A LOW ESCAPE FRACTION OF IONIZING PHOTONS OF L > L* LYMAN BREAK GALAXIES AT z=3.3

We present an upper limit for the relative escape fraction (f(esc)(rel)) of ionizing radiation at z similar to 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 angstrom and 900 angstrom, respectively, obtaining a limit in the U band image of >= 30.7 mag (AB) at 1 sigma. We derive a 1 sigma upper limit of f(esc)(rel) similar to 5%, which is one of the lowest values found in the literature so far at z similar to 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 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 similar to 3 LBG samples at similar or even greater depths are necessary to confirm these results on a more firm statistical basis.