The Evolution of Optical Depth in the Lyα Forest: Evidence Against Reionization at z~6

By George D. Becker, M. Rauch, W. Sargent. 2006

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We examine the evolution of the IGM Lyα optical depth distribution using the transmitted flux probability distribution function (PDF) in a high-resolution sample of 55 QSOs spanning absorption redshifts 1.7 < z < 5.8. The data are compared to two theoretical τ distributions: a model distribution based on the density distribution of Miralda-Escudé et al. (2000; MHR00) and a lognormal distribution. Assuming a spatially uniform UV background and an isothermal IGM, as was done in previous works where transmitted flux statistics have been used to infer an end to cosmic reionization at z ~ 6, the MHR00 model fails to reproduce the observed flux PDFs at redshifts where the optical depth distribution is well sampled unless large continuum corrections are applied. A lognormal τ distribution, in contrast, fits the data well at all redshifts with only minor continuum adjustments. Extrapolating the evolution of the lognormal distribution at z < 5.4 predicts the observed upturn in the Lyα and Lyβ effective optical depths at z > 5.7, while simultaneously reproducing the mean transmitted flux down to z = 1.6. In this empirical sense, the evolution of the Lyα forest at z ~ 6 is consistent with observed trends at lower redshift. If the evolution of the forest at z ≲ 5 reflects a slowly evolving density field, temperature, and UV background, then no sudden change in the IGM, such as one due to late reionization, appears necessary to explain the disappearance of transmitted flux at z ~ 6. If the MHR00 density distribution is correct, then a nonuniform UV background and/or IGM temperature may be required to produce the correct distribution of optical depths. We find that an inverse temperature-density relation for the MHR00 model significantly improves the PDF fits, but with a large scatter in the equation-of-state index.

Subfields: Astronomy and Astrophysics, Global and Planetary Change, Spectroscopy

View Paper: https://www.semanticscholar.org/paper/531404f6822e768f458326bc77afa1f16b0b1cb5

