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

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 Lyα-emitting galaxies at redshift z ≃ 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 that there is a 98% probability that ≤2 of our detections could be affected by foreground contamination. Fitting multiband 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%-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.