CDFS-6664: A Candidate of Lyman-continuum Emission at z ∼ 3.8 Detected by the Hubble Deep UV Legacy Survey

Abstract

We report the detection of Lyman continuum (LyC) emission from the galaxy, CDFS-6664, at z = 3.797 in a sample of Lyman break galaxies with detected [O iii] emission lines. The LyC emission is detected with a significance ∼5σ in the F336W band of the Hubble Deep UV Legacy Survey, corresponding to the 650–770 Å rest frame. The light centroid of the LyC emission is offset from the galaxy center by about 0.″2 (1.4 pkpc). The Hubble deep images at longer wavelengths show that the emission is unlikely provided by low-redshift interlopers. The photometric and spectroscopic data show that the possible contribution of an active galactic nucleus is quite low. Fitting the spectral energy distribution of this source to stellar population synthesis models, we find that the galaxy is young (∼50 Myr) and actively forming stars with a rate of 52.1 ± 4.9 M ⊙ yr−1. The significant star formation and the spatially offset LyC emission support a scenario where the ionizing photons escape from the low-density cavities in the ISM excavated by massive young stars. From the nebular model, we estimate the escape fraction of LyC photons to be 38% ± 7% and the corresponding intergalactic medium (IGM) transmission to be 60%, which deviates more than 3σ from the average transmission. The unusually high IGM transmission of LyC photons in CDFS-6664 may be related to a foreground type-2 quasar, CDF-202, at z = 3.7, with a projected separation of 1.′2 only. The quasar may have photoevaporated optically thick absorbers and enhance the transmission on the sightline of CDFS-6664.

Lyman Continuum Emission from Spectroscopically Confirmed Lyα Emitters at z ∼ 3.1

“…However, IGM absorption has large line-of-sight variations, and galaxies surrounded by more transparent environments tend to have higher LyC escape fractions (Bassett et al 2021). Therefore, the usage of a uniform IGM absorption or transmission introduces bias for individual LyC or LAE detections (Fletcher et al 2019;Byrohl & Gronke 2020;Bassett et al 2021;Yuan et al 2021). This is particularly important for the five LyC galaxies here.…”

“…Our LyC candidates either have intrinsically high f esc or have special environmental conditions. For example, a clear IGM path can be caused by a nearby AGN (Yuan et al 2021). In addition, f esc can also be overestimated by the average model if the intrinsic ionizing to nonionizing flux ratio is higher than usual.…”

“…UV images covering LyC emission provide an alternative method to find LyC galaxies and determine their f esc . Deep UV imaging observations have obtained a number of LyC galaxies or candidates (Vanzella et al 2012(Vanzella et al , 2016Yuan et al 2021;Saxena et al 2022b). These galaxies are often LBGs or Lyα emitters (LAEs) with strong resonant lines like Lyα.…”