2011ApJ...736...41B

We present an upper limit for the relative escape fraction (f ^rel _esc) 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 redshifts in the range 3.27 < z < 3.35, from three independent fields. We stacked the images of our sources in the R and U bands, which correspond to an effective rest-frame wavelength of 1500 Å and 900 Å, respectively, obtaining a limit in the U band image of ≥ 30.7 mag (AB) at 1σ. We derive a 1σ upper limit of f ^rel_esc∼ 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 (Γ_–12< 0.3/s) is not enough to keep the intergalactic medium ionized and a substantial contribution to the UV background by faint active galactic nuclei 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.