An uncontaminated measurement of the escaping Lyman continuum at z ∼ 3

Observations of reionization-era analogues at z similar to 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 similar to 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 < f(900)/f(1500)>(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 < f(900)/f(1500)>(out) out and Ly alpha equivalent width (W-lambda(Lya alpha) and a negative correlation between < f(900)/f(1500)>(out) out and UV luminosity. As in previous work, we interpret the relationship between < f(900)/f(1500)>(out) out and W-lambda(Ly 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-lambda(Ly alpha)-weighted < f(900)/f(1500)>(out) out to estimate an ionizing emissivity from star-forming galaxies at z similar to 3 as epsilon(LyC) similar or equal to 5.5 x 10(24) 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 similar to 3 is comparable to that of active galactic nuclei.