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(Lyα⁠)) 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(Lyα⁠) 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(Lyα⁠)-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.

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