The Lyman Continuum Escape Survey: Ionizing Radiation from [O III]-Strong Sources at a Redshift of 3.1

TL;DR

This study uses HST observations to measure the escape fraction of ionizing radiation from z~3.1 galaxies, revealing significant leakage in some sources and setting upper limits for others, informing reionization models.

Contribution

First direct measurement of ionizing photon escape fractions from [O III]-strong galaxies at z~3.1, linking low redshift analogs to reionization-era galaxies.

Findings

20% of sources show Lyman continuum leakage

Average escape fraction estimated at 20% for detected sources

Stacking non-detections constrains average escape fraction to less than 0.5%

Abstract

We present results from the LymAn Continuum Escape Survey (LACES), a Hubble Space Telescope (HST) program designed to characterize the ionizing radiation emerging from a sample of Lyman alpha emitting galaxies at redshift $z\simeq 3.1$. As many show intense [O III] emission characteristic of $z>6.5$ star-forming galaxies, they may represent valuable low redshift analogs of galaxies in the reionization era. Using HST Wide Field Camera 3 / UVIS $F336W$ to image Lyman continuum emission, we investigate the escape fraction of ionizing photons in this sample. For 61 sources, of which 77% are spectroscopically confirmed and 53 have measures of [O III] emission, we detect Lyman continuum leakage in 20%, a rate significantly higher than is seen in individual continuum-selected Lyman break galaxies. We estimate there is a 98% probability that $\leq 2$ of our detections could be affected by foreground contamination. Fitting multi-band spectral energy distributions (SEDs) to take account of the varying stellar populations, dust extinctions and metallicities, we derive individual Lyman continuum escape fractions corrected for foreground intergalactic absorption. We find escape fractions of 15 to 60% for individual objects, and infer an average 20% escape fraction by fitting composite SEDs for our detected samples. Surprisingly however, even a deep stack of those sources with no individual $F336W$ detections provides a stringent upper limit on the average escape fraction of less than 0.5%. We examine various correlations with source properties and discuss the implications in the context of the popular picture that cosmic reionization is driven by such compact, low metallicity star-forming galaxies.