揭露宇宙再電離史（cosmic reionisation history）是近代宇宙學研究的一大目標。紅移（z）大於5的類星體（quasi-stellar object，QSO，或稱 Quasar）自其初被發現以降，被用作量測其前景星系間介質（intergalactic medium，IGM）於再電離時期的電離化程度演化史之背景光源。然而，受限於此種高紅移且高光度星體的稀少性（至今已知約130個），科學家未能用其精確地限縮宇宙再電離史。本研究利用近期由Subaru高紅移低光度類星體探測計畫（ Subaru High-z Exploration of Low-Luminosity Quasars project，SHELLQs）新發現的93個紅移介於5.5到7.1之間的類星體限縮宇宙再電離史。此樣本數（93個類星體）為至今所有利用類星體限縮再電離史的研究中最大的樣本數。此外，這項研究所使用的類星體紅移分佈也最廣
（5.5<z<7.1），這有助於描繪一個更完整的再電離史。\\
此研究從SHELLQs類星體的紫外光譜量測了宇宙再電離中後期的星系間介質之Ly$\alpha$光子平均通透率、類星體的電離氫區（HII region，或稱near zone、proximity zone）大小、及紫外背景輻射對中性氫的光致游離速率（HI background photoionisation rate ，$\Gamma_{\rm HI}$）。我們發現星系間介質之Ly$\alpha$光子平均通透率在紅移小於6時升至0以上，這意味著宇宙再電離的結束；SHELLQs類星體的電離氫區大小與其他研究模擬中，年齡介於一百萬到一千萬年間的類星體之電離氫區大小相符；而受限於現有紫外光譜的低信噪比及以類星體Ly$\alpha$發射線判定的紅移之不確定性對測得的電離氫區大小造成的誤差，我們無法判定其隨紅移的大小演化是否與宇宙再電離相關；紫外背景輻射對中性氫的光致游離速率隨紅移的演化則與由低光度之恆星形成星系（star-forming galaxy）造成的宇宙再電離模型相符。

Revealing the cosmic hydrogen reionisation history is one of the main goals in modern cosmology. z$>$5 quasars (QSOs) have been used as back-lights to study the evolution of the intervening intergalactic medium (IGM) during the cosmic reionisation since their first discovery. However, due to the small population of luminous QSOs ($\sim$130 QSOs known to date), a tight constraint on the reionisation history has not yet been placed. We aim to tighten the constraint using the 93 QSOs recently discovered in the Subaru High-z Exploration of Low-Luminosity Quasars (SHELLQS) project. This is the largest QSO sample used on constraining the EoR. Besides, it covers the widest redshift range (5.5$<$z$<$7.1). We measure the mean IGM Ly$\alpha$ transmission, the QSO near zone size (or observed HII bubble size), and the HI background photoionisation rate ($\Gamma_{\rm HI}$) using the UV spectra of these QSOs. The mean IGM Ly$\alpha$ transmission rises above zero at z$\lesssim$6, indicating the end of the reionisation. The near zone sizes of the SHELLQs QSOs are consistent with sizes spanned by QSOs of lifetime t$_{\rm q}\sim$1-100 Myr in simulations. Due to the scatter created by the low signal-to-noise spectra and large Ly$\alpha$ redshift uncertainty, we cannot conclude whether the redshift evolution of the near zone size is affected by the reionisation effect. The redshift evolution of $\Gamma_{\rm HI}$ at 6$\lesssim$z$\lesssim$7 is consistent with models which assume a reionisation driven by faint star-forming galaxies.

1 Introduction 1
1.1 Cosmic reionization . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.1.1 Source term . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.1.2 IGM term . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.1.3 Aims of this study . . . . . . . . . . . . . . . . . . . . . . . . 4
2 Data 6
2.1 UV spectra . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.2 MgII redshift . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.3 [CII] redshift . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.4 Spectral quality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
3 Analysis 8
3.1 Continuum fitting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
3.1.1 Power-law plus Ly훼 emission as the continuum . . . . . . . . . 11
3.2 The IGM opacity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
3.2.1 Lya transmission and optical depth . . . . . . . . . . . . . . . 11
3.2.2 Spatial fluctuations of tau_eff . . . . . . . . . . . . . . . . . . . . 12
3.3 HII regions of QSOs . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
3.3.1 Near zone size RNZ . . . . . . . . . . . . . . . . . . . . . . . . 13
3.3.2 Proximity zone Req and Ionising Background . . . . . . . . . . 14
4 Result and discussion 16
4.1 The evolution of the mean IGM opacity at 5.54.2 The QSO ionised bubbles and the inferred QSO lifetime or UVB . . . . 18
4.2.1 RNZ, corrected and the inferred QSO lifetime . . . . . . . . . . . . 22
4.2.2 Req and the inferred UVB . . . . . . . . . . . . . . . . . . . . 26
5 Conclusion 32
A Information of the QSOs in this work 34
B Continuum prediction using Principle component analysis 36
C The relation between QSO Lya equivalent width and their near zone sizes 37
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