(Nearly) Model-independent Constraints on the Neutral Hydrogen Fraction in the Intergalactic Medium at z ∼ 5–7 Using Dark Pixel Fractions in Lyα and Lyβ Forests

Cosmic reionization was the last major phase transition of hydrogen from neutral to highly ionized in the intergalactic medium (IGM). Current observations show that the IGM is significantly neutral at z > 7 and largely ionized by z ∼ 5.5. However, most methods to measure the IGM neutral fraction are highly model dependent and are limited to when the volume-averaged neutral fraction of the IGM is either relatively low ( x¯HI≲10−3 ) or close to unity ( x¯HI∼1 ). In particular, the neutral fraction evolution of the IGM at the critical redshift range of z = 6–7 is poorly constrained. We present new constraints on x¯HI at z ∼ 5.1–6.8 by analyzing deep optical spectra of 53 quasars at 5.73 < z < 7.09. We derive model-independent upper limits on the neutral hydrogen fraction based on the fraction of “dark” pixels identified in the Lyα and Lyβ forests, without any assumptions on the IGM model or the intrinsic shape of the quasar continuum. They are the first model-independent constraints on the IGM neutral hydrogen fraction at z ∼ 6.2–6.8 using quasar absorption measurements. Our results give upper limits of x¯HI(z=6.3)<0.79±0.04 (1σ), x¯HI(z=6.5)<0.87±0.03 (1σ), and x¯HI(z=6.7)<0.94−0.09+0.06 (1σ). The dark pixel fractions at z > 6.1 are consistent with the redshift evolution of the neutral fraction of the IGM derived from Planck 2018.

[1]  Miguel de Val-Borro,et al.  The Astropy Project: Sustaining and Growing a Community-oriented Open-source Project and the Latest Major Release (v5.0) of the Core Package , 2022, The Astrophysical Journal.

[2]  Xiaohui Fan,et al.  Long Dark Gaps in the Lyβ Forest at z < 6: Evidence of Ultra-late Reionization from XQR-30 Spectra , 2022, The Astrophysical Journal.

[3]  F. Davies,et al.  IGM damping wing constraints on reionisation from covariance reconstruction of two $z\gtrsim7$ QSOs , 2021, 2112.04091.

[4]  Xiaohui Fan,et al.  Chasing the Tail of Cosmic Reionization with Dark Gap Statistics in the Lyα Forest over 5 < z < 6 , 2021, The Astrophysical Journal.

[5]  J. Bolton,et al.  The mean free path of ionizing photons at 5 < z < 6: evidence for rapid evolution near reionization , 2021, Monthly Notices of the Royal Astronomical Society.

[6]  C. Scarlata,et al.  The Evolution of the Lyman-alpha Luminosity Function during Reionization , 2021, The Astrophysical Journal.

[7]  Feige Wang,et al.  Measurements of the z ∼ 6 Intergalactic Medium Optical Depth and Transmission Spikes Using a New z > 6.3 Quasar Sample , 2020, The Astrophysical Journal.

[8]  S. Finkelstein,et al.  Texas Spectroscopic Search for Lyα Emission at the End of Reionization. III. The Lyα Equivalent-width Distribution and Ionized Structures at z > 7 , 2020, The Astrophysical Journal.

[9]  Linhua Jiang,et al.  Pōniuā‘ena: A Luminous z = 7.5 Quasar Hosting a 1.5 Billion Solar Mass Black Hole , 2020, The Astrophysical Journal.

[10]  F. Davies,et al.  A comparison of quasar emission reconstruction techniques for z ≥ 5.0 Lyman α and Lyman β transmission , 2020, 2006.10744.

[11]  Jaime Fern'andez del R'io,et al.  Array programming with NumPy , 2020, Nature.

[12]  Xiaohui Fan,et al.  A Significantly Neutral Intergalactic Medium Around the Luminous z = 7 Quasar J0252–0503 , 2020, The Astrophysical Journal.

[13]  Steward Observatory,et al.  PypeIt: The Python Spectroscopic Data Reduction Pipeline , 2019, J. Open Source Softw..

[14]  Anson D’Aloisio,et al.  Observing the tail of reionization: neutral islands in the z = 5.5 Lyman-α forest , 2019, Monthly Notices of the Royal Astronomical Society.

[15]  R. Naidu,et al.  Rapid Reionization by the Oligarchs: The Case for Massive, UV-bright, Star-forming Galaxies with High Escape Fractions , 2019, The Astrophysical Journal.

[16]  Joel Nothman,et al.  SciPy 1.0-Fundamental Algorithms for Scientific Computing in Python , 2019, ArXiv.

[17]  F. Davies,et al.  Anomaly in the Opacity of the Post-reionization Intergalactic Medium in the Lyα and Lyβ Forest , 2019, The Astrophysical Journal.

[18]  S. Finkelstein,et al.  Conditions for Reionizing the Universe with a Low Galaxy Ionizing Photon Escape Fraction , 2019, The Astrophysical Journal.

[19]  A. Fontana,et al.  Inferences on the timeline of reionization at z ∼ 8 from the KMOS Lens-Amplified Spectroscopic Survey , 2019, Monthly Notices of the Royal Astronomical Society.

