An uncontaminated measurement of the escaping Lyman continuum at $z\sim3$

TL;DR

This study measures the escape fraction of ionizing radiation from $z\,sim3$ galaxies using Hubble imaging and spectroscopy, revealing correlations with Ly$ ext{a}$ properties and estimating their contribution to cosmic reionization.

Contribution

It introduces a method to remove foreground contamination and provides the first robust measurement of the absolute escape fraction at $z\,sim3$, linking Ly$ ext{a}$ features to ionizing photon escape.

Findings

Sample-averaged escape fraction of 0.06±0.01.

Strong correlation between Ly$ ext{a}$ equivalent width and ionizing flux ratio.

Galaxies' ionizing emissivity comparable to active galactic nuclei at $z\,sim3$.

Abstract

Observations of reionization-era analogs at $z\sim3$ 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 $z\sim3$ 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 $f_{esc,abs}=0.06\pm0.01$ and a sample-averaged ratio of ionizing to non-ionizing ultraviolet flux density of $<f_{900}/f_{1500}>_{out}=0.040\pm0.006$, 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$\alpha$ equivalent width (W$_\lambda$(Ly$\alpha$)) 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$_\lambda$(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 $<f_{900}/f_{1500}>_{out}$ to estimate an ionizing emissivity from star-forming galaxies at $z\sim3$ as $\epsilon_{\rm LyC}\simeq5.5\times10^{24}$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 $z\sim3$ is comparable to that of active galactic nuclei.