Model-independent constraints on the hydrogen-ionizing emissivity at z > 6

Charlotte A. Mason, Rohan P. Naidu, Sandro Tacchella, Joel Leja

Research output: Contribution to journalArticlepeer-review

42 Scopus citations


Modelling reionization often requires significant assumptions about the properties of ionizing sources. Here, we infer the total output of hydrogen-ionizing photons (the ionizing emissivity, N ion) at z = 4-14 from current reionization constraints, being maximally agnostic to the properties of ionizing sources. We use a Bayesian analysis to fit for a non-parametric form of Nion, allowing us to flexibly explore the entire prior volume. We infer a declining N ion with redshift at z > 6, which can be used as a benchmark for reionization models. Model-independent reionization constraints from the cosmic microwave background (CMB) optical depth and Ly α and Ly β forest dark pixel fraction produce Nion evolution (d log10 N ion/dz|z=6→8 = -0.31 ± 0.35 dex) consistent with the declining UV luminosity density of galaxies, assuming constant ionizing photon escape fraction and efficiency. Including measurements from Ly α damping of galaxies and quasars produces a more rapid decline: D log10 N ion/dz|z=6→8 = -0.44 ± 0.22 dex, steeper than the declining galaxy luminosity density (if extrapolated beyond MUV ≳ -13), and constrains the mid-point of reionization to z = 6.93 ± 0.14.

Original languageEnglish (US)
Pages (from-to)2669-2676
Number of pages8
JournalMonthly Notices of the Royal Astronomical Society
Issue number2
StatePublished - Oct 21 2019

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


Dive into the research topics of 'Model-independent constraints on the hydrogen-ionizing emissivity at z > 6'. Together they form a unique fingerprint.

Cite this