[Paper Review] Cosmic Reionization after Planck: The Derived Growth of the Ionizing Background now matches the Growth of the Galaxy UV Luminosity Density

R. J. Bouwens, G. D. Illingworth+5 more|arXiv (Cornell University)|Mar 27, 2015

Galaxies: Formation, Evolution, Phenomena14 references21 citations

TL;DR

This paper uses Planck's optical depth measurements and observational constraints from quasars and high-redshift galaxies to empirically reconstruct the evolution of the cosmic ionizing background above z ≈ 6. It finds that the redshift evolution of the ionizing background closely matches that of the galaxy UV luminosity density, strongly supporting galaxies as the primary drivers of cosmic reionization, while quasars/AGN are ruled out as dominant sources.

ABSTRACT

Thomson optical depth measurements from Planck provide new insights into the reionization of the universe. To obtain new model-independent constraints on the properties of the ionizing sources, we determine the empirical evolution of the ionising background. We use a simple two-parameter model to map out the evolution in this background at z>~6 from the new Planck optical depth tau measurements and from the constraints provided by quasar absorption spectra and the prevalence of Ly-alpha emission in z~7-8 galaxies. We find the redshift evolution in the ionising background N_{ion} required by the observations to be dlog_{10} N_{ion}/dz(z=8)=-0.19_{-0.11}^{+0.09}, largely independent of the assumed clumping factor C_{HII} and entirely independent of the identity of the ionizing sources. The trend in N_{ion} is well-matched by the evolution of the galaxy UV-luminosity density (dlog_{10} rho_{UV}/dz=-0.11+/-0.04) to a magnitude limit >~-13 mag, suggesting that galaxies are the sources that drive the reionization of the universe. The role of galaxies is further strengthened by the conversion from the UV luminosity density to N_{ion}(z) being possible for physically plausible values of the escape fraction f_{esc}, the Lyman-continuum photon production efficiency xi_{ion}, and faint-end cut-off M_{lim} to the LF. Lastly, we use the inferred evolution in the ionizing background to estimate the z~10 UV luminosity density, finding this luminosity density to be 12_{-7}^{+21}x lower than at z~6, consistent with current measurements at z~10. Quasars/AGN appear to match neither the redshift evolution nor normalization of the ionizing background. This new approach of contrasting the inferred evolution of the ionising background with that of the galaxy UV luminosity density adds to the growing observational evidence that galaxies are the sources that drive the reionization of the universe.

Motivation & Objective

• To derive model-independent constraints on the evolution of the cosmic ionizing background at z > 6 using Planck's optical depth measurements.
• To test whether the observed evolution of the ionizing background matches the evolution of the galaxy UV luminosity density.
• To assess the viability of galaxies versus quasars/AGN as the primary sources of reionization by comparing their luminosity and ionizing output evolution.
• To estimate the UV luminosity density at z ≈ 10 based on the inferred ionizing background evolution.
• To evaluate the consistency of the inferred ionizing background with physical parameters such as escape fraction, ionizing efficiency, and faint-end magnitude cutoff in the luminosity function.

Proposed method

• Using Planck's measured Thomson optical depth (τ) to constrain the total ionization optical depth and infer the average ionizing background evolution at high redshift.
• Applying observational constraints from quasar absorption spectra and the prevalence of Lyα emission in z ≈ 7–8 galaxies to anchor the ionizing background evolution at high redshift.
• Employing a two-parameter model to map the redshift evolution of the ionizing background N_ion(z) for z > 6, independent of source identity or clumping factor.
• Comparing the inferred evolution of N_ion(z) with the observed evolution of the galaxy UV luminosity density ρ_UV(z) to test consistency with galaxies as the dominant ionizing sources.
• Converting UV luminosity density to ionizing background using physically plausible values of the escape fraction f_esc, ionizing photon production efficiency ξ_ion, and magnitude limit M_lim of the luminosity function.
• Projecting the UV luminosity density at z ≈ 10 using the inferred ionizing background evolution and comparing it with current observational constraints.

Experimental results

Research questions

• RQ1 Does the observed redshift evolution of the ionizing background at z > 6 match the evolution of the galaxy UV luminosity density?
• RQ2 Can the inferred evolution of the ionizing background be consistently explained by galaxies as the dominant ionizing sources, given plausible values of f_esc, ξ_ion, and M_lim?
• RQ3 Do quasars or AGN match both the normalization and redshift evolution of the observed ionizing background?
• RQ4 What is the predicted UV luminosity density at z ≈ 10 based on the inferred ionizing background evolution?
• RQ5 How independent is the derived ionizing background evolution from assumptions about the clumping factor C_HII or the nature of the ionizing sources?

Key findings

• The redshift evolution of the ionizing background is constrained to dlog₁₀N_ion/dz(z=8) = -0.19⁺⁰·⁰⁹₋₀·¹¹, with minimal dependence on the clumping factor or source identity.
• This evolution is in excellent quantitative agreement with the observed evolution of the galaxy UV luminosity density, which has dlog₁₀ρ_UV/dz = -0.11 ± 0.04 down to a magnitude limit of M > -13 mag.
• The consistency between the ionizing background evolution and the UV luminosity density evolution strongly supports galaxies as the primary drivers of cosmic reionization.
• The conversion from UV luminosity density to ionizing background is physically plausible for standard values of f_esc, ξ_ion, and M_lim, reinforcing the galaxy-driven reionization scenario.
• The inferred UV luminosity density at z ≈ 10 is estimated to be 12⁻⁷⁺²¹ times lower than at z ≈ 6, consistent with current observational constraints.
• Quasars and AGN are ruled out as dominant sources, as their ionizing output evolution and normalization do not match the observed ionizing background.