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

Abstract

We present an upper limit for the relative escape fraction (f relesc) 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 1500Å 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 relesc ∼ 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-1) 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. © 2011. The American Astronomical Society. All rights reserved.