2018ApJ...853..191S

We present observations of escaping Lyman Continuum (LyC) radiation from 34 massive star-forming galaxies (SFGs) and 12 weak AGN with reliably measured spectroscopic redshifts at z≃2.3–4.1. We analyzed Hubble Space Telescope (HST) Wide Field Camera 3 (WFC3) mosaics of the Early Release Science (ERS) field in three UVIS filters to sample the rest-frame LyC over this redshift range. With our best current assessment of the WFC3 systematics, we provide 1σ upper limits for the average LyC emission of galaxies at <z>= 2.35, 2.75, and 3.60 to ∼28.5, 28.1, and 30.7 mag in image stacks of 11-15 galaxies in the WFC3/UVIS F225W, F275W, and F336W, respectively. The LyC flux of weak AGN at <z>= 2.62 and 3.32 are detected at 28.3 and 27.4 mag with S/Ns of ∼2.7 and 2.5 in F275W and F336W for stacks of 7 and 3 AGN, respectively, while AGN at <z>= 2.37 are constrained to >=27.9 mag at 1σ in a stack of 2 AGN. The stacked AGN LyC light profiles are flatter than their corresponding non-ionizing UV continuum profiles out to radii of r 0′′ / . 9, which may indicate a radial dependence of porosity in the ISM. With synthetic stellar SEDs fit to UV continuum measurements longward of Lyαand IGM transmission models, we constrain the absolute LyC escape fractions to f_esc^abs≃22_–22⁺⁴⁴% at <z>= 2.35 and <=55% at <z>= 2.75 and 3.60, respectively. All available data for galaxies, including published work, suggests a more sudden increase of f_esc with redshift at z≃2. Dust accumulating in (massive) galaxies over cosmic time correlates with increased H I column density, which may lead to reducing f_esc more suddenly at z 2. This may suggest that SFGs collectively contributed to maintaining cosmic reionization at redshifts z 2–4, while AGN likely dominated reionization at z 2.