Hubble Space Telescope Wide Field Camera 3 Observations of Escaping Lyman Continuum Radiation from Galaxies and Weak AGN at Redshifts z~2.3--5
(18 votes from 14 institutions)
We present observations of escaping Lyman Continuum (LyC) radiation from 50 massive star-forming galaxies and 14 weak AGN with reliable spectroscopic redshifts at z~2.3--5.8. We analyzed HST WFC3/UVIS mosaics of the ERS field in three UV filters, and ACS B in the GOODS-South field to sample the rest-frame LyC over these redshifts. The average LyC emission of galaxies at z_mean=2.38, 2.68, 3.47, and 5.02 is detected at the >=3sigma level in image stacks of 11--15 galaxies in the WFC3/UVIS F225W, F275W, F336W, and ACS/WFC F435W filters. Their average LyC flux corresponds to AB~29.5--30.7 mag. The LyC flux of weak AGN is typically ~1 mag brighter at z~2.3--4.8, but averaged over ~4x fewer galaxies. The stacked galaxy LyC profiles are flatter than their non-ionizing UV-continuum profiles out to r~0".7, possibly indicating a radial porosity dependence in the ISM. The average LyC emission from AGN is more extended and sometimes more elongated compared to galaxies without AGN, possibly due to the viewing-angle at which LyC escapes. With SED fits to UV-continuum longwards of Lya and IGM transmission models, we find absolute LyC escape fractions of fesc_abs~0.1--18% at z~2.3--3.9, and marginal constraints at z=5.02. Available data for galaxies, including published work, suggests a sudden increase of f_esc at z~3. Our models imply that dust accumulating in (massive) galaxies may be a major factor in reducing f_esc at z<=3. Our best fits of f_esc(z) suggest that star-forming galaxies collectively contributed to maintaining cosmic reionization at redshifts z>=3--6, while AGN likely dominated reionization at z<=2.5.