An uncontaminated measurement of the escaping Lyman continuum at $z\sim 3$

Abstract

Observations of reionization-era analogs at $z\sim 3$ 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 $z\sim 3$ galaxies from the Keck Lyman Continuum Spectroscopic Survey, with sensitive spectroscopic measurements of the Lyman continuum (LyC) 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 LyC sources and 107 individually-undetected sources, including a sample-averaged absolute escape fraction of $f_{esc,abs}=0.06\pm 0.01$ and a sample-averaged ratio of ionizing to non-ionizing ultraviolet flux density of $<f_{900}/f_{1500}{>}_{out}=0.040\pm 0.006$, corrected for attenuation from the intergalactic and circumgalactic media. Based on composite spectra, we also recover a strong positive correlation between $<f_{900}/f_{1500}{>}_{out}$ and Ly$\hat{I}\pm$ equivalent width (W${}_{\hat{I}}»$(Ly$\hat{I}\pm$)) and a negative correlation between $<f_{900}/f_{1500}{>}_{out}$ and UV luminosity. As in previous work, we interpret the relationship between $<f_{900}/f_{1500}{>}_{out}$ and W${}_{\hat{I}}»$(Ly$\hat{I}\pm$) 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${}_{\hat{I}}»$(Ly$\hat{I}\pm$)-weighted $<f_{900}/f_{1500}{>}_{out}$ to estimate an ionizing emissivity from star-forming galaxies at $z\sim 3$ as $\hat{I}{\mu }_{\mathrm{LyC}}\simeq 5.5\times 10^{24}$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 $z\sim 3$ is comparable to that of active galactic nuclei.

23 pages, 10 figures, accepted for publication in MNRAS