CDFS-6664: A candidate of Lyman-continuum Emission at z~3.8 detected by HDUV

TL;DR

This study reports the detection of Lyman Continuum emission from a galaxy at z~3.8, suggesting significant ionizing photon escape possibly facilitated by local star formation and a nearby quasar, with implications for cosmic reionization.

Contribution

First detection of LyC emission at z~3.8 with detailed analysis of escape fraction and IGM transmission, highlighting the role of local environment and foreground quasars.

Findings

LyC emission detected with ~5σ significance.

Estimated LyC escape fraction of 38±7%.

High IGM transmission possibly influenced by a nearby quasar.

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 [OIII] emission lines. The LyC emission is detected with a significance ~$5\sigma$ in the F336W band of the Hubble Deep UV Legacy Survey, corresponding to the 650-770 Angstrom 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$\pm$4.9 $M_{\odot}\mathrm{yr^{-1}}$. 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$\pm$7% and the corresponding IGM transmission to be 60%, which deviates more than 3$\sigma$ 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.