How Neutral is the Intergalactic Medium at z ~ 6?

Abstract

Recent observations of high redshift quasar spectra reveal long gaps with little flux. A small or no detectable flux does not by itself imply the intergalactic medium (IGM) is neutral. Inferring the average neutral fraction from the observed absorption requires assumptions about clustering of the IGM, which the gravitational instability model supplies. Our most stringent constraint on the neutral fraction at z ~ 6 is derived from the mean Lyman-beta transmission measured from the z=6.28 SDSS quasar of Becker et al. -- the neutral hydrogen fraction at mean density has to be larger than 4.7 times 10^{-4}. This is substantially higher than the neutral fraction of ~ 3-5 times 10^{-5} at z = 4.5 - 5.7, suggesting that dramatic changes take place around or just before z ~ 6, even though current constraints are still consistent with a fairly ionized IGM at z ~ 6. An interesting alternative method to constrain the neutral fraction is to consider the probability of having many consecutive pixels with little flux, which is small unless the neutral fraction is high. This constraint is slightly weaker than the one obtained from the mean transmission. We show that while the derived neutral fraction at a given redshift is sensitive to the power spectrum normalization, the size of the jump around z ~ 6 is not. We caution that systematic uncertainties include spatial fluctuations in the ionizing background, and the continuum placement. Tests are proposed. In particular, the sightline to sightline dispersion in mean transmission might provide a useful diagnostic. We express the dispersion in terms of the transmission power spectrum, and develop a method to calculate the dispersion for spectra that are longer than the typical simulation box.

20 pages, 5 figures; ApJ accepted version; constraints revised due to a revised power spectrum normalization in fiducial model

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• The Universe Was Reionized Twice
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• The Reionization History at High Redshifts I: Physical Models and New Constraints from CMB Polarization
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• Evidence for a Cosmological Stromgren Surface and for Significant Neutral Hydrogen Surrounding the Quasar SDSS J1030+0524
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• The Effects of Photoionization on Galaxy Formation -- III: Environmental Dependence in the Luminosity Function
• Constraining Reionization with the Evolution of the Luminosity Function of Lyman Alpha mitting Galaxies
• How Universal is the Gunn-Peterson Trough at z~6?: A Closer Look at the Quasar SDSS J1148+5251
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• A Snapshot Survey for Gravitational Lenses Among z>=4.0 Quasars: I. The z>5.7 Sample
• The Topology of Cosmological Reionization
• Constraining the second half of reionization with the Lyman-$\beta$ forest
• Probing the Reionization History Using the Spectra of High-Redshift Sources
• Hubble Advanced Camera for Surveys Observations of the z=6.42 Quasar SDSS 1148+5251: A Leak in the Gunn-Peterson Trough
• Anomaly in the Opacity of the Post-Reionization Intergalactic Medium in the Ly$\alpha$ and Ly$\beta$ Forest
• Beyond Lyman-alpha: Constraints and Consistency Tests from the Lyman-beta Forest
• The Formation and Evolution of the First Massive Black Holes
• Galaxies Inside Stromgren Spheres of Luminous Quasars at z>6: Detection of The First Galaxies
• Constraining the reionization history with CMB and spectroscopic observations