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Citations in the corpus

Science with the Murchison Widefield Array (2012, background): “In addition, several studies have interpreted the large saturated regions at z > 6 in these absorption spectra as evidence for the end of reionisation (e.g., Fan et al. 2006), but this conclusion is disputed (Becker et al. 2007; Mesinger 2010).”
Concordance models of reionization: implications for faint galaxies and escape fraction evolution (2012, background): “…galaxies can actually reionize the Universe by z ∼ 6 (as required by the transmission of the Lyα forest at lower redshifts; Fan et al. 2002; Becker et al. 2007, although see McGreer et al. 2011) and account for the Thomson scattering optical depth to the microwave background implied by…”
THE EVOLUTION OF LYMAN LIMIT ABSORPTION SYSTEMS TO REDSHIFT SIX (2010, unknown): “This procedure is very uncertain at the high redshifts where the transmission is dominated by the extreme underdense tail of the distribution function, which is poorly described by the log normal function (Becker et al. 2007).”
Four Quasars above Redshift 6 Discovered by the Canada-France High-z Quasar Survey (2007, background): “Becker et al. (2007) have cast doubt on this form for the expected evolution in transmission.”
STATISTICS OF 207 Lyα EMITTERS AT A REDSHIFT NEAR 7: CONSTRAINTS ON REIONIZATION AND GALAXY FORMATION MODELS (2010, background): “Although the increase of Gunn-Peterson (GP) optical depths may be contiguous from low z to z > 6 (Becker et al. 2007), the evolution of GP optical depths clearly show the steep rise at z ∼ 6 towards high-z (Fan et al. 2006).”
He ii REIONIZATION AND ITS EFFECT ON THE INTERGALACTIC MEDIUM (2008, methodology): “Bolton et al. (2005), Becker et al. (2007), and especially Faucher-Giguère et al. (2008b) measured the hydrogen photoionization rate (ΓHI) to be flat as a function of redshift using the flux decrement method on 2 < z < 4 HI Lyα forest data and to within a factor of ∼ 2 given by ΓHI(z) = 10 −12 s−1.”
LARGE AREA SURVEY FOR z = 7 GALAXIES IN SDF AND GOODS-N: IMPLICATIONS FOR GALAXY FORMATION AND COSMIC REIONIZATION (2009, background): “Although this increase can be explained by the extrapolation from low z to z > 6 via adoption of log-normal distribution for optical depths (Becker et al. 2007), it is also true that GP optical depths appear to increase significantly at around z = 6.”
The Giant Gemini GMOS survey of zem > 4.4 quasars – I. Measuring the mean free path across cosmic time (2014, background): “Studies of the IGM across cosmic time have revealed redshift evolution in many properties of the Lyα forest: temperature (e.g. Lidz et al. 2010; Becker et al. 2011), line density (e.g. Kim et al. 2013), and the flux PDF (e.g. Becker et al. 2007; Kim et al. 2007).”
ON LYMAN-LIMIT SYSTEMS AND THE EVOLUTION OF THE INTERGALACTIC IONIZING BACKGROUND (2011, background): “14 Alternative (lognormal) models for P (∆) have also been invoked to explain this evolution (Becker et al. 2007).”
A refined measurement of the mean transmitted flux in the Lyα forest over 2 < z < 5 using composite quasar spectra (2012, methodology): “At the highest redshifts (z > 5), F (z) measurements have generally relied on extrapolating a power-law continuum from redward of the Lyα emission line (Songaila 2004; Fan et al. 2006; Becker et al. 2007).”
The observed ionization rate of the intergalactic medium and the ionizing emissivity at z≥ 5: evidence for a photon-starved and extended epoch of reionization (2007, background): “Approaching z = 6 there is some debate whether an abrupt transition (Fan et al. 2002, 2006) or smooth progression (Songaila 2004; Becker et al. 2007) in the redshift evolution of τ eff is a better fit to the observational data.”
Comparison of reionization models: radiative transfer simulations and approximate, seminumeric models (2010, background): “This makes the high-redshift Lyα forest spectra difficult to interpret (Lidz et al. 2006; Becker et al. 2007), and the data may even be consistent with reionization completing at redshifts z < 6 (Lidz et al. 2007; Mesinger 2009).”
Detection of extended He II reionization in the temperature evolution of the intergalactic medium (2010, background): “While tentative evidence evidence for an inverted temperature-density relation has been found in the flux probability distribution function (Becker et al. 2007; Bolton et al. 2008), it remains a theoretical challenge to simultaneously achieve this over the entire IGM, even during He ii reionization…”
COSMIC MICROWAVE BACKGROUND CONSTRAINTS ON THE DURATION AND TIMING OF REIONIZATION FROM THE SOUTH POLE TELESCOPE (2011, methodology): “In the lower panel, we plot constraints on the residual neutral fraction after reionization from the Ly-α forest (Fan et al. 2006b; Becker et al. 2007).”
A DEEP HUBBLE SPACE TELESCOPE SEARCH FOR ESCAPING LYMAN CONTINUUM FLUX AT z ∼ 1.3: EVIDENCE FOR AN EVOLVING IONIZING EMISSIVITY (2010, background): “Recent investigations of the ionizing background at high redshift (Bolton et al. 2005; Becker et al. 2007) note that the Hi–photoionization rate is fairly flat between 2 < z < 4, which implies an increase in the ionizing emissitivity at higher redshifts (Faucher-Giguère et al. 2008).”
How neutral is the intergalactic medium surrounding the redshift z = 7.085 quasar ULAS J1120+0641? (2011, background): “The intergalactic Lyα opacity rapidly rises with increasing redshift, and consequently there has been much debate whether these quasars have been observed before the Universe was fully reionised (e.g. Becker et al. 2007; Lidz et al. 2007; Mesinger 2010).”