Hubble Space Telescope Wide Field Camera 3 Observations of Escaping Lyman Continuum Radiation from Galaxies and Weak AGN at Redshifts z ∼ 2.3-4.1

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 1s 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 1s 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 Lya and 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 fesc 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.