An uncontaminated measurement of the escaping Lyman continuum at z 3

Abstract

Observations of reionization-era analogues at z ~ 3 are a powerful tool for constraining reionization. Rest-ultraviolet observations are particularly useful, in which both direct and indirect tracers of ionizing-photon production and escape can be observed. We analyse a sample of 124 z ~ 3 galaxies from the Keck Lyman Continuum Spectroscopic Survey, with sensitive spectroscopic measurements of the Lyman continuum region. We present a method of removing foreground contamination from our sample using high-resolution, multiband Hubble Space Telescope imaging. We re-measure the global properties of the cleaned sample of 13 individually detected Lyman continuum sources and 107 individually undetected sources, including a sample-averaged absolute escape fraction of f_{esc, abs} = 0.06 ± 0.01 and a sample-averaged ratio of ionizing to non-ionizing ultraviolet flux density of ⟨f₉₀₀/f₁₅₀₀⟩_out = 0.040 ± 0.006, corrected for attenuation from the intergalactic and circumgalactic media. Based on composite spectra, we also recover a strong positive correlation between ⟨f₉₀₀/f₁₅₀₀⟩_out and Lyα equivalent width (W_λ(Ly$\rm \alpha$)) and a negative correlation between ⟨f₉₀₀/f₁₅₀₀⟩_out and UV luminosity. As in previous work, we interpret the relationship between ⟨f₉₀₀/f₁₅₀₀⟩_out and W_λ(Ly$\rm \alpha$) in terms of the modulation of the escape of ionizing radiation from star-forming galaxies based on the covering fraction of neutral gas. We also use a W_λ(Ly$\rm \alpha$)-weighted ⟨f₉₀₀/f₁₅₀₀⟩_out to estimate an ionizing emissivity from star-forming galaxies at z ~ 3 as ϵ_LyC ≃ 5.5 × 10²⁴ erg s^-1 Hz^-1 Mpc^-3. This estimate, evaluated using the uncontaminated sample of this work, affirms that the contribution of galaxies to the ionizing background at z ~ 3 is comparable to that of active galactic nuclei.