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~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.