[20]  T. Treu,et al.  Constraining the Neutral Fraction of Hydrogen in the IGM at Redshift 7.5 , 2019, The Astrophysical Journal.

[21]  F. Davies,et al.  The Opacity of the Intergalactic Medium Measured along Quasar Sightlines at z ∼ 6 , 2018, The Astrophysical Journal.

[22]  E. Bañados,et al.  Constraints on reionization from the z = 7.5 QSO ULASJ1342+0928 , 2018, Monthly Notices of the Royal Astronomical Society.

[23]  T. Nagao,et al.  CHORUS. II. Subaru/HSC Determination of the Lyα Luminosity Function at z = 7.0: Constraints on Cosmic Reionization Model Parameter , 2018, The Astrophysical Journal.

[24]  M. Millea,et al.  Cosmic microwave background constraints in light of priors over reionization histories , 2018, Astronomy & Astrophysics.

[25]  Adrian M. Price-Whelan,et al.  Binary Companions of Evolved Stars in APOGEE DR14: Search Method and Catalog of ∼5000 Companions , 2018, The Astronomical Journal.

[26]  Xiaohui Fan,et al.  New constraints on lyman-α opacity with a sample of 62 quasars at z > 5.7 , 2018, 1802.08177.

[27]  H. Rix,et al.  Quantitative Constraints on the Reionization History from the IGM Damping Wing Signature in Two Quasars at z > 7 , 2018, The Astrophysical Journal.

[28]  R. B. Barreiro,et al.  Planck 2018 results , 2018, Astronomy & Astrophysics.

[29]  H. Rix,et al.  An 800-million-solar-mass black hole in a significantly neutral Universe at a redshift of 7.5 , 2017, Nature.

[30]  T. Treu,et al.  The Universe Is Reionizing at z ∼ 7: Bayesian Inference of the IGM Neutral Fraction Using Lyα Emission from Galaxies , 2017, 1709.05356.

[31]  T. Nagao,et al.  SILVERRUSH. IV. Lyα luminosity functions at z = 5.7 and 6.6 studied with ∼1300 Lyα emitters on the 14–21 deg2 sky , 2017, 1705.01222.

[32]  T. Nagao,et al.  Systematic Identification of LAEs for Visible Exploration and Reionization Research Using Subaru HSC (SILVERRUSH). I. Program strategy and clustering properties of ∼2000 Lyα emitters at z = 6–7 over the 0.3–0.5 Gpc2 survey area , 2017, 1704.07455.

[33]  J. Prochaska,et al.  Implications of z ∼ 6 Quasar Proximity Zones for the Epoch of Reionization and Quasar Lifetimes , 2017, 1703.02539.

[34]  P. Hewett,et al.  Observations of the Lyman series forest towards the redshift 7.1 quasar ULAS J1120+0641 , 2017, 1702.03687.

[35]  I. McGreer,et al.  Lyα emission-line reconstruction for high-z QSOs , 2016, 1605.09388.

[36]  A. Mesinger,et al.  The clustering of Lyman α emitters at z ≈ 7: implications for reionization and host halo masses , 2015 .

[37]  I. McGreer,et al.  Model-independent evidence in favour of an end to reionization by z ≈ 6 , 2014, 1411.5375.

[38]  P. Madau,et al.  Evidence of patchy hydrogen reionization from an extreme Lyα trough below redshift six , 2014, 1407.4850.

[39]  George D. Becker,et al.  The Giant Gemini GMOS survey of zem > 4.4 quasars – I. Measuring the mean free path across cosmic time , 2014, 1402.4154.

[40]  Prasanth H. Nair,et al.  Astropy: A community Python package for astronomy , 2013, 1307.6212.

[41]  Z. Haiman,et al.  Evidence of Gunn–Peterson damping wings in high-z quasar spectra: strengthening the case for incomplete reionization at z ∼ 6–7 , 2012, 1204.2838.

[42]  Richard G. McMahon,et al.  A luminous quasar at a redshift of z = 7.085 , 2011, Nature.

[43]  Xiaohui Fan,et al.  The first (nearly) model-independent constraint on the neutral hydrogen fraction at z ∼ 5–6 , 2011, 1101.3314.

[44]  Melbourne.,et al.  Measurements of the UV background at 4.6 < z < 6.4 using the quasar proximity effect , 2010, 1011.5850.

[45]  Richard S. Ellis,et al.  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 , 2010, 1003.5244.

[46]  A. Mesinger Was reionization complete by z∼ 5–6? , 2009, 0910.4161.

[47]  Robert H. Becker,et al.  Constraining the Evolution of the Ionizing Background and the Epoch of Reionization with z ∼ 6 Quasars. II. A Sample of 19 Quasars , 2005, astro-ph/0512082.

[48]  Jordi Miralda-Escude,et al.  Reionization of the Intergalactic Medium and the Damping Wing of the Gunn-Peterson Trough , 1997, astro-ph/9708253.

[49]  D. Osterbrock,et al.  NIGHT-SKY HIGH-RESOLUTION SPECTRAL ATLAS OF OH AND O2 EMISSION LINES FOR ECHELLE SPECTROGRAPH WAVELENGTH CALIBRATION , 1996 .