A low escape fraction of ionizing photons of L>L* Lyman break galaxies at z=3.3

TL;DR

This study places a stringent upper limit on the escape fraction of ionizing photons from Lyman Break Galaxies at z~3.3, suggesting they contribute minimally to cosmic reionization, and highlights the need for larger samples.

Contribution

First direct measurement of the upper limit of ionizing photon escape fraction in LBGs at z~3.3 using deep U-band imaging and stacking techniques.

Findings

Escape fraction upper limit ~5% at 1 sigma.

Ionizing photon contribution insufficient for IGM ionization.

Faint AGNs likely contribute significantly to UV background.

Abstract

We present an upper limit for the relative escape fraction (f_{esc}^{rel}) of ionizing radiation at z~3.3 using a sample of 11 Lyman Break Galaxies (LBGs) with deep imaging in the U band obtained with the Large Binocular Camera, mounted on the prime focus of the Large Binocular Telescope. We selected 11 LBGs with secure redshift in the range 3.27<z<3.35, from 3 independent fields. We stacked the images of our sources in the R and U band, which correspond to an effective rest-frame wavelength of 1500\AA and 900\AA respectively, obtaining a limit in the U band image of >=30.7(AB)mag at 1 sigma. We derive a 1 sigma upper limit of f_{esc}^{rel}~5%, which is one of the lowest values found in the literature so far at z~3.3. Assuming that the upper limit for the escape fraction that we derived from our sample holds for all galaxies at this redshift, the hydrogen ionization rate that we obtain (Gamma_{-12}<0.3 s^{-1}) is not enough to keep the IGM ionized and a substantial contribution to the UV background by faint AGNs is required. Since our sample is clearly still limited in size, larger z~3 LBG samples, at similar or even greater depths are necessary to confirm these results on a more firm statistical basis.