiv:1407.4850. doi:10.1093/mnras/stu2646. ISSN 1365-2966.
  4. ^ Zhu, Yongda; Becker, George D.; Bosman, Sarah E. I.; Keating, Laura C.; D’Odorico, Valentina; Davies, Rebecca L.; Christenson, Holly M.; Bañados, Eduardo; Bian, Fuyan; Bischetti, Manuela; Chen, Huanqing; Davies, Frederick B.; Eilers, Anna-Christina; Fan, Xiaohui; Gaikwad, Prakash (2022-06-01). "Long Dark Gaps in the Lyβ Forest at z < 6: Evidence of Ultra-late Reionization from XQR-30 Spectra". The Astrophysical Journal. 932 (2): 76. arXiv:2205.04569. Bibcode:2022ApJ...932...76Z. doi:10.3847/1538-4357/ac6e60. ISSN 0004-637X.
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  6. ^ Dore, O.; et al. (2007). "Signature of patchy reionization in the polarization anisotropy of the CMB". Physical Review D. 76 (4): 043002. arXiv:astro-ph/0701784. Bibcode:2007PhRvD..76d3002D. doi:10.1103/PhysRevD.76.043002. S2CID 119360903.
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  11. ^ Miralda‐Escude, Jordi; Rees, Martin J. (1998-04-10). "Searching for the Earliest Galaxies Using the Gunn‐Peterson Trough and the Lyα Emission Line". The Astrophysical Journal. 497 (1): 21–27. doi:10.1086/305458. ISSN 0004-637X.
  12. ^ Stark, Daniel P.; Ellis, Richard S.; Chiu, Kuenley; Ouchi, Masami; Bunker, Andrew (2010-11-01). "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". Monthly Notices of the Royal Astronomical Society. 408 (3): 1628–1648. doi:10.1111/j.1365-2966.2010.17227.x.
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  15. ^ Malhotra, Sangeeta; Rhoads, James E. (2004-12-10). "Luminosity Functions of Lyα Emitters at Redshifts z = 6.5 and z = 5.7: Evidence against Reionization at z ≤ 6.5". The Astrophysical Journal. 617 (1): L5 – L8. doi:10.1086/427182. ISSN 0004-637X.
  16. ^ Hu, E. M.; Cowie, L. L.; Barger, A. J.; Capak, P.; Kakazu, Y.; Trouille, L. (2010-12-10). "AN ATLAS OF z = 5.7 AND z = 6.5 Lyα EMITTERS,". The Astrophysical Journal. 725 (1): 394–423. doi:10.1088/0004-637X/725/1/394. ISSN 0004-637X.
  17. ^ Kashikawa, Nobunari; Shimasaku, Kazuhiro; Matsuda, Yuichi; Egami, Eiichi; Jiang, Linhua; Nagao, Tohru; Ouchi, Masami; Malkan, Matthew A.; Hattori, Takashi; Ota, Kazuaki; Taniguchi, Yoshiaki; Okamura, Sadanori; Ly, Chun; Iye, Masanori; Furusawa, Hisanori (2011-06-20). "COMPLETING THE CENSUS OF Lyα EMITTERS AT THE REIONIZATION EPOCH $^,$". The Astrophysical Journal. 734 (2): 119. doi:10.1088/0004-637X/734/2/119. ISSN 0004-637X.
  18. ^ Ouchi, Masami; Shimasaku, Kazuhiro; Furusawa, Hisanori; Saito, Tomoki; Yoshida, Makiko; Akiyama, Masayuki; Ono, Yoshiaki; Yamada, Toru; Ota, Kazuaki; Kashikawa, Nobunari; Iye, Masanori; Kodama, Tadayuki; Okamura, Sadanori; Simpson, Chris; Yoshida, Michitoshi (2010-11-01). "STATISTICS OF 207 Lyα EMITTERS AT A REDSHIFT NEAR 7: CONSTRAINTS ON REIONIZATION AND GALAXY FORMATION MODELS". The Astrophysical Journal. 723 (1): 869–894. doi:10.1088/0004-637X/723/1/869. ISSN 0004-637X.
  19. ^ Wold, Isak G. B.; Malhotra, Sangeeta; Rhoads, James; Wang, Junxian; Hu, Weida; Perez, Lucia A.; Zheng, Zhen-Ya; Khostovan, Ali Ahmad; Walker, Alistair R.; Barrientos, L. Felipe; González-López, Jorge; Harish, Santosh; Infante, Leopoldo; Jiang, Chunyan; Pharo, John (2022-03-01). "LAGER Lyα Luminosity Function at z ∼ 7: Implications for Reionization". The Astrophysical Journal. 927 (1): 36. doi:10.3847/1538-4357/ac4997. ISSN 0004-637X.{{cite journal}}: CS1 maint: unflagged free DOI (link)
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  21. ^ Tilvi, V.; Malhotra, S.; Rhoads, J. E.; Coughlin, A.; Zheng, Z.; Finkelstein, S. L.; Veilleux, S.; Mobasher, B.; Wang, J.; Probst, R.; Swaters, R.; Hibon, P.; Joshi, B.; Zabl, J.; Jiang, T. (2020-03-01). "Onset of Cosmic Reionization: Evidence of an Ionized Bubble Merely 680 Myr after the Big Bang". The Astrophysical Journal Letters. 891 (1): L10. doi:10.3847/2041-8213/ab75ec. ISSN 2041-8205.{{cite journal}}: CS1 maint: unflagged free DOI (link)
  22. ^ Hu, Weida; Wang, Junxian; Infante, Leopoldo; Rhoads, James E.; Zheng, Zhen-Ya; Yang, Huan; Malhotra, Sangeeta; Barrientos, L. Felipe; Jiang, Chunyan; González-López, Jorge; Prieto, Gonzalo; Perez, Lucia A.; Hibon, Pascale; Galaz, Gaspar; Coughlin, Alicia (2021-01-25). "A Lyman-α protocluster at redshift 6.9". Nature Astronomy. 5 (5): 485–490. doi:10.1038/s41550-020-01291-y. ISSN 2397-3366.
